Susan L Beck

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• Kaviani, A., Sandvol, E., Ku, W. F., Beck, S. L., Turkelli, N., Ozacar, A. A., & Delph, J. R. (2022). Seismic attenuation tomography of the Sn phase beneath the Turkish-Iranian Plateau and the Zagros mountain belt:. Geosphere, 18(4), 1377– 1393. doi:https://doi.org /10.1130 /GES02503.1.
• Lynner, C., Delph, J. R., Portner, D. E., Beck, S. L., Sandvol, E., & Ozacar, A. A. (2022). Slab induced mantle upwelling beneath the Anatolian plateau. Geophysical Research Letters, 49. doi:doi.org/10.1029/2021GL097451
• Eakin, C. M., Villegas-lanza, J. C., Tavera, H., Lynner, C., Franca, G. S., Eakin, C. M., Condori, C., & Beck, S. L. (2021). Variable seismic anisotropy across the Peruvian flat-slab subduction zone with implications for upper plate deformation. Journal of South American Earth Sciences, 106. doi:10.1016/j.jsames.2020.103053
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  Abstract We performed shear wave splitting analyses to investigate seismic anisotropy across the northern extent of the Peruvian flat-slab subduction region. We used core-mantle refracted SKS, SKKS and PKS phases from teleseismic events (88° > Δ
• Rodriguez, E. E., Portner, D. E., Rocha, M. P., Rodriguez, E. E., Portner, D. E., Rocha, M. P., Fuck, R. A., Portner, D. E., Costa, I. S., Fuck, R. A., Costa, I. S., Beck, S. L., Assumpcao, M., Beck, S. L., Albuquerque, D. F., Assumpcao, M., Affonso, G. M., Albuquerque, D. F., & Affonso, G. M. (2021). Lithospheric architecture of the Paranapanema Block and adjacent nuclei using multiple‐frequency P‐wave seismic tomography. Journal of Geophysical Research, 126(4). doi:10.1029/2020jb021183
• Samaniego, P., Ruiz, M., Mothes, P., Meltzer, A., Lynner, C., Koch, C. D., Hidalgo, S., Hernandez, S., Delph, J. R., & Beck, S. L. (2021). Crustal thickness and magma storage beneath the Ecuadorian arc. Journal of South American Earth Sciences, 110, 103331. doi:10.1016/j.jsames.2021.103331
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  Abstract The Northern Andes of Ecuador contain some of the most active volcanic systems in the Andes and extend over a broad region from the Western Cordillera to the Subandean Zone. While it is known that the arc straddles a range of basement compositions, from accreted mafic oceanic terranes in the west to silicic continental terranes in the east, the details of the crustal structure beneath the arc is unclear despite being critical for understanding magmatic and tectonic processes in this portion of the Andes. To gain insight into these processes, we create two 3D models of crustal and upper mantle seismic properties throughout the region. The first highlights the discontinuity structure using receiver functions, which allows for the recovery of crustal thickness beneath the Ecuadorian Andes. We observe a range from ~50 to 65 km under the high elevations, with thicker crust beneath the lower elevation Western Cordillera compared to the higher elevation Eastern Cordillera. This can largely be explained by density variations within the crust that are consistent with observed terranes at the surface, implying these terranes extend to depth. The second model combines our receiver functions with Rayleigh wave dispersion data from ambient noise measurements in a joint inversion to construct a 3-D shear wave velocity model. This model shows several mid-crustal (5–20 km below sea-level) low velocity zones beneath Ecuadorian arc volcanoes that contain a maximum of ~14% melt. These low velocity zones likely represent zones of long-term magma storage in predominantly crystalline reservoirs, consistent with “mush zones”. Furthermore, the depth of the inferred reservoirs below several of the volcanic centers (e.g., Chiles-Cerro Negro and Tungurahua) are in broad agreement with previous geobarometry and geodetic modeling. Our results provide new observations of possible long-term magma reservoirs below other less-studied volcanic systems in the Ecuadorian arc as well, and further contributes to a mounting number of observations indicating long-term magma storage at low melt percentages in the mid-crust beneath active arc systems.
• Vaca, S., Soto-cordero, L., Segovia, M., Ruiz, M., Rolandone, F., Rietbrock, A., Regnier, M., Nocquet, J., Meltzer, A., Lynner, C., Leon-rios, S., Hoskins, M., Hernandez, S., Galve, A., Font, Y., Charvis, P., Chalumeau, C., Beck, S. L., Barros, L. D., , Alvarado, A., et al. (2021). Repeating Earthquakes at the Edge of the Afterslip of the 2016 Ecuadorian MW 7.8 Pedernales Earthquake. Journal of Geophysical Research, 126(5). doi:10.1029/2021jb021746
• Vaca, S., Stachnik, J. C., Soto-cordero, L., Ruiz, M., Rolandone, F., Rietbrock, A., Regnier, M., Nocquet, J., Meltzer, A., Lynner, C., Leon-rios, S., Hoskins, M., Hernandez, S., Font, Y., Charvis, P., Beck, S. L., Alvarado, A., & Agurto-detzel, H. (2021). Triggered crustal earthquake swarm across subduction segment boundary after the 2016 Pedernales, Ecuador megathrust earthquake. Earth and Planetary Science Letters, 553. doi:10.1016/j.epsl.2020.116620
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  Abstract Megathrust ruptures and the ensuing postseismic deformation cause stress changes that may induce seismicity on upper plate crustal faults far from the coseismic rupture area. In this study, we analyze seismic swarms that occurred in the north Ecuador area of Esmeraldas, beginning two months after the 2016 Mw 7.8 Pedernales, Ecuador megathrust earthquake. The Esmeraldas region is 70 km from the Pedernales rupture area in a separate segment of the subduction zone. We characterize the Esmeraldas sequence, relocating the events using manual arrival time picks and a local a-priori 3D velocity model. The earthquake locations from the Esmeraldas sequence outline an upper plate fault or shear zone. The sequence contains one major swarm and several smaller swarms. Moment tensor solutions of several events include normal and strike-slip motion and non-double-couple components. During the main swarm, earthquake hypocenters increase in distance from the first event over time, at a rate of a few hundred meters per day, consistent with fluid diffusion. Events with similar waveforms occur within the sequence, and a transient is seen in time series of nearby GPS stations concurrent with the seismicity. The events with similar waveforms and the transient in GPS time series suggest that slow aseismic slip took place along a crustal normal fault during the sequence. Coulomb stress calculations show a positive Coulomb stress change in the Esmeraldas region, consistent with seismicity being triggered by the Pedernales mainshock and large aftershocks. The characteristics of the seismicity indicate that postseismic deformation involving fluid flow and slow slip activated upper plate faults in the Esmeraldas area. These findings suggest the need for further investigation into the seismic hazard potential of shallow upper plate faults and the potential for megathrust earthquakes to trigger slow-slip and shallow seismicity across separate segments of subduction zones.
• Woollam, J., Ruiz, M., Rietbrock, A., Oregioni, D., Meltzer, A., Leon-rios, S., Laigle, M., Hoskins, M., Hidalgo, S., Galve, A., Font, Y., Charvis, P., Bie, L., Beck, S. L., Alvarado, A., & Agurto-detzel, H. (2021). 3D Local Earthquake Tomography of the Ecuadorian Margin in the Source Area of the 2016 Mw 7.8 Pedernales Earthquake. Journal of Geophysical Research, 126(3). doi:10.1029/2020jb020701
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  Based on manually analyzed waveforms recorded by the permanent Ecuadorian network and our large aftershock deployment installed after the Pedernales earthquake, we derive three-dimensional Vp and V...
• Kumar, A., Wagner, L. S., Long, M. D., Kumar, A., Caddick, M. J., & Beck, S. L. (2020). Effects of Oceanic Crustal Thickness on Intermediate Depth Seismicity. Frontiers in Earth Science, 8. doi:10.3389/feart.2020.00244
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  The occurrence of intermediate depth seismicity (70 – 300 km) is commonly attributed to the dehydration of hydrous phases within the downgoing oceanic plate. While some water is incorporated into the oceanic crust at formation, a significant amount of water is introduced into the plate immediately before subduction along outer-rise faults. These faults have been shown to extend to depths of over 30 km and can channel water to depths of 20 km or more beneath the seafloor. However, the amount of water introduced into the oceanic mantle lithosphere, and the role of that water in the formation of intermediate depth seismicity, has been the topic of ongoing research. Here we compile evidence from areas where the subducted oceanic crust is likely thicker than the penetration depth of water into the downgoing plate. These regions comprise aseismic plateaus and ridges (hot spot tracks) that can be compared directly to adjacent segments of the oceanic plate where oceanic crust of normal thickness is subducted. Regions with thick oceanic crust show little to no seismicity at intermediate depths, whereas adjacent regions with normal oceanic crust (~6-8 km thick) have significant seismicity at similar depths and distances from the trench. We hypothesize that intermediate depth earthquakes observed in regions with thinner oceanic crust are caused by mantle dehydration reactions that are not possible in regions where the oceanic mantle was never hydrated because the thickness of the oceanic crust exceeded the penetration depth of water into the plate. We compare our observations to phase diagrams of hydrous basalt and depleted peridotite to determine pressures and temperatures that would be consistent with our observations. These can provide valuable constraints, not only on the degree of hydration and dehydration in the downgoing plate, but also as ground-truth for thermal models of these regions, all of which have complex, three-dimensional, time-variant subduction geometries and thermal histories.
• Meltzer, A., Hoskins, M., Stachnik, J. C., Alvarado, A., Charvis, P., Font, Y., Hayes, G. P., Hernandez, S., Lynner, C., Regnier, M., Rietbrock, A., Rolandone, F., Ruiz, M., Soto-cordero, L., Nocquet, J., Leon-rios, S., Bergman, E. A., Beck, S. L., & Agurto-detzel, H. (2020). Structural Control on Megathrust Rupture and Slip Behavior: Insights From the 2016 Mw 7.8 Pedernales Ecuador Earthquake. Journal of Geophysical Research, 125(2). doi:10.1029/2019jb018001
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  The heterogeneous seafloor topography of the Nazca Plate as it enters the Ecuador subduction zone provides an opportunity to document the influence of seafloor roughness on slip behavior and megathrust rupture. The 2016 M$_{w}$ 7.8 Pedernales Ecuador earthquake was followed by a rich and active postseismic sequence. An internationally coordinated rapid response effort installed a temporary seismic network to densify coastal stations of the permanent Ecuadorian national seismic network. A combination of 82 onshore short and intermediate period and broadband seismic stations and six ocean bottom seismometers recorded the postseismic Pedernales sequence for over a year after the mainshock. A robust earthquake catalog combined with calibrated relocations for a subset of magnitude ≥4 earthquakes shows pronounced spatial and temporal clustering. A range of slip behavior accommodates postseismic deformation including earthquakes, slow slip events, and earthquake swarms. Models of plate coupling and the consistency of earthquake clustering and slip behavior through multiple seismic cycles reveal a segmented subduction zone primarily controlled by subducted seafloor topography, accreted terranes, and inherited structure. The 2016 Pedernales mainshock triggered moderate to strong earthquakes (5 ≤ M ≤ 7) and earthquake swarms north of the mainshock rupture close to the epicenter of the 1906M$_{w}$ 8.8 earthquake and in the segment of the subduction zone that ruptured in 1958 in a M$_{w}$ 7.7 earthquake.
• Meltzer, A., Stachnik, J. C., Soto-cordero, L., Ruiz, M., Rietbrock, A., Regnier, M., Meltzer, A. S., Lynner, C., Koch, C. D., Hoskins, M., Font, Y., Delph, J. R., Charvis, P., Beck, S. L., Alvarado, A., & Agurto-detzel, H. (2020). Structure of the Ecuadorian forearc from the joint inversion of receiver functions and ambient noise surface waves. Geophysical Journal International, 222(3), 1671-1685. doi:10.1093/gji/ggaa237
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  The Ecuadorian forearc is a complex region of accreted terranes with a history of largemegathrust earthquakes. Most recently, aMw7.8 megathrust earthquake ruptured the plateboundary offshore of Pedernales, Ecuador on 16 April 2016. Following this event, an inter-national collaboration arranged by the Instituto Geofisico at the Escuela Polit ecnica Nacionalmobilized a rapid deployment of 65 seismic instruments along the Ecuadorian forearc. Wecombine this new seismic data set with 14 permanent stations from the Ecuadorian nationalnetwork to better understand how variations in crustal structure relate to regional seismichazards along the margin. Here, we present receiver function adaptive common conversionpoint stacks and a shear velocity model derived from the joint inversion of receiver functionsand surface wave dispersion data obtained through ambient noise cross-correlations for theupper 50 km of the forearc. Beneath the forearc crust, we observe an eastward dipping slowvelocity anomaly we interpret as subducting oceanic crust, which shallows near the projectedcentre of the subducting Carnegie Ridge. We also observe a strong shallow positive conversionin the Ecuadorian forearc near the Borbon Basin indicating a major discontinuity at a depth of∼7 km. This conversion is not ubiquitous and may be the top of the accreted terranes. We alsoobserve significant north–south changes in shear wave velocity. The velocity changes indicatevariations in the accreted terranes and may indicate an increased amount of hydration beneaththe Manab i Basin. This change in structure also correlates geographically with the southernrupture limit of multiple high magnitude megathrust earthquakes. The earthquake record alongthe Ecuadorian trench shows that no event with aMw>7.4 has ruptured south of∼0.5◦Sinsouthern Ecuador or northern Peru. Our observations, along with previous studies, suggestthat variations in the forearc crustal structure and subducting oceanic crust may influance theoccurrence and spatial distribution of high magnitude seismicity in the region.
• Venerdini, A., Podesta, M., Lopez, L., Linkimer, L., Fuentes, F., Beck, S. L., Ammirati, J. B., & Alvarado, P. (2020). Crustal seismicity in the Andean Precordillera of Argentina using seismic broadband data. Tectonophysics, 786, 228450. doi:10.1016/j.tecto.2020.228450
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  Abstract In this study, we analyze 100 crustal Precordilleran earthquakes recorded in 2008 and 2009 by 52 broadband seismic stations from the SIEMBRA and ESP, two temporary experiments deployed in the Pampean flat slab region, between the Andean Cordillera and the Sierras Pampeanas in the Argentine Andean backarc region. In order to determine more accurate hypocenters, focal mechanisms and regional stress orientations, we relocated 100 earthquakes using the JHD technique and a local velocity model. The focal depths of our relocated events vary between 6 and 50 km. We estimated local magnitudes between 0.4 ≤ ML ≤ 5.3 and moment magnitudes between 1.3 ≤ Mw ≤ 5.3. Focal mechanisms were determined from new hypocenter relocations and first motion P-wave polarities. Our solutions exhibit a majority of the earthquakes with reverse faulting mechanism. Regional stress tensor from the inversion of P- and T-axis orientations, shows a maximum stress axis (σ1) almost horizontal with a strike of 85° and a minimum stress axis (σ3) almost vertical. We correlate this small-to-moderate magnitude seismicity with the presence of large basement structures beneath the Iglesia-Calingasta Basin in the west and the Eastern Precordillera in the east. The nucleation of deep earthquakes beneath the Iglesia Basin could be related to the presence of a major ramp accommodating the crustal shortening between the Frontal Cordillera and the Precordillera. The crustal seismicity beneath the Precordillera seems to correlate with west-dipping structures rooting deep into the Cuyania basement suggesting a thick-skinned basement deformation system beneath the Precordillera and its shallow thin-skinned fold and thrust belt.
• Zandt, G., Anderson, M., Zhang, H., Zandt, G., Linkimer, L., Gilbert, H., Beck, S. L., Anderson, M. L., & Alvarado, P. (2020). Lithospheric structure of the Pampean flat slab region from double-difference tomography. Journal of South American Earth Sciences, 97, 102417. doi:10.1016/j.jsames.2019.102417
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  Abstract We obtain earthquake locations and a detailed three-dimensional velocity model of the flat slab subduction zone in west-central Argentina (latitudes: 32-30°S and longitudes: 70-66°W) using a regional-scale double-difference tomography algorithm with earthquake data recorded by the SIEMBRA (2007–2009) and ESP (2008–2010) broadband seismic networks. In this region, the flat subduction of the Nazca Plate including the Juan Fernandez Ridge is spatially correlated with a volcanic gap and the basement-cored uplifts of the Sierras Pampeanas in the overriding South American Plate. Our results show the subducting Nazca Plate as a continuous band of mostly increased P-wave velocities coinciding with the Wadati-Benioff Zone. In the overriding South American Plate, the lithospheric mantle appears to be heterogeneous but mostly characterized by a ratio between P- and S-wave velocities (Vp/Vs) of 1.75–1.77, which is consistent with depleted peridotites. Two Vp/Vs anomalies deviate from this mantle with lower (1.70–1.73) and higher (1.78–1.82) Vp/Vs, which are interpreted as localized dry and hydrated regions, respectively. The lower Vp/Vs is consistent with an enrichment of 40–80% of orthopyroxene and the higher Vp/Vs with up to 5% mantle hydration. The size, orientation, and location of these seismic anomalies suggest the progressive eastward dehydration of the subducting slab and the presence of an east-dipping large-scale lithospheric suture, which is interpreted as evidence of an ancient subduction zone and also as a weak zone that facilitates the hydration of the upper plate. Our inversion results suggest a thicker South American crust in the Western Sierras Pampeanas and the partial eclogitization of the lower crust beneath that region where velocities match three types of eclogites at depths of 40–60 km. In the middle-to-upper crust, velocities are reduced in the Precordillera and Vp/Vs is higher in the Cuyania and Chilenia terranes (>1.75) than in the Pampia terrane (1.67–1.75). These observations are consistent with the presence of a thick carbonate sequence in the Precordillera, mafic-ultramafic rocks in Cuyania and Chilenia, and felsic rocks in Pampia. The higher variability in Vp/Vs and strong velocity changes at crustal depths within the Precordillera and the Cuyania Terrane agree with more complexity in crustal structure for these regions and reveal two mid-crustal discontinuities as well as the Chilenia-Precordillera suture zone. Finally, the relocated slab earthquakes refine the slab geometry and suggest that at depths of ~100 km, the flat slab segment is ~240 km wide and has a slight westward dip (~2°) before it resumes its descent into the mantle with a steep angle (~25°). The observation of a wider flat slab segment than the width of the Juan Fernandez Ridge offshore (~100 km) implies that there might be additional contributing factors for the flattening besides the subduction of the overthickened oceanic crust.
• Zandt, G., Zandt, G., Scire, A. C., Ruiz, M., Rodriguez, E. E., Rocha, M. P., Portner, D. E., Franca, G. S., Condori, C., Bianchi, M. B., Beck, S. L., & Alvarado, P. (2020). Detailed Structure of the Subducted Nazca Slab into the Lower Mantle Derived From Continent-Scale Teleseismic P Wave Tomography. Journal of Geophysical Research, 125(5). doi:10.1029/2019jb017884
• Leon-rios, S., Agurto-detzel, H., Rietbrock, A., Alvarado, A., Charvis, P., Edwards, B., Font, Y., Garth, T., Hoskins, M., Lynner, C., Meltzer, A., Nocquet, J. M., Regnier, M., Rolandone, F., Ruiz, M., Soto-cordero, L., & Beck, S. L. (2019). 1D-velocity structure and seismotectonics of the Ecuadorian margin inferred from the 2016 Mw7.8 Pedernales aftershock sequence. Tectonophysics, 767, 228165. doi:10.1016/j.tecto.2019.228165
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  Abstract On April 16th 2016 a Mw 7.8 earthquake ruptured the central coastal segment of the Ecuadorian subduction zone. Shortly after the earthquake, the Instituto Geofisico de la Escuela Politecnica Nacional of Ecuador, together with several international institutions deployed a dense, temporary seismic network to accurately categorize the post-seismic aftershock sequence. Instrumentation included short-period and broadband sensors, along with Ocean Bottom Seismometers. This deployment complemented the permanent Ecuadorian seismic network and recorded the developing aftershock sequence for a period of one year following the main-shock. A subset of 345 events with ML > 3.5, were manually picked in the period of May to August 2016, providing highly accurate P- and S-onset times. From this catalogue, a high-quality dataset of 227 events, with an azimuthal gap
• Zandt, G., Zandt, G., Bishop, B. T., & Beck, S. L. (2019). Segmentation in continental forearcs: Links between large-scale overriding plate structure and seismogenic behavior associated with the 2010 Mw 8.8 Maule, Chile earthquake. Tectonophysics, 767, 228164. doi:10.1016/j.tecto.2019.228164
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  Abstract Subduction along the active margin of a continental plate occurs in a context where the overriding plate's crust and lithospheric mantle may contain inherited structures significantly predating the present tectonic conditions of the margin. These structures are persistent over very long-term time scales (>105 to >106 years) and are thought to play an important role in both seismogenic processes on the megathrust and development of topography along coastlines. We use receiver functions calculated from broadband seismic data collected along the Chilean forearc between ~33°S and 38.5°S in the vicinity of the 2010 Mw 8.8 Maule earthquake to determine the structure of the overriding South American continental plate and subducting Nazca oceanic plate along and inboard of the seismogenic portion of the megathrust. We show that the Chilean forearc is divided into three structurally distinct zones: a northern zone where the continental crust intersects the subducting plate well inboard of the coast at ~35–40 km depth, a central zone where the continental crust tapers to
• Zandt, G., Zandt, G., Wagner, L. S., Tavera, H., Long, M. D., Beck, S. L., & Bar, N. (2019). Receiver function analysis reveals layered anisotropy in the crust and upper mantle beneath southern Peru and northern Bolivia. Tectonophysics, 753, 93-110. doi:10.1016/j.tecto.2019.01.007
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  Abstract Subduction systems play a key role in plate tectonics, but the deformation of the crust and uppermost mantle during continental subduction remains poorly understood. Observations of seismic anisotropy can provide constraints on dynamic processes in the crust and uppermost mantle in subduction systems. The subduction zone beneath Peru and Bolivia, where the Nazca plate subducts beneath South America, represents a particularly interesting location to study subduction-related deformation, given the along-strike transition from flat to normally dipping subduction. In this study we constrain seismic anisotropy within and above the subducting slab (including the overriding plate) beneath Peru and Bolivia by examining azimuthal variations in radial and transverse component receiver functions. Because anisotropy-aware receiver function analysis has good lateral resolution and depth constraints, it is complementary to previous studies of anisotropy in this region using shear wave splitting or surface wave tomography. We examine data from long-running permanent stations NNA (near Lima, Peru) and LPAZ (near La Paz, Bolivia), and two dense lines of seismometers from the PULSE and CAUGHT deployments in Peru and Bolivia, respectively. The northern line overlies the Peru flat slab, while the southern line overlies the normally dipping slab beneath Bolivia. We applied harmonic decomposition modeling to constrain the presence, depth, and characteristics of dipping and/or anisotropic interfaces within the crust and upper mantle. We found evidence for varying multi-layer anisotropy, in some cases with dipping symmetry axes, underneath both regions. The presence of multiple layers of anisotropy with distinct geometries that change with depth suggests a highly complex deformation regime associated with subduction beneath the Andes. In particular, our identification of depth-dependent seismic anisotropy within the overlying plate crust implies a change in deformation geometry, dominant mineralogy, and/or rheology with depth, shedding light on the nature of deep crustal deformation during orogenesis.
• Bishop, B. T., Beck, S. L., Zandt, G., Wagner, L. S., Long, M. D., & Tavera, H. (2018). Foreland uplift during flat subduction: Insights from the Peruvian Andes and Fitzcarrald Arch. TECTONOPHYSICS, 731, 73-84.
• Lynner, C., Beck, S. L., Zandt, G., Porritt, R. W., Lin, F., & Eilon, Z. C. (2018). Midcrustal Deformation in the Central Andes Constrained by Radial Anisotropy. JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, 123(6), 4798-4813.
• Portner, D. E., Delph, J. R., Biryol, C. B., Beck, S. L., Zandt, G., Ozacar, A. A., Sandvol, E., & Turkelli, N. (2018). Subduction termination through progressive slab deformation across Eastern Mediterranean subduction zones from updated P-wave tomography beneath Anatolia. GEOSPHERE, 14(3), 907-925.
• Abgarmi, B., Delph, J. R., Ozacar, A. A., Beck, S. L., Zandt, G., Sandvol, E., Turkelli, N., & Biryol, C. B. (2017). Structure of the crust and African slab beneath the central Anatolian plateau from receiver functions: New insights on isostatic compensation and slab dynamics. GEOSPHERE, 13(6), 1774-1787.
• Bishop, B. T., Beck, S. L., Zandt, G., Wagner, L., Long, M., Antonijevic, S. K., Kumar, A., & Tavera, H. (2017). Causes and consequences of flat-slab subduction in southern Peru. GEOSPHERE, 13(5), 1392-1407.
• Delph, J. R., Abgarmi, B., Ward, K. M., Beck, S. L., Ozacar, A. A., Zandt, G., Sandvol, E., Turkelli, N., & Kalafat, D. (2017). The effects of subduction termination on the continental lithosphere: Linking volcanism, deformation, surface uplift, and slab tearing in central Anatolia. GEOSPHERE, 13(6), 1788-1805.
• Delph, J. R., Ward, K. M., Zandt, G., Ducea, M. N., & Beck, S. L. (2017). Imaging a magma plumbing system from MASH zone to magma reservoir. EARTH AND PLANETARY SCIENCE LETTERS, 457, 313-324.
• Garzione, C. N., McQuarrie, N., Perez, N. D., Ehlers, T. A., Beck, S. L., Kar, N., Eichelberger, N., Chapman, A. D., Ward, K. M., Ducea, M. N., Lease, R. O., Poulsen, C. J., Wagner, L. S., Saylor, J. E., Zandt, G., & Horton, B. K. (2017). Tectonic Evolution of the Central Andean Plateau and Implications for the Growth of Plateaus. ANNUAL REVIEW OF EARTH AND PLANETARY SCIENCES, VOL 45, 45, 529-559.
• Lynner, C., Anderson, M. L., Portner, D. E., Beck, S. L., & Gilbert, H. (2017). Mantle flow through a tear in the Nazca slab inferred from shear wave splitting. GEOPHYSICAL RESEARCH LETTERS, 44(13), 6735-6742.
• Portner, D. E., Beck, S., Zandt, G., & Scire, A. (2017). The nature of subslab slow velocity anomalies beneath South America. GEOPHYSICAL RESEARCH LETTERS, 44(10), 4747-4755.
• Scire, A., Zandt, G., Beck, S., Long, M., & Wagner, L. (2017). The deforming Nazca slab in the mantle transition zone and lower mantle: Constraints from teleseismic tomography on the deeply subducted slab between 6 degrees S and 32 degrees S. GEOSPHERE, 13(3), 665-680.
• Tavera, H. J., Franca, G. S., Condori, C., Bishop, B. T., Beck, S. L., & Albuquerque, D. F. (2017). Crustal structure of north Peru from analysis of teleseismic receiver functions. Journal of South American Earth Sciences, 76, 11-24. doi:10.1016/j.jsames.2017.02.006
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  Abstract In this study, we present results from teleseismic receiver functions, in order to investigate the crustal thickness and Vp/Vs ratio beneath northern Peru. A total number of 981 receiver functions were analyzed, from data recorded by 28 broadband seismic stations from the Peruvian permanent seismic network, the regional temporary SisNort network and one CTBTO station. The Moho depth and average crustal Vp/Vs ratio were determined at each station using the H-k stacking technique to identify the arrival times of primary P to S conversion and crustal reverberations (PpPms, PpSs + PsPms). The results show that the Moho depth correlates well with the surface topography and varies significantly from west to east, showing a shallow depth of around 25 km near the coast, a maximum depth of 55–60 km beneath the Andean Cordillera, and a depth of 35–40 km further to the east in the Amazonian Basin. The bulk crustal Vp/Vs ratio ranges between 1.60 and 1.88 with the mean of 1.75. Higher values between 1.75 and 1.88 are found beneath the Eastern and Western Cordilleras, consistent with a mafic composition in the lower crust. In contrast values vary from 1.60 to 1.75 in the extreme flanks of the Eastern and Western Cordillera indicating a felsic composition. We find a positive relationship between crustal thickness, Vp/Vs ratio, the Bouguer anomaly, and topography. These results are consistent with previous studies in other parts of Peru (central and southern regions) and provide the first crustal thickness estimates for the high cordillera in northern Peru.
• Ward, K. M., Delph, J. R., Zandt, G., Beck, S. L., & Ducea, M. N. (2017). Magmatic evolution of a Cordilleran flare-up and its role in the creation of silicic crust. SCIENTIFIC REPORTS, 7.
• Zandt, G., Wagner, L. S., Tavera, H., Minaya, E., Long, M. D., Deng, J., Creasy, N., & Beck, S. L. (2017). Lowermost mantle anisotropy near the eastern edge of the Pacific LLSVP: constraints from SKS–SKKS splitting intensity measurements. Geophysical Journal International, 210(2), 774-786. doi:10.1093/gji/ggx190
• Ammirati, J., Perez, L. S., Alvarado, P., Beck, S., Rocher, S., & Zandt, G. (2016). High-resolution images above the Pampean flat slab of Argentina (31-32 degrees S)from local receiver functions: Implications on regional tectonics. EARTH AND PLANETARY SCIENCE LETTERS, 450, 29-39.
• Antonijevic, S. K., Wagner, L. S., Beck, S. L., Long, M. D., Zandt, G., & Tavera, H. (2016). Effects of change in slab geometry on the mantle flow and slab fabric in Southern Peru. JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, 121(10), 7252-7270.
• Eakin, C. M., Long, M. D., Scire, A., Beck, S. L., Wagner, L. S., Zandt, G., & Tavera, H. (2016). Internal deformation of the subducted Nazca slab inferred from seismic anisotropy. NATURE GEOSCIENCE, 9(1), 56-+.
• Kumar, A., Wagner, L. S., Beck, S. L., Long, M. D., Zandt, G., Young, B., Tavera, H., & Minaya, E. (2016). Geometry and state of stress in the central and southern Peruvian flat slab. Earth and Planetary Science Letters, 441, 71-80.
• Kumar, A., Wagner, L. S., Beck, S. L., Long, M. D., Zandt, G., Young, B., Tavera, H., & Minaya, E. (2016). Seismicity and state of stress in the central and southern Peruvian flat slab. EARTH AND PLANETARY SCIENCE LETTERS, 441, 71-80.
• Long, M. D., Biryol, C. B., Eakin, C. M., Beck, S. L., Wagner, L. S., Zandt, G., Minaya, E., & Tavera, H. (2016). Overriding plate, mantle wedge, slab, and subslab contributions to seismic anisotropy beneath the northern Central Andean Plateau. GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, 17(7), 2556-2575.
• Perkins, J. P., Ward, K. M., de, S., Zandt, G., Beck, S. L., & Finnegan, N. J. (2016). Surface uplift in the Central Andes driven by growth of the Altiplano Puna Magma Body. NATURE COMMUNICATIONS, 7.
• Ryan, J., Beck, S. L., Zandt, G., Wagner, L., & Minaya, E. (2016). Central Andean crustal structure from receiver function analysis. Tectonophysics, 682, 120-133. doi:doi:10.1016/0040-1951(96)00024-8
• Scire, A., Zandt, G., Beck, S., Long, M., Wagner, L., Minaya, E., & Tavera, H. (2015). Imaging the transition from flat to normal subduction: variations in the structure of the Nazca slab and upper mantle under southern Peru and northwestern Bolivia. GEOPHYSICAL JOURNAL INTERNATIONAL, 204(1), 457-479.
• Ward, K. M., Zandt, G., Beck, S. L., Wagner, L. S., & Tavera, H. (2016). Lithospheric structure beneath the northern Central Andean Plateau from the joint inversion of ambient noise and earthquake-generated surface waves. JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, 121(11), 8217-8238.
• Zandt, G., Zandt, G., Wagner, L. S., Tavera, H., Scire, A. C., Minaya, E., Long, M. D., & Beck, S. L. (2016). Imaging the transition from flat to normal subduction: variations in the structure of the Nazca slab and upper mantle under southern Peru and northwestern Bolivia. Geophysical Journal International, 204(1), 457-479. doi:10.1093/gji/ggv452
• Ammirati, J. B., Zandt, G., Porter, R. C., Perez, S. B., Beck, S. L., & Alvarado, P. (2015). High resolution crustal structure for the region between the Chilenia and Cuyania terrane above the Pampean flat slab of Argentina from local receiver function and petrological analyses. earth and Planetary Science Letters.
• Ammirati, J., Alvarado, P., & Beck, S. L. (2015). A lithospheric velocity model for the flat slab region of Argentina from joint inversion of Rayleigh wave phase velocity dispersion and teleseismic receiver functions. GEOPHYSICAL JOURNAL INTERNATIONAL, 202(1), 224-241.
• Antonijevic, S. K., Wagner, L. S., Kumar, A., Beck, S. L., Long, M. D., Zandt, G., Tavera, H., & Condori, C. (2015). The role of ridges in the formation and longevity of flat slabs. NATURE, 524(7564), 212-+.
• Beck, S. L., Zandt, G., Ward, K. M., & Scire, A. (2015). Multiple styles and scales of lithospheric foundering beneath the Puna Plateau, Central Andes. Geodynamics of a Cordilleran Orogenic System: The Central Andes of Argentina and Northern Chile: Geological Society of America Memoir.
• DeCelles, P. G., Zandt, G., Beck, S. L., Currie, C. A., Ducea, M. N., Kapp, P., Gehrels, G. E., Carrapa, B., Quade, J., & Schoenbohm, L. M. (2015). Cyclical Orogenic Processes in the Cenozoic Central Andes. Geodynamics of a Cordilleran Orogenic System: The Central Andes of Argentina and Northern Chile: Geological Society of America Memoir 212., 212, 459-490.
• Delph, J. R., Biryol, C. B., Beck, S. L., Zandt, G., & Ward, K. M. (2015). Shear wave velocity structure of the Anatolian Plate: anomalously slow crust in southwestern Turkey. GEOPHYSICAL JOURNAL INTERNATIONAL, 202(1), 261-276.
• Delph, J. R., Zandt, G., & Beck, S. L. (2015). A new approach to obtaining a 3D shear wave velocity model of the crust and upper mantle: An application to eastern Turkey. TECTONOPHYSICS, 665, 92-100.
• Delph, J., Biryol, C. B., Beck, S. L., Zandt, G., & Ward, K. M. (2015). Shear-wave velocity structure of the Anatolian Plate: Anomalously slow crust in southwestern Turkey. Geophysical Journal International.
• Eakin, C. M., Long, M. D., Wagner, L. S., Beck, S. L., & Tavera, H. (2015). Upper mantle anisotropy beneath Peru from SKS splitting: Constraints on flat slab dynamics and interaction with the Nazca Ridge. EARTH AND PLANETARY SCIENCE LETTERS, 412, 152-162.
• Eakin, C. M., Long, M. D., Wagner, L. S., Beck, S. L., & Tavera, H. (2015). Upper mantle anisotropy beneath Peru from SKS splitting: Constraints on flat slab dynamics and interactions with the Nazca Ridge. Earth and Planetary Science Letters, 412, 152-162.
• Eichelberger, N., McQuarrie, N., Ryan, J., Karimi, B., Beck, S., & Zandt, G. (2015). Evolution of crustal thickening in the central Andes, Bolivia. EARTH AND PLANETARY SCIENCE LETTERS, 426, 191-203.
• Knezevic Antonijevic, S., Wagner, L., Kumar, A., Beck, S. L., Long, M., Zandt, G., Tavera, H., & Condori, C. (2015). The Role of Slab Buoyancy in the Formation of Flat Slabs. Nature Geosciences.
• Perkins, J. P., Zandt, G., Ward, K. M., Silva, S. L., Finnegan, N. J., & Beck, S. L. (2015). Melt production constrained by the topographic signature of the Altiplano-Puna Magma Body. Geoscience Communication.
• Scire, A., Biryol, C. B., Zandt, G., & Beck, S. L. (2015). Imaging the Nazca slab and surrounding mantle to 700 km depth beneath the Central Andes. Geodynamics of a Cordilleran Orogenic System: The Central Andes of Argentina and Northern Chile: Geological Society of America Memoir.
• Beck, S. L., Rietbrock, A., Tilmann, F., Barrientos, S., Meltzer, A., Oncken, O., Bataille, K., Roecker, S., Vilotte, J., & Russo, R. M. (2014). Advancing Subduction Zone Science After a Big Quake. EOS, 95(23), 193-194.
• Eakin, C. M., Long, M. D., Beck, S. L., Wagner, L. S., Tavera, H., & Condori, C. (2014). Response of the Mantle to Flat Slab Evolution: Insights from Local S Splitting beneath Peru. Geophysical Research Letters, 41, 3438-3446.
• Eakin, C. M., Long, M. D., Beck, S. L., Wagner, L. S., Tavera, H., & Condori, C. (2014). Response of the mantle to flat slab evolution: Insights from local S splitting beneath Peru. GEOPHYSICAL RESEARCH LETTERS, 41(10), 3438-3446.
• Karasozen, E., Ozacar, A. A., Biryol, C. B., & Beck, S. L. (2014). Seismicity, focal mechanisms and active stress field around the central segment of the North Anatolian Fault in Turkey. GEOPHYSICAL JOURNAL INTERNATIONAL, 196(1), 405-421.
• Ryan, J., Zandt, G., Wagner, L. S., Tavera, H., Minaya, E., & Beck, S. L. (2014). Constraining the Lithospheric Structure of the Central Andes Using P- and S- wave Receiver Functions. Tectonophysics. doi:10.1016/0040-1951(96)00024-8
• Ward, K. M., Porter, R. C., Zandt, G., Beck, S. L., Wagner, L. S., Minaya, E., & Tavera, H. (2014). Ambient noise tomography across the Central Andes (vol 194, pg 1559, 2013). GEOPHYSICAL JOURNAL INTERNATIONAL, 196(2), 1264-1265.
• Ward, K. M., Zandt, G., Beck, S. L., Christensen, D. H., & McFarlin, H. (2014). Seismic imaging of the magmatic underpinnings beneath the Altiplano-Puna volcanic complex from the joint inversion of surface wave dispersion and receiver functions. EARTH AND PLANETARY SCIENCE LETTERS, 404, 43-53.
• Ward, K. M., Zandt, G., Beck, S. L., Christensen, D. H., & McFarlin, H. (2015). Seismic Imaging of the Magmatic Underpinnings Beneath the Altiplano-Puna Volcanic Complex from the Joint Inversion of Surface Wave Dispersion and Receiver Functions. Earth and Planetary Science Letters, 404, 43-53.
• Biryol, C. B., Leahy, G. M., Zandt, G., & Beck, S. L. (2013). Imaging the shallow crust with local and regional earthquake tomography. JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, 118(5), 2289-2306.
• Biryol, C. B., Leahy, G. M., Zandt, G., & Beck, S. L. (2013). Imaging the shallow crust with local and regional earthquake tomography. Journal of Geophysical Research B: Solid Earth, 118(5), 2289-2306.
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  Abstract: While active-source imaging (seismic reflection, refraction) is typically used to image the shallow crust, these techniques tend to suffer from energy penetration problems in complex tectonic regimes, resulting in poor imaging. Further, active sources (such as air guns or vibroseis) tend to be band limited, resulting in poor signal-to-noise ratio at low frequencies (1-10 Hz). Recent studies suggest that earthquake data may be able to solve these imaging problems. However, conventional earthquake tomography typically aims to image the upper mantle and lithosphere, where typical station spacing (tens of km) and array aperture (hundreds of km) have resulted in the maximum resolution. In this study, we take advantage of the small-scale and 250 m station spacing of the LaBarge Passive Seismic Experiment to determine whether local and regional earthquake tomographies can be used to constrain structure in the top 5 km of the crust. We also study how the inclusion of finite-frequency effects impacts the final images. Our results indicate that local and regional events provide substantial improvements over teleseismic events alone, with approximately 500 m resolution both vertically and laterally in the upper most 5 km. We also find that inclusion of finite-frequency data between 1 and 10 Hz plays a key role in maintaining resolution in the shallowest portion of the model. © 2013. American Geophysical Union.
• Karasözen, E., Özacar, A. A., Biryol, C. B., & Beck, S. L. (2013). Seismicity, focal mechanisms and active stress field around the central segment of the north anatolian fault in turkey. Geophysical Journal International, 196(1), 405-421.
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  Abstract: We analysed locations and focal mechanisms of events with magnitude ≥3, which are recordedby 39 broad-band seismic stations deployed during the North Anatolian Passive SeismicExperiment (2005-2008) around central segment of the North Anatolian Fault (NAF). UsingP- and S-arrival times, earthquakes are relocated and a new 1-D seismic velocity model of the region is derived. Relocated events in the area are mainly limited to a depth of 15 km andpresent seismicity in the southern block indicates widespread continental deformation. In thenext step, focal mechanisms are derived from first motions (P, SH) and amplitude ratios (SH/P)using a grid-search algorithm in an iterative scheme. Analysis of our well-constrained focalmechanisms indicate mainly strike-slip motions apart from some normal and few thrust eventsthat are related to complex local fault geometry. Calculated pressure/tension axes are mainlysubhorizontal and maximum horizontal stress directions (SH max) are oriented predominantlyin NW-SE direction which corresponds well with the slip character of NAF and its splays.In the east, E-W trending splays show right-lateral strike-slip mechanisms similar to themain strand whereas in the west, antithetic N-S trending faults show left lateral strike-slipmotions. The seismic cluster that converged near Çorum after relocation indicates a dominantright-lateral strike-slip mechanism along the E-W trending fault. These focal mechanisms areused to perform stress tensor inversion across the region to map out the stress field in detail.Overall, maximum (σ1) and minimum (σ3) principal stresses are found to be subhorizontaland the intermediate principle stress (σ2) is vertically orientated, consistent with a dominantstrike-slip regime. These directions point to the clockwise rotation of stress trajectories from N to S where NW-SE directed σ1 in the north turns towards N-S in the south away from theNAF. Moreover, the 200-km-long Ezinepazar-Sungurlu Fault which is previously mapped asan active strike-slip fault is characterized by minor seismic activity and trends perpendicularto the computed maximum stress direction in the southwest away from the main strand of NAF suggesting that the Sungurlu segment is either compressional in nature or inactive. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.
• Ward, K. M., Porter, R. C., Zandt, G., Beck, S. L., Wagner, L. S., Minaya, E., & Tavera, H. (2013). Ambient noise tomography across the Central Andes. GEOPHYSICAL JOURNAL INTERNATIONAL, 194(3), 1559-1573.
• Ward, K. M., Porter, R. C., Zandt, G., Beck, S. L., Wagner, L. S., Minaya, E., & Tavera, H. (2013). Ambient noise tomography across the Central Andes. Geophysical Journal International, 194(3), 1559-1573.
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  Abstract: The Central Andes of southern Peru, Bolivia, Argentina and Chile (between 12°S and 42°S) comprise the largest orogenic plateau in the world associated with abundant arc volcanism, the Central Andean Plateau, as well as multiple segments of flat-slab subduction making this part of the Earth a unique place to study various aspects of active plate tectonics. The goal of this continental-scale ambient noise tomography study is to incorporate broad-band seismic data from 20 seismic networks deployed incrementally in the Central Andes from 1994 May to 2012 August, to image the vertically polarized shear wave velocity (Vsv) structure of the South American Cordillera. Using dispersion measurements calculated from the cross-correlation of 330 broad-band seismic stations, we construct Rayleigh wave phase velocity maps in the period range of 8-40 s and invert these for the shear wave velocity (Vsv) structure of the Andean crust. We provide a dispersion misfit map as well as uncertainty envelopes for our Vsv model and observe striking first-order correlations with our shallow results (~5 km) and the morphotectonic provinces as well as subtler geological features indicating our results are robust. Our results reveal for the first time the full extent of the mid-crustal Andean lowvelocity zone that we tentatively interpret as the signature of a very large volume Neogene batholith. This study demonstrates the efficacy of integrating seismic data from numerous regional broad-band seismic networks to approximate the high-resolution coverage previously only available though larger networks such as the EarthScope USArray Transportable Array in the United States. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.
• Ward, K. M., Porter, R. C., Zandt, G., Beck, S. L., Wagner, L. S., Minaya, E., & Tavera, H. (2013). Erratum to Ambient noise tomography across the Central Andes [Ambient noise tomography across the Central Andes [Geophys. J. Int., 194, (2013), 1559]. Geophysical Journal International, 196(2), 1264-1265.
• Warren, L. M., Beck, S. L., Biryol, C. B., Ozacar, A., & Yang, Y. (2013). Crustal velocity structure of Central and Eastern Turkey from ambient noise tomography. Geophysical Journal International, 194(3), 1941-1954.
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  Abstract: In eastern Turkey, the ongoing convergence of the Arabian and African plates with Eurasia has resulted in the westward extrusion of the Anatolian Plate. To better understand the current state and the tectonic history of this region, we image crust and uppermost mantle structure with ambient noise tomography. Our study area extends from longitudes of 32° to 44°E. We use continuous data from two temporary seismic deployments, our 2006-2008 North Anatolian Fault Passive Seismic Experiment and the 1999-2001 Eastern Turkey Seismic Experiment, as well as from additional seismographs in the region. We compute daily cross-correlations of noise records between all station pairs and stack them over the entire time period for which they are available, as well as in seasonal subsets, to obtain interstation empirical Green's functions. After selecting interstation cross-correlations with high signal-to-noise ratios and measuring interstation phase velocities, we compute phase velocity maps at periods ranging from 8 to 40 s. At all periods, the phase velocity mapsare similar for winter and summer subsets of the data, indicating that seasonal variations in noise sources do not bias our results. Across the study area, we invert the phase velocity estimates for shear velocity as a function of depth. The shear velocity model, which extends to 50 km depth, highlights tectonic features apparent at the surface: the Eastern Anatolian Plateau is a prominent low-velocity anomaly whereas the Kirşehir Massif has relatively fast velocities. There is a large velocity jump across the Inner Tauride Suture/Central Anataolian Fault Zone throughout the crust whereas the North Anatolian Fault does not have a consistent signature. In addition, in the southeastern part of our study area, we image a high velocity region below 20 km depth which may be the northern tip of the underthrusting Arabian Plate. copy; The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.
• Warren, L. M., Beck, S. L., Biryol, C. B., Zandt, G., Ozacar, A. A., & Yang, Y. (2013). Crustal velocity structure of Central and Eastern Turkey from ambient noise tomography. GEOPHYSICAL JOURNAL INTERNATIONAL, 194(3), 1941-1954.
• Kumar, A., Zandt, G., Wagner, L. S., Long, M. D., & Beck, S. L. (2012). GEOMETRY AND STATE OF STRESS OF THE SLAB BENEATH THE NORTH CENTRAL ANDES. Earth and Planetary Science Letters.
• Lange, D., Tilmann, F., Barrientos, S. E., Contreras-Reyes, E., Methe, P., Moreno, M., Heit, B., Agurto, H., Bernard, P., Vilotte, J., & Beck, S. (2012). Aftershock seismicity of the 27 February 2010 Mw 8.8 Maule earthquake rupture zone. EARTH AND PLANETARY SCIENCE LETTERS, 317, 413-425.
• Lange, D., Tilmann, F., Barrientos, S. E., Contreras-Reyes, E., Methe, P., Moreno, M., Heit, B., Agurto, H., Bernard, P., Vilotte, J., & Beck, S. (2012). Aftershock seismicity of the 27 February 2010 Mw 8.8 Maule earthquake rupture zone. Earth and Planetary Science Letters, 317-318, 413-425.
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  Abstract: On 27 February 2010 the M w 8.8 Maule earthquake in Central Chile ruptured a seismic gap where significant strain had accumulated since 1835. Shortly after the mainshock a dense network of temporary seismic stations was installed along the whole rupture zone in order to capture the aftershock activity. Here, we present the aftershock distribution and first motion polarity focal mechanisms based on automatic detection algorithms and picking engines. By processing the seismic data between 15 March and 30 September 2010 from stations from IRIS, IPGP, GFZ and University of Liverpool we determined 20,205 hypocentres with magnitudes M w between 1 and 5.5. Seismic activity occurs in six groups: 1.) Normal faulting outer rise events 2.) A shallow group of plate interface seismicity apparent at 25-35km depth and 50-120km distance to the trench with some variations between profiles. Along strike, the aftershocks occur largely within the zone of coseismic slip but extend ~50km further north, and with predominantly shallowly dipping thrust mechanisms. Along dip, the events are either within the zone of coseismic slip, or downdip from it, depending on the coseismic slip model used. 3.) A third band of seismicity is observed further downdip at 40-50km depth and further inland at 150-160km trench perpendicular distance, with mostly shallow dipping (~28°) thrust focal mechanisms indicating rupture of the plate interface significantly downdip of the coseismic rupture, and presumably above the intersection of the continental Moho with the plate interface. 4.) A deep group of intermediate depth events between 80 and 120km depth is present north of 36°S. Within the Maule segment, a large portion of events during the inter-seismic phase originated from this depth range. 5.) The magmatic arc exhibits a small amount of crustal seismicity but does not appear to show significantly enhanced activity after the M w 8.8 Maule 2010 earthquake. 6.) Pronounced crustal aftershock activity with mainly normal faulting mechanisms is found in the region of Pichilemu (~34.5°S). These crustal events occur in a ~30km wide region with sharp inclined boundaries and oriented oblique to the trench. The best-located events describe a plane dipping to the southwest, consistent with one of the focal planes of the large normal-faulting aftershock (M w=6.9) on 11 March 2010. © 2011 Elsevier B.V.
• Porter, R., Gilbert, H., Zandt, G., Beck, S., Warren, L., Calkins, J., Alvarado, P., & Anderson, M. (2012). Shear wave velocities in the Pampean flat-slab region from Rayleigh wave tomography: Implications for slab and upper mantle hydration. JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, 117.
• Porter, R., Gilbert, H., Zandt, G., Beck, S., Warren, L., Calkins, J., Alvarado, P., & Anderson, M. (2012). Shear wave velocities in the Pampean flat-slab region from Rayleigh wave tomography: Implications for slab and upper mantle hydration. Journal of Geophysical Research B: Solid Earth, 117(11).
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  Abstract: The Pampean flat-slab region, located in central Argentina and Chile between 29 and 34S, is considered a modern analog for Laramide flat-slab subduction within western North America. Regionally, flat-slab subduction is characterized by the Nazca slab descending to ∼100 km depth, flattening out for ∼300 km laterally before resuming a more "normal" angle of subduction. Flat-slab subduction correlates spatially with the track of the Juan Fernandez Ridge, and is associated with the inboard migration of deformation and the cessation of volcanism within the region. To better understand flat-slab subduction we combine ambient-noise tomography and earthquake-generated surface wave measurements to calculate a regional 3D shear velocity model for the region. Shear wave velocity variations largely relate to changes in lithology within the crust, with basins and bedrock exposures clearly defined as low- and high-velocity regions, respectively. We argue that subduction-related hydration plays a significant role in controlling shear wave velocities within the upper mantle. In the southern part of the study area, where normal-angle subduction is occurring, the slab is visible as a high-velocity body with a low-velocity mantle wedge above it, extending eastward from the active arc. Where flat-slab subduction is occurring, slab velocities increase to the east while velocities in the overlying lithosphere decrease, consistent with the slab dewatering and gradually hydrating the overlying mantle. The hydration of the slab may be contributing to the excess buoyancy of the subducting oceanic lithosphere, helping to drive flat-slab subduction. © 2012. American Geophysical Union. All Rights Reserved.
• Yolsal-Cevikbilen, S., Biryol, C. B., Beck, S., Zandt, G., Taymaz, T., Adiyaman, H. E., & Ozacar, A. A. (2012). 3-D crustal structure along the North Anatolian Fault Zone in north-central Anatolia revealed by local earthquake tomography. GEOPHYSICAL JOURNAL INTERNATIONAL, 188(3), 819-849.
• Yolsal-Çevikbilen, S., Biryol, C. B., Beck, S., Zandt, G., Taymaz, T., Adiyaman, H. E., & Özacar, A. A. (2012). 3-D crustal structure along the North Anatolian Fault Zone in north-central Anatolia revealed by local earthquake tomography. Geophysical Journal International, 188(3), 819-849.
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  Abstract: 3-D P-wave velocity structure and V p/V s variations in the crust along the North Anatolian Fault Zone (NAFZ) in north-central Anatolia were investigated by the inversion of local P- and S-wave traveltimes, to gain a better understanding of the seismological characteristics of the region. The 3-D local earthquake tomography inversions included 5444 P- and 3200 S-wave readings obtained from 168 well-located earthquakes between 2006 January and 2008 May. Dense ray coverage yields good resolution, particularly in the central part of the study area. The 3-D V p and V p/V s tomographic images reveal clear correlations with both the surface geology and significant tectonic units in the region. We observed the lower limit of the seismogenic zone for north-central Anatolia at 15 km depth. Final earthquake locations display a distributed pattern throughout the study area, with most of the earthquakes occurring on the major splays of the NAFZ, rather than its master strand. We identify three major high-velocity blocks in the mid-crust separated by the I.zmir-Ankara-Erzincan Suture and interpret these blocks to be continental basement fragments that were accreted onto the margin following the closure of Neo-Tethyan Ocean. These basement blocks may have in part influenced the rupture propagations of the historical 1939, 1942 and 1943 earthquakes. In addition, large variations in the V p/V s ratio in the mid-crust were observed and have been correlated with the varying fluid contents of the existing lithologies and related tectonic structures. © 2012 The Authors Geophysical Journal International © 2012 RAS.
• Biryol, C. B., Beck, S. L., Zandt, G., & Ozacar, A. A. (2011). Segmented African lithosphere beneath the Anatolian region inferred from teleseismic P-wave tomography. GEOPHYSICAL JOURNAL INTERNATIONAL, 184(3), 1037-1057.
• Biryol, C. B., Beck, S. L., Zandt, G., & Özacar, A. A. (2011). Segmented African lithosphere beneath the Anatolian region inferred from teleseismic P-wave tomography. Geophysical Journal International, 184(3), 1037-1057.
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  Abstract: Lithospheric deformation throughout Anatolia, a part of the Alpine-Himalayan orogenic belt, is controlled mainly by collision-related tectonic escape of the Anatolian Plate and subduction roll-back along the Aegean Subduction Zone. We study the deeper lithosphere and mantle structure of Anatolia using teleseismic, finite-frequency, P-wave traveltime tomography. We use data from several temporary and permanent seismic networks deployed in the region. Approximately 34 000 P-wave relative traveltime residuals, measured in multiple frequency bands, are inverted using approximate finite-frequency sensitivity kernels. Our tomograms reveal segmented fast seismic anomalies beneath Anatolia that corresponds to the subducted portion of the African lithosphere along the Cyprean and the Aegean trenches. We identify these anomalies as the subducted Aegean and the Cyprus slabs that are separated from each other by a gap as wide as 300 km beneath Western Anatolia. This gap is occupied by slow velocity perturbations that we interpret as hot upwelling asthenosphere. The eastern termination of the subducting African lithosphere is located near the transition from central Anatolia to the Eastern Anatolian Plateau or Arabian-Eurasian collision front that is underlain by large volumes of hot, underplating asthenosphere marked by slow velocity perturbations. Our tomograms also show fast velocity perturbations at shallow depths beneath northwestern Anatolia that sharply terminates towards the south at the North Anatolian Fault Zone (NAFZ). The associated velocity contrast across the NAFZ persists down to a depth of 100-150 km. Hence, our study is the first to investigate and interpret the vertical extent of deformation along this nascent transform plate boundary. Overall, the resolved upper-mantle structure of Anatolia is directly related with the geology and tectonic features observed at the surface of the Anatolian Plate and suggest that the segmented nature of the subducted African lithosphere plays an important role in the evolution of Anatolia and distribution of its tectonic provinces. © 2011 The Authors Geophysical Journal International © 2011 RAS.
• Gans, C. R., Beck, S. L., Zandt, G., Gilbert, H., Alvarado, P., Anderson, M., & Linkimer, L. (2011). Continental and oceanic crustal structure of the Pampean flat slab region, western Argentina, using receiver function analysis: New high-resolution results. Geophysical Journal International, 186(1), 45-58.
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  Abstract: The Pampean flat slab of central Chile and Argentina (30°-32°S) has strongly influenced Cenozoic tectonics in western Argentina, which contains both the thick-skinned, basement-cored uplifts of the Sierras Pampeanas and the thin-skinned Andean Precordillera fold and thrust belt. In this region of South America, the Nazca Plate is subducting nearly horizontally beneath the South American Plate at ∼100km depth. To gain a better understanding of the deeper structure of this region, including the transition from flat to 'normal' subduction to the south, three IRIS-PASSCAL arrays of broad-band seismic stations have been deployed in central Argentina. Using the dense SIEMBRA array, combined with the broader CHARGE and ESP arrays, the flat slab is imaged for the first time in 3-D detail using receiver function (RF) analysis. A distinct pair of RF arrivals consisting of a negative pulse that marks the top of the oceanic crust, followed by a positive pulse, which indicates the base of the oceanic crust, can be used to map the slab's structure. Depths to Moho and oceanic crustal thicknesses estimated from RF results provide new, more detailed regional maps. An improved depth to continental Moho map shows depths of more than 70km in the main Cordillera and ∼50km in the western Sierras Pampeanas, that shallow to ∼35km in the eastern Sierras Pampeanas. Depth to Moho contours roughly follow terrane boundaries. Offshore, the hotspot seamount chain of the Juan Fernández Ridge (JFR) is thought to create overthickened oceanic crust, providing a mechanism for flat slab subduction. By comparing synthetic RFs, based on various structures, to the observed RF signal we determine that the thickness of the oceanic crust at the top of the slab averages at least ∼13-19km, supporting the idea of a moderately overthickened crust to provide the additional buoyancy for the slab to remain flat. The overthickened region is broader than the area directly aligned with the path of the JFR, however, and indicates, along with the slab earthquake locations, that the flat slab area is wider than the JFR volcanic chain observed in the offshore bathymetry. Further, RFs indicate that the subducted oceanic crust in the region directly along the path of the subducted ridge is broken by trench-parallel faults. One explanation for these faults is that they are older structures within the oceanic crust that were created when the slab subducted. Alternatively, it is possible that faults formed recently from tectonic underplating caused by increased interplate coupling in the flat slab region. © 2011 The Authors. Geophysical Journal International © 2011 RAS.
• Gans, C. R., Beck, S. L., Zandt, G., Gilbert, H., Alvarado, P., Anderson, M., & Linkimer, L. (2011). Continental and oceanic crustal structure of the Pampean flat slab region, western Argentina, using receiver function analysis: new high-resolution results. GEOPHYSICAL JOURNAL INTERNATIONAL, 186(1), 45-58.
• Biryol, C. B., Zandt, G., Beck, S. L., Ozacar, A. A., Adiyaman, H. E., & Gans, C. R. (2010). Shear wave splitting along a nascent plate boundary: the North Anatolian Fault Zone. GEOPHYSICAL JOURNAL INTERNATIONAL, 181(3), 1201-1213.
• Biryol, C. B., Zandt, G., Beck, S. L., Ozacar, A. A., Adiyaman, H. E., & Gans, C. R. (2010). Shear wave splitting along a nascent plate boundary: the North Anatolian Fault Zone. Geophysical Journal International, 181(3), 1201-1213.
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  Abstract: The North Anatolian Fault Zone (NAFZ) is a transform structure that constitutes the boundary between the Anatolian Plate to the south and the Eurasia Plate to the north. We analysed the properties of the upper-mantle strain field and mantle anisotropy in the vicinity of NAFZ via splitting of SKS and SKKS phases. We used data from the North Anatolian Fault (NAF) passive seismic experiment. This is the first study that analyses the upper-mantle anisotropy in this region and our results indicate that the observed upper-mantle strain field is uniform underneath the array with consistent NE-SW polarization directions for fast split waves. The measured lag times between the arrivals of the fast and slow split waves varies from 0.5 to 1.6 s for the study area. Smaller lag times are obtained consistently in the eastern part of the array. However, we do not observe any significant variation in either the polarization directions or the delay times across the plate boundary (NAFZ).The uniformity of the fast polarization directions throughout the study area and the strength of anisotropy favour an asthenospheric source for the anisotropy. The regional tectonic framework favours a SW direction of asthenospheric flow due to the forces acting on the upper-mantle exerted by the slab-roll-back taking place along the Aegean and the Cyprean Subduction Zones. © 2010 The Authors Journal compilation © 2010 RAS.
• Linkimer, L., Beck, S. L., Schwartz, S. Y., Zandt, G., & Levin, V. (2010). Nature of crustal terranes and the Moho in northern Costa Rica from receiver function analysis. GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, 11.
• Özacar, A. A., Zandt, G., Gilbert, H., & Beck, S. L. (2010). Seismic images of crustal variations beneath the East Anatolian Plateau (Turkey) from teleseismic receiver functions. Geological Society Special Publication, 340, 485-496.
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  Abstract: We used teleseismic P-wave receiver functions recorded by the Eastern Turkey Seismic Experiment to determine the crustal structure across an active continent-continent collision zone. Moho depth and V p/V s variations in the region are mapped by incorporating crustal multiples and later two-dimsional (2-D) seismic profiles are produced using a common conversion point technique with our crustal V p/V s estimates. Moho depths do not correlate with surface topography and reveal a relatively thin crust consistent with the high plateau being supported by hot asthenosphere near the base of the crust. Under the Arabian plate, the crust is thinnest (c. 35 km) and exhibits high V p/V s (≥1.8) associated with mafic compositions. In the east, the crust gradually becomes thicker towards the north and exceeds 45 km in the northeastern side whereas in the west, the crust thickens sharply near the Bitlis suture and displays pronounced Moho topography within the Anatolian plate that suggests the presence of multiple fragments. V p/V s variations show an anomalously high V p/V s corridor (≥1.85) along the North Anatolian Fault and near the youngest volcanic units (c. 3 Ma) and support the presence of partial melt. This corridor is spatially limited from both north and south by low V p/V s regions implying a change in crustal composition. Near the Bitlis suture, a layered V p/V s model points to the source of low V p/V s in the lower crust that may be rich in quartz. Furthermore, the seismic profiles indicate a prominent low velocity zone in the lower crust across a large area beneath the plateau that may act as a decoupling zone between the crust and upper mantle. © 2010 The Geological Society of London.
• Alvarado, P., Barrientos, S., Saez, M., Astroza, M., & Beck, S. (2009). Source study and tectonic implications of the historic 1958 Las Melosas crustal earthquake, Chile, compared to earthquake damage. PHYSICS OF THE EARTH AND PLANETARY INTERIORS, 175(1-2), 26-36.
• Alvarado, P., Barrientos, S., Saez, M., Astroza, M., & Beck, S. (2009). Source study and tectonic implications of the historic 1958 Las Melosas crustal earthquake, Chile, compared to earthquake damage. Physics of the Earth and Planetary Interiors, 175(1-2), 26-36.
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  Abstract: The 4 September 1958 earthquake has been the largest event recorded at shallow depths in the western flank of the Andes on Chilean territory. New estimates of fault orientation, depth and size have been carried out using modern techniques of body-wave modeling. Two possible fault planes solutions with right-lateral displacement on an east-west fault or left-lateral displacement on a north-south fault nucleated at 5-9 km depth produce the best fit to teleseismic recordings. A seismic moment M0 of 0.227 × 1019 N m associated with a moment-magnitude Mw of 6.3 has been estimated with these techniques, which is a more reliable estimation of earthquake size than the 0.4-0.7 units larger surface-wave magnitude Ms earlier reported. Although no surface rupture for the 1958 Las Melosas crustal earthquake was reported, the displacement along east-west structures like that one suggested for one of the fault plane in our focal mechanism solution seems to be an efficient mechanism to accommodate differences in shortening from north to south in the High Andean Cordillera. Reports on damage, landslide effects as well as re-analysis of intensities associated with the new seismic source estimations for the 1958 Las Melosas earthquake are presented to further estimate the hazard to which this zone, and others along the western foothills of the Andes, is exposed. © 2009 Elsevier B.V.
• Alvarado, P., Pardo, M., Gilbert, H., Miranda, S., Anderson, M., Saez, M., & Beck, S. (2009). Flat-slab subduction and crustal models for the seismically active Sierras Pampeanas region of Argentina. BACKBONE OF THE AMERICAS: SHALLOW SUBDUCTION, PLATEAU UPLIFT, AND RIDGE AND TERRANE COLLISION, 204, 261-278.
• Alvarado, P., Pardo, M., Gilbert, H., Miranda, S., Anderson, M., Saez, M., & Beck, S. (2009). Flat-slab subduction and crustal models for the seismically active Sierras Pampeanas region of Argentina. Memoir of the Geological Society of America, 204, 261-278.
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  Abstract: The Sierras Pampeanas in the west-central part of Argentina are a modern analog for Laramide uplifts in the western United States. In this region, the Nazca plate is subducting beneath South America almost horizontally at about ∼100 km depth before descending into the mantle. The flat-slab geometry correlates with the inland prolongation of the subducted oceanic Juan Fernández Ridge. This region of Argentina is characterized by the termination of the volcanic arc and uplift of the active basementcored Sierras Pampeanas. The upper plate shows marked differences in seismic properties that are interpreted as variations in crustal composition in agreement with the presence of several Neoproterozoic to Paleozoic accreted terranes. In this paper, we combine the results from the CHile-AKgentina Geophysical Experiment (CHARGE) and the CHile-ARgentina Seismology Measurement Experiment (CHARSME) passive broadband arrays to better characterize the flat-slab subduction and the lithospheric structure. Stress tensor orientations indicate that the horizontal slab is in extension, whereas the upper plate backarc crust is under compression. The Cuyania terrane crust exhibits high P-wave seismic velocities (Yp -6.4 km/s), high P- to S-wave seismic velocity ratios (Vp/Vs = 1.80-1.85), and 55-60 km crustal thickness. In addition, the Cuyania terrane has a high-density and high-seismicvelocity lower crust. In contrast, the Pampia terrane crust has a lower Vp value of 6.0 km/s, a lower Vp/Vs ratio of 1.73, and a thinner crust of ∼35 km thickness. We integrate seismic and gravity studies to evaluate crustal models that can explain the unusually low elevations of the western Sierras Pampeanas. Flat-slab subduction models based on CHARGE and CHARSME seismic data and gravity observations show a good correlation with the predicted Juan Fernández Ridge path beneath South America, the deep Moho depths in the Andean backarc, and the high-density and high-seismic- velocity lower crust of the Cuyania terrane. The Cuyania terrane is also the region characterized by more frequent and largermagnitude crustal earthqu. © 2009 The Geological Society of America. All rights reserved.
• Gans, C. R., Beck, S. L., Zandt, G., Biryol, C. B., & Ozacar, A. A. (2009). Detecting the limit of slab break-off in central Turkey: New high-resolution Pn tomography results. Geophysical Journal International, 179(3), 1566-1572.
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  Abstract: Inversion of Pn traveltime residuals from a 39-station broad-band array provides a high-resolution image of the velocity structure in the uppermost mantle beneath central Turkey. Individually picked Pn phase arrivals from events recorded by the North Anatolian Fault Passive Seismic Experiment and the Kandilli Observatory were combined with additional events associated with the Eastern Turkey Seismic Experiment. Tomography results show no change in Pn velocity across the North Anatolian Fault, although longitudinal variations are evident. A region of very low Pn velocities (8.1 km s-1) west of the fault. The sharp transition along the CAFZ, which follows the palaeotectonic Inner-Tauride Suture, may represent the location of the edge of the slab window, created when the oceanic slab broke off along the Bitlis-Zagros Suture around 11 Ma, as the Arabian plate collided with the Eurasian plate. © 2009 The Authors Journal compilation © 2009 RAS.
• Gans, C. R., Beck, S. L., Zandt, G., Biryol, C. B., & Ozacar, A. A. (2009). Detecting the limit of slab break-off in central Turkey: new high-resolution Pn tomography results. GEOPHYSICAL JOURNAL INTERNATIONAL, 179(3), 1566-1572.
• Jeanloz, R., Beck, S. L., Lisowski, M., Lorenzo, J. M., Mora, C. I., Rimstidt, J. D., Shirey, S. B., Stein, S., & Wirth, K. (2008). Earth science Instrumentation and Facilities program review. Eos, 89(7), 61-.
• Wagner, L. S., Anderson, M. L., Jackson, J. M., Beck, S. L., & Zandt, G. (2008). Seismic evidence for orthopyroxene enrichment in the continental lithosphere. GEOLOGY, 36(12), 935-938.
• Wagner, L. S., Anderson, M. L., Jackson, J. M., Beck, S. L., & Zandt, G. (2008). Seismic evidence for orthopyroxene enrichment in the continental lithosphere. Geology, 36(12), 935-938.
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  Abstract: We assess the ability of predicted seismic velocities (Vp, compressional wave; Vs, shear wave; and Vp/Vs) to identify regions in the upper mantle that are enriched in orthopyroxene relative to normal melt-depleted peridotite compositions. Orthopyroxene enrichment has been found in mantle xenoliths from a number of locations, including the Colorado Plateau and the Kaapvaal craton. We find that the Vp/Vs ratio is very sensitive to orthopyroxene concentration, but it is not sensitive to depletion level. We compare these predicted velocities and Vp/Vs ratios to the high Vs, low Vp/Vs anomaly found above the central Chile-Argentina flat slab. Within error, the predicted velocities of some of the orthopyroxene-enriched xenoliths match the observed velocities from the Chile-Argentina upper mantle. Because the anomaly above the central Chile-Argentina flat slab does not conform to known terrane boundaries but does align with the downgoing Juan Fernandez ridge track, we suggest that the orthopyroxene enrichment in this area may be related to flat slab processes, which would have implications for our understanding of lithospheric silica enrichment in the western U.S. and elsewhere. © 2008 The Geological Society of America.
• Alvarado, P., Beck, S., & Zandt, G. (2007). Crustal structure of the south-central Andes Cordillera and backarc region from regional waveform modelling. Geophysical Journal International, 170(2), 858-875.
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  Abstract: We investigate the crustal structure in the Andes Cordillera and its backarc region using regional broadband waveforms from crustal earthquakes. We consider seismic waveforms recorded at regional distances by the CHile-ARgentina Geophysical Experiment (CHARGE) during 2000-2002 and utilize previous seismic moment tensor inversion results. For each single station-earthquake pair, we fixed the source parameters and performed forward waveform modelling using ray paths that sample the crust of the highest elevation Cordillera and the accreted terranes in the backarc region. Our investigation indicates that synthetic seismograms for our earthquake-station geometry are most sensitive to crustal parameters and less sensitive to mantle parameters. We performed a grid search around crustal thickness, P-wave seismic velocity (Vp) and P- to S-wave seismic velocity ratio (Vp/Vs), fixing mantle parameters. We evaluated this waveform analysis by estimating an average correlation coefficient between observed and synthetic data over the three broadband components. We identified all acceptable crustal models that correspond to high correlation coefficients that provide best overall seismogram fits for the data and synthetic waveforms filtered mainly between 10 and 80 s. Our results indicate along strike variations in the crustal structure for the north-south high Cordillera with higher P -wave velocity and thickness in the northern segment (north of 33°S), and persistently high Vp/Vs ratio (>1.85) in both segments. This is consistent with a colder mafic composition for the northern segment and a region of crustal thickening above the flat slab region. In contrast, the results for the current volcanic arc (south of 33°S) agree with a warmer crust consistent with partial melt related to Quaternary volcanism presumably of an intermediate to mafic composition. A distinctive feature in the backarc region is the marked contrast between the seismic properties of the Cuyania and Pampia terranes that correlates with their heterogeneous crustal composition. The Cuyania terrane, composed of mafic-ultramafic rocks, exhibits high Vp, high Vp/ Vs and a thicker layered crust versus the thinner more quartz-rich crust of the eastern Sierras Pampeanas associated with low Vp and low Vp/Vs. These differences may have some effect on the mechanism that unevenly generates crustal seismicity in the upper ∼30 km in this active compressional region. In particular, the seismic properties of the Cuyania terrane, which shows evidence for a high Vp and high Vp/Vs crust, may be reflecting the complex tectonic evolution history of this terrane including accretion-rifting and re-accretion processes since the Palaeozoic that promote a high level of crustal seismicity in the upper ∼30 km, enhanced by the flat slab subduction in the segment between 31°S and 32°S. Another possible mechanism could be directly related to the presence of a strong lower crust above the flat slab that efficiently transfer stresses from the slab to the upper crust generating higher seismicity in the Cuyania terrane between 30°S and 34°S. © 2007 The Authors Journal compilation © 2007 RAS.
• Anderson, M., Alvarado, P., Zandt, G., & Beck, S. (2007). Geometry and brittle deformation of the subducting Nazca Plate, Central Chile and Argentina. GEOPHYSICAL JOURNAL INTERNATIONAL, 171(1), 419-434.
• Anderson, M., Alvarado, P., Zandt, G., & Beck, S. (2007). Geometry and brittle deformation of the subducting Nazca Plate, Central Chile and Argentina. Geophysical Journal International, 171(1), 419-434.
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  Abstract: We use data from the Chile Argentina Geophysical Experiment (CHARGE) broad-band seismic deployment to refine past observations of the geometry and deformation within the subducting slab in the South American subduction zone between 30°S and 36°S. This region contains a zone of flat slab subduction where the subducting Nazca Plate flattens at a depth of ∼100 km and extends ∼300 km eastward before continuing its descent into the mantle. We use a grid-search multiple-event earthquake relocation technique to relocate 1098 events within the subducting slab and generate contours of the Wadati-Benioff zone. These contours reflect slab geometries from previous studies of intermediate-depth seismicity in this region with some small but important deviations. Our hypocentres indicate that the shallowest portion of the flat slab is associated with the inferred location of the subducting Juan Fernández Ridge at 31°S and that the slab deepens both to the south and the north of this region. We have also determined first motion focal mechanisms for ∼180 of the slab earthquakes. The subhorizontal T-axis solutions for these events are almost entirely consistent with a slab pull interpretation, especially when compared to our newly inferred slab geometry. Deviations of T-axes from the direction of slab dip may be explained with a gap within the subducting slab below 150 km in the vicinity of the transition from flat to normal subducting geometry around 33°S. © 2007 The Authors Journal compilation © 2007 RAS.
• Alvarado, P., & Beck, S. (2006). Source characterization of the San Juan (Argentina) crustal earthquakes of 15 January 1944 (M_w 7.0) and 11 June 1952 (M_w 6.8). Earth and Planetary Science Letters, 243(3-4), 615-631.
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  Abstract: The backarc region of the Andes in the vicinity of San Juan, Argentina, is one of the most seismically active fold and thrust belt regions in the world. Four large damaging crustal earthquakes (1894, 1944, 1952 and 1977) occurred during the last 111 yr between 30°S and 32°S. We have determined the source parameters for two of these important earthquakes, the 1944 and 1952 events, using historic seismic records. The earthquake on 15 January 1944 had an epicentral location between the eastern thin-skinned Precordillera fold and thrust belt and the thick-skinned Sierras Pampeanas basement-cored uplifts. The 11 June 1952 earthquake occurred in the eastern Precordillera about 35 km southwest of the 1944 epicenter location. The P-wave first motions, long-period teleseismic P waveform modeling, and SV/SH amplitude ratio indicate a thrust focal mechanism for the 1944 event (strike N45°E, dip 35° to the southeast, and rake 110deg;) with M0 3.01 × 10 19N m and Mw = 7.0. The 1952 earthquake focal mechanism solution indicates a more oblique mechanism (strike N40°E, dip 75° to the southeast, and rake 30°) with M0 = 2.20 × 1019 N m and Mw = 6.8. Both the 1944 and 1952 earthquakes have focal depths
• Calkins, J. A., Zandt, G., Gilbert, H. J., & Beck, S. L. (2006). Crustal images from San Juan, Argentina, obtained using high frequency local event receiver functions. Geophysical Research Letters, 33(7).
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  Abstract: Images of the crust and upper mantle obtained using teleseismic receiver functions exhibit a weak P-S conversion from the Moho in Western Argentina, where a segment of the subducting Nazca plate flattens out beneath the overriding South American plate. To better estimate depth to the Moho and search for mid-crustal impedance contrasts, we calculate high frequency receiver functions using 37 intermediate-depth local earthquakes. Radial receiver functions from a station located near San Juan, Argentina, show a strong signal from the Moho at 7 seconds corresponding to a depth near 50 km, and conversions from interfaces within the crust at depths of ∼20 and 35 km. The higher frequency results obtained using local events have twice the vertical resolution of the teleseismic results and the combined data sets indicate a gradational increase in S velocity in the lower crust that cannot be detected in the lower frequency teleseismic data alone. Copyright 2006 by the American Geophysical Union.
• Frassetto, A., Gilbert, H., Zandt, G., Beck, S., & Fouch, M. I. (2006). Support of high elevation in the southern Basin and Range based on the composition and architecture of the crust in the Basin and Range and Colorado Plateau. EARTH AND PLANETARY SCIENCE LETTERS, 249(1-2), 62-73.
• Frassetto, A., Gilbert, H., Zandt, G., Beck, S., & Fouch, M. J. (2006). Support of high elevation in the southern Basin and Range based on the composition and architecture of the crust in the Basin and Range and Colorado Plateau. Earth and Planetary Science Letters, 249(1-2), 62-73.
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  Abstract: To explore the nature of how the structure and physical properties of the crust vary from extended to relatively unextended domains we present teleseismic receiver functions which measure crustal thickness, shear wavespeed structure and the Vp/Vs ratio at 12 seismic stations in eastern Arizona. The crustal thickness is ∼ 28 km, increases slightly eastward, and remains nearly uniform beneath the varying elevations in the southern Basin and Range. The observed Vp/Vs ratio in the Basin and Range (∼ 1.78) exceeds the global average. The southern Colorado Plateau exhibits thicker crust (∼ 40 km) and a slightly greater observed Vp/Vs (∼ 1.81). A discrete region in the Colorado Plateau generates an unusually high Vp/Vs ratio (1.90) and contains low wavespeed zones which serve as evidence of partial melt related to Quaternary volcanism. The metamorphic core complexes in the southern Basin and Range likewise exhibit anomalously high Vp/Vs values (1.79-1.87) and lack locally compensating crustal roots. Density models show that ∼ 85 kg/m3 lighter crust or ∼ 35 kg/m3 lighter mantle than that of the surrounding Basin and Range helps these metamorphic core complexes maintain their high elevation. Compositional modeling of intrusive bodies exposed throughout the Catalina-Rincon metamorphic core complex indicates that the observed high Vp/Vs ratio and modeled low density could result from substantial amounts of a plagioclase-rich, quartz-poor rock. These Vp/Vs data are evidence that significant compositional heterogeneity of the crust can occur over a short distance and provide a clue as to how these areas that underwent significant Tertiary extension may have been preconditioned for orogenic collapse. © 2006 Elsevier B.V. All rights reserved.
• Fromm, R., Alvarado, P., Beck, S. L., & Zandt, G. (2006). The April 9, 2001 Juan Fernández ridge (M_w 6.7) tensional outer-rise earthquake and its aftershock sequence. Journal of Seismology, 10(2), 163-170.
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  Abstract: On April 9, 2001 a Mw 6.7 earthquake occurred offshore of the Chilean coast close to the intersection of the subducting Juan Fernández Ridge (JFR) and the trench near 33°S. The mainshock as well as an unprecedented number of aftershocks were recorded on regional broad-band and short-period seismic networks. We obtained a regional moment tensor solution of the mainshock that indcates a tensional focal mechanism consistent with the Harvard CMT solution. Based on waveform modeling and relocation, the depth of the mainshock was found to be 10-12 km. We relocated 142 aftershocks, which are strongly clustered and restricted to 10-30 km in depth. The seismicity distribution indicates a conjugate normal fault system extending into the lithospheric mantle that correlates with ridge-parallel fractures observed by previous seismic and bathymetric surveys. In conjunction with the historic regional distribution of outer-rise and large interplate seismicity, our results indicate that, with the exception of anomalously large thrust events, preexisting fractures associated with large bathymetric features like ridges have to exist to allow the generation of outer-rise seismicity along the Chilean margin. Hence, flexural bending and time-dependent interplate earthquakes can locally affect the nucleation of outer-rise events. The occurrence of the outer-rise seismicity in the oceanic mantle suggests the existence of lithospheric scale faults which might act as conduits to hydrate the subducting slab. © Springer Science+Business Media, Inc. 2006.
• Gilbert, H., Beck, S., & Zandt, G. (2006). Lithospheric and upper mantle structure of central Chile and Argentina. GEOPHYSICAL JOURNAL INTERNATIONAL, 165(1), 383-398.
• Gilbert, H., Beck, S., & Zandt, G. (2006). Lithospheric and upper mantle structure of central Chile and Argentina. Geophysical Journal International, 165(1), 383-398.
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  Abstract: The tectonics of central Chile and Argentina have been greatly affected by the shallow dips of the subducting Nazca plate, which controlled patterns of magmatism and deformation nearly 1000 km away from the plate boundary. We calculate receiver functions from data recorded by the CHARGE array, which transected the Andes and Sierras Pampeanas in central Chile and Argentina, to better constrain the crustal structure of this region. Beneath the northern transect of the CHARGE array, where the Nazca slab flattens near 100 km, we find the crust is over 60 km thick beneath the Andes and thin to the east. The thick crust, however, extends ∼200 kmto the east of the high elevations. Estimates of VP/ VS obtained from receiver functions vary along ancient terrane boundaries exhibiting higher values to the west. Interestingly, we observe that the amplitude of the phase corresponding to the Moho on receiver functions diminishes to the west, complicating our images of crustal structure. We proposed that the observations presented here of thickened crust within a region of low elevations, diminished receiver function arrivals, and reports of high shear-wave speeds atop of the mantle wedge overlying the shallowly subducted Nazca slab, can be explained by partial eclogitization of the lower crust. The Moho appears simpler across the southern transect where it can be identified near 50 km depth at its deepest point beneath the Andes and shallows eastwards. Volcanism remains active near the latitudes of our southern transect and we observe multiple crustal lowvelocity zones indicative of regions of partial melt near the centres of volcanism. Signals related to the Nazca slab remain more elusive, suggestive of a small impedance contrast between the slab and overlying mantle. © 2006 The Authors Journal compilation © 2006 RAS.
• Wagner, L. S., Beck, S., Zandt, G., & Ducea, M. N. (2006). Depleted lithosphere, cold, trapped asthenosphere, and frozen melt puddles above the flat slab in central Chile and Argentina. EARTH AND PLANETARY SCIENCE LETTERS, 245(1-2), 289-301.
• Wagner, L. S., Beck, S., Zandt, G., & Ducea, M. N. (2006). Depleted lithosphere, cold, trapped asthenosphere, and frozen melt puddles above the flat slab in central Chile and Argentina. Earth and Planetary Science Letters, 245(1-2), 289-301.
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  Abstract: Recent studies of the upper mantle above the flat slab in central Chile and Argentina indicate the seismic velocity structures in this area are very different from those found in subduction zones with "normal," steeper slab geometries. The mantle above the horizontal section of the flat slab is characterized by low P-wave velocities, high S-wave velocities, and low Vp / Vs ratios. As the slab begins to transition to a more normal dip to the south, the mantle above it changes as well. Above this "transition zone", the mantle is characterized by high P-wave velocities, high S-wave velocities, and generally high Vp / Vs ratios. Above this high velocity anomaly, in the corner between the south-dipping slab of the transition zone and the east-dipping slab of the normal subduction zone, lies a small shallow anomaly distinguished by its very low P- and S-wave velocities and its moderately high Vp / Vs ratio. We interpret these anomalies to represent very depleted cold lithosphere, cooled trapped asthenosphere, and frozen pooled melt, respectively. Recent research looking at the elastic properties of cratonic xenoliths indicate that a low Vp, high Vs, and low Vp / Vs signature is indicative of cratonic lithosphere. This suggests that the material above the flat slab may be ancient Laurentian lithosphere, or other melt-depleted, dry lithospheric material. The presence of frozen asthenosphere to the south of the flat slab may indicate that large volumes of asthenosphere were displaced in advance of the eastward progression of the flattening slab between 10 and 2 Ma, forcing a change in flow patterns from trench normal to SSW around the south eastern corner of the flattening slab. As flattening progressed this asthenosphere became trapped above the transition zone. Eventually, temperatures dropped sufficiently to freeze the last remaining pocket of extracted melt which had collected in the corner of the descending slab. © 2006 Elsevier B.V. All rights reserved.
• Alvarado, P., Beck, S., Zandt, G., Araujo, M., & Triep, E. (2005). Crustal deformation in the south-central Andes backarc terranes as viewed from regional broad-band seismic waveform modelling. Geophysical Journal International, 163(2), 580-598.
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  Abstract: The convergence between the Nazca and South America tectonic plates generates a seismically active backarc region near 31°S. Earthquake locations define the subhorizontal subducted oceanic Nazca plate at depths of 90-120 km. Another seismic region is located within the continental upper plate with events at depths 1.80, th = 45-55 km) than those of the eastern Sierras Pampeanas (Vp = 6.0-6.2 km s-1, Vp/Vs < 1.70, th 27 = 35 km). In addition, we observed an apparent distribution of reverse crustal earthquakes along the suture that connects those terranes. Finally, we estimated average P and T axes over the CHARGE period. The entire region showed P and T-axis orientations of 275° and 90°, plunging 6° and 84°, respectively. © 2005 RAS.
• Alvarado, P., Castro, B., & Beck, S. (2005). Comparative seismic and petrographic crustal study between the Western and Eastern Sierras Pampeanas region (31°S). Revista de la Asociacion Geologica Argentina, 60(4), 787-796.
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  Abstract: The ancient Sierras Pampeanas in the central west part of Argentina are a seismically active region in the back-arc of the Andes. Their crystalline basement cored uplifts extend up to 800 km east of the oceanic trench over the flat subduction segment of the Nazca plate. Approximately 40 felt crustal earthquakes, are reported per year for this region. Historic and modern seismicity indicates that the Western Sierras Pampeanas (WSP) have more crustal earthquakes of greater-size than the Eastern Sierras Pampeanas (ESP). Remarkable changes in composition and structure also characterize the WSP and ESP basements. We have quantitatively compared both regions using seismological constrains. A recent regional study of moderate earthquakes shows reverse and thrust focal mechanisms occurring at depths down to 25 km in the WSP. In contrast, the ESP have reverse and strike-slip focal mechanisms of shallower depths (
• Lay, T., Kanamori, H., Ammon, C. J., Nettles, M., Ward, S. N., Aster, R. C., Beck, S. L., Bilek, S. L., Brudzinski, M. R., Butler, R., Deshon, H. R., Ekström, G., Satake, K., & Sipkin, S. (2005). The great Sumatra-Andaman earthquake of 26 December 2004. Science, 308(5725), 1127-1133.
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  PMID: 15905392;Abstract: The two largest earthquakes of the past 40 years ruptured a 1600-kilometer-long portion of the fault boundary between the Indo-Australian and southeastern Eurasian plates on 26 December 2004 [seismic moment magnitude (MW) = 9.1 to 9.3] and 28 March 2005 (MW = 8.6). The first event generated a tsunami that caused more than 283,000 deaths. Fault slip of up to 15 meters occurred near Banda Aceh, Sumatra, but to the north, along the Nicobar and Andaman Islands, rapid slip was much smaller. Tsunami and geodetic observations indicate that additional slow slip occurred in the north over a time scale of 50 minutes or longer.
• Wagner, L. S., Beck, S., & Zandt, G. (2005). Upper mantle structure in the south central Chilean subduction zone (30° to 36°S). Journal of Geophysical Research B: Solid Earth, 110(1), 1-20.
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  Abstract: Regional P and S wave travel time data were used to obtain three-dimensional seismic tomography models for Vp, Vs, and Vp/Vs above the subducting slab in central Chile and Argentina. In this region, there is an abrupt change from a normal subduction geometry south of 33°S to a flat subduction geometry to the north. We find low Vp, low Vs, and high Vp/Vs ratios in the southern half of our study area directly beneath the modern active volcanic arc, which we interpret as localized pockets of melt. In the northern half of our study area, above where the subducting Nazca plate flattens at 100 km depth, we find low Vp, high Vs, and low Vp/Vs ratios. These unusual results point to a lack of melt or hydrated mineralogies such as serpentine, both of which are characterized by high Vp/Vs values. The only mantle rocks that have low Vp/Vs and high Vs are Mg-rich compositions, such as dehydrated serpentinite or orthopyroxenite. We suggest that significant portions of the mantle overlying the flat slab consist of orthopyroxenite, formed by a transient fluxing of silica-rich fluids. Such fluids may have come from sediments that were subducted during the initiation of flat subduction at this latitude at ∼10 Ma. This would imply that the hydration of mantle material above a flat slab can be a transient phenomenon, which leaves little residual-free water behind but significantly alters the mantle chemistry. Copyright 2005 by the American Geophysical Union.
• Zandt, G., Decelles, P. G., Zandt, G., Mcquarrie, N., Horton, B. K., Decelles, P. G., & Beck, S. L. (2005). Lithospheric evolution of the Andean fold-thrust belt, Bolivia, and the origin of the central Andean plateau. Tectonophysics, 399(1), 15-37. doi:10.1016/j.tecto.2004.12.013
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  We combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central Andean plateau between 188 and 208 S from Late Cretaceous to present. By integrating geophysical results of upper mantle structure, crustal thickness, and composition with recently published structural, stratigraphic, and thermochronologic data, we emphasize the importance of both the crust and upper mantle in the evolution of the central Andean plateau. Four key steps in the evolution of the Andean plateau are as follows. 1) Initiation of mountain building by ~70 Ma suggested by the associated foreland basin depositional history. 2) Eastward jump of a narrow, early fold–thrust belt at 40 Ma through the eastward propagation of a 200–400-km-long basement thrust sheet. 3) Continued shortening within the Eastern Cordillera from 40 to 15 Ma, which thickened the crust and mantle and established the eastern boundary of the modern central Andean plateau. Removal of excess mantle through lithospheric delamination at the Eastern Cordillera–Altiplano boundary during the early Miocene appears necessary to accommodate underthrusting of the Brazilian shield. Replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the Eastern Cordillera and Altiplano at 24–23 Ma, and further volcanism recorded by 12–7 Ma crustal ignimbrites. 4) After ~20 Ma, deformation waned in the Eastern Cordillera and Interandean zone and began to be transferred into the Subandean zone. Long-term rates of shortening in the fold–thrust belt indicate that the average shortening rate has remained fairly constant (~8–10 mm/year) through time with possible slowing (~5–7 mm/year) in the last 15–20 myr. We suggest that Cenozoic deformation within the mantle lithosphere has been focused at the Eastern Cordillera–Altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination. D 2005 Elsevier B.V. All rights reserved.
• Anderson, M. L., Zandt, G., Triep, E., Fouch, M., & Beck, S. (2004). Anisotropy and mantle flow in the Chile-Argentina subduction zone from shear wave splitting analysis. Geophysical Research Letters, 31(23), 1-4.
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  Abstract: We examine shear wave splitting in teleseismic phases to observe seismic anisotropy in the South American subduction zone. Data is from the CHARGE network, which traversed Chile and western Argentina across two transects between 30°S and 36°S. Beneath the southern and northwestern parts of the network, fast polarization direction is consistently trench-parallel, while in the northeast is trench-normal; the transition between these two zones is gradual. We infer that anisotropy sampled by teleseismic phases is localized within or below the subducting slab. We explain our observations with a model in which eastward, Nazca-entrained asthenospheric flow is deflected by retrograde motion of the subducting Nazca plate. Resulting southward flow through this area produces N-S observed in the south and northwest; E-W result from interaction of this flow with the local slab geometry producing eastward mantle flow under the actively flattening part of the slab. Copyright 2004 by the American Geophysical Union.
• Fromm, R., Zandt, G., & Beck, S. L. (2004). Crustal thickness beneath the Andes and Sierras Pampeanas at 30°S inferred from Pn apparent phase velocities. Geophysical Research Letters, 31(6), L06625 1-4.
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  Abstract: We obtained a 2D crustal model beneath central Chile and western Argentina using apparent Pn phase velocities recorded along an EW trending transect at 30°S. This model is characterized by a thick (65 km) crustal root beneath the High Cordillera, a 60 km thick crust beneath the Precordillera, and a gradual shallowing of Moho depths beneath the Sierras Pampeanas from 55 km in the west to 40 km in the east. Beneath the Sierras Pampeanas, where very little crustal shortening is observed, the thick crust must be explained by means other than simple tectonic shortening. Isostatically balanced crustal thickness is consistently shallower than our model shows, suggesting the existence of dynamic forces possibly linked to the flat slab. Copyright 2004 by the American Geophysical Union.
• Ozacar, A. A., & Beck, S. L. (2004). The 2002 denali fault and 2001 kunlun fault earthquakes: Complex rupture processes of two large strike-slip events. Bulletin of the Seismological Society of America, 94(6 SUPPL. B), S278-S292.
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  Abstract: We studied the source processes of two large continental earthquakes, the 3 November 2002 Denali fault earthquake and the 14 November 2001 Kunlun fault earthquake, associated with strike-slip faulting along ancient sutures. We inverted teleseismic P waveforms using a pulse-stripping method for multiple time windows with different focal mechanisms and derived composite source models. According to our results, the 2002 Denali fault earthquake began with initial thrusting (M w 7.3) along a 40-km-long segment of the north-northwest-dipping Susitna Glacier fault and later ruptured a 300-km-long segment along the Denali and Totschunda faults with a right-lateral strike-slip mechanism (M w 7.7). In contrast, the 2001 Kunlun fault earthquake nucleated near an extensional step-over with a subevent pair consisting of 30-km-long strike-slip (M w 6.9) event and 40-km-long normal (M w 6.8) faulting event and later ruptured a 350-km-long segment along the Kunlun fault with a left-lateral strike-slip mechanism (M w 7.7). Both earthquakes propagated primarily unilaterally to the east and released most of their energy along slip patches (asperities) far from the hypocenter locations. We find that both the Denali fault and Kunlun fault earthquakes had high-average rupture velocities of 3.2 km/sec and 3.4 km/sec, respectively. We also compared the source properties of these two earthquakes with other strike-slip earthquakes. For scaling purposes, large strike-slip earthquakes were classified as interplate, oceanic intraplate, or continental intraplate events. By using this classification the Denali fault and Kunlun fault earthquakes have an interplate signature that suggests overall weak faulting.
• Ozacar, A. A., Beck, S. L., & Christensen, D. H. (2003). Source process of the 3 November 2002 Denali fault earthquake (central Alaska) from teleseismic observations. Geophysical Research Letters, 30(12), 40-1.
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  Abstract: The November 3, 2002 Denali fault earthquake, which is the largest inland event ever recorded in central Alaska, occurred along an arcuate segment of the right-lateral strike-slip Denali fault. We use first-motion P wave polarities and inversions of teleseismic P waveforms for a fixed focal mechanism to constrain the rupture process. We find clear evidence for a substantial reverse component near the hypocenter at the beginning of the rupture. Twenty-five seconds later, rupture propagated unilaterally to the east on a strike-slip fault and released most of the seismic moment along an asperity located 170 km east of the hypocenter with a maximum slip of 8 m. This earthquake had a duration of ∼120 s and ruptured more than 300 km in length. Correlation with gravity anomalies suggests a relation between moment distribution and physical properties of subsurface rock units that may support a weaker middle fault segment marked by fewer aftershocks.
• Baumont, D., Paul, A., Zandt, G., Beck, S. L., & Pedersen, H. (2002). Lithospheric structure of the central Andes based on surface wave dispersion. Journal of Geophysical Research B: Solid Earth, 107(12), ESE 18-1 - 18-13.
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  Abstract: Shear wave velocity is very sensitive to temperature anomalies and partial melt and can provide important insights on the state of the lithosphere. With that aim in mind, phase velocities of Rayleigh and Love waves have been inverted for the regionalized shear wave velocity structure of the lithosphere across the central Andes. This inversion reveals strong lateral variations of Vs both across the range and along-strike in the Altiplano crust. In the upper crust, the main features of our models are prominent low-velocity anomalies probably related to partial melt below the Los Frailes ignimbrite complex and the southern Altiplano. At lower crustal level, the Altiplano is characterized by lower Vs than the surrounding regions. We find that the transition from the Altiplano to the Puna is associated with a 7-km thickening of the crust. At mantle depths, the Nazca plate is found to be overlaid by a dipping low-velocity zone with decreasing intensity with depth. Our results favor the idea of a cold mantle lid underlying the whole central Andes.
• Beck, S. L., & Zandt, G. (2002). The nature of orogenic crust in the central Andes. Journal of Geophysical Research B: Solid Earth, 107(10), ESE 7-1 - 7-16.
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  Abstract: The central Andes (16°-22°S) are part of an active continental margin mountain belt and the result of shortening of the weak western edge of South America between the strong lithospheres of the subducting Nazca plate and the underthrusting Brazilian shield. We have combined receiver function and surface wave dispersion results from the BANJO-SEDA project with other geophysical studies to characterize the nature of the continental crust and mantle lithospheric structure. The major results are as follows: (1) The crust supporting the high elevations is thick and has a felsic to intermediate bulk composition. (2) The relatively strong Brazilian lithosphere is underthrusting as far west (65.5°W) as the high elevations of the western part of the Eastern Cordillera (EC) but does not underthrust the entire Altiplano. (3) The subcrustal lithosphere is delaminating piecemeal under the Altiplano-EC boundary but is not completely removed beneath the central Altiplano. The Altiplano crust is characterized by a brittle upper crust decoupled from a very weak lower crust that is dominated by ductile deformation, leading to lower crustal flow and flat topography. In contrast, in the high-relief, inland-sloping regions of the EC and sub-Andean zone, the upper crust is still strongly coupled across the basal thrust of the fold-thrust belt to the underthrusting Brazilian Shield lithosphere. Subcrustal shortening between the Altiplano and Brazilian lithosphere appears to be accommodated by delamination near the Altiplano-EC boundary. Our study suggests that orogenic reworking may be an important part of the "felsification" of continental crust.
• Giovanni, M. K., Beck, S. L., & Wagner, L. (2002). The June 23, 2001 Peru earthquake and the southern Peru subduction zone. Geophysical Research Letters, 29(21), 14-1.
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  Abstract: The plate boundary between the South American and Nazca plate along the south-central Peru coast has been the site of large destructive earthquakes for many centuries, including the June 23, 2001 (Mw = 8.4) event. This underthrusting event has a fault area of 320 km by 100 km based on relocated aftershocks during the first three weeks following the mainshock. Modeling of the teleseismic broadband P waves of the 2001 Peru earthquake indicates two pulses of moment release with the larger second pulse located 130 km southeast of the mainshock initiation, indicating a unilateral rupture to the southeast. Based on intensity and tsunami reports, previous earthquakes in 1868 and 1604 were larger than the 2001 earthquake, while an event in 1784 was smaller. This provides further evidence that the size of earthquakes along the Peru coast has changed between successive earthquake cycles.
• Baumont, D., Paul, A., Zandt, G., & Beck, S. L. (2001). Inversion of Pn travel times for lateral variations of Moho geometry beneath the central Andes and comparison with the receiver functions. Geophysical Research Letters, 28(8), 1663-1666.
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  Abstract: We inverted the Pn travel times to characterize the geometry of the Moho along a profile across the Central Andes (20°S) where previous workers have estimated the crustal thickness using receiver functions. Contrary to receiver functions, this technique is not sensitive to the crustal V3. Therefore, the comparison of the two approaches provides valuable complementary information. Overall, our results are in good agreement with those based on receiver functions. However, some important discrepancies are observed beneath the Western Cordillera and the Subandes, where we find crusts 10-km thinner than in previous models. We confirm that the central part of the orogen appears to be isostatically compensated by the presence of a thick crust. However, at both edges, the topography probably requires additional support, low-density mantle beneath the Western Cordillera and a strong flexural support of the Brazilian shield beneath the Subandes.
• Polet, J., Silver, P. G., Beck, S., Wallace, T., Zandt, G., Ruppert, S., Kind, R., & Rudloff, A. (2000). Shear wave anisotropy beneath the Andes from the BANJO, SEDA, and PISCO experiments. Journal of Geophysical Research B: Solid Earth, 105(3), 6287-6304.
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  Abstract: We present the results of a detailed shear wave splitting analysis of data collected by three temporary broadband deployments located in central western South America: the Broadband Andean Joint experiment (BANJO), a 1000-km-long east-west line at 20°S, and the Projecto de Investigacion Sismologica de la Cordillera Occidental (PISCO) and Seismic Exploration of the Deep Altiplano (SEDA), deployed several hundred kilometers north and south of this line. We determined the splitting parameters φ (fast polarization direction) and δt (splitting delay time) for waves that sample the above- and below-slab regions: teleseismic (*)KS and S, ScS waves from local deep-focus events, as well as S waves from intermediate-focus events that sample only the above-slab region. All but one of the (*)KS stacks for the BANJO stations show E-W fast directions with δt varying between 0.4 and 1.5 s. However, for (*)KS recorded at most of the SEDA and PISCO stations, and for local deep-focus S events north and south of BANJO, there is a rotation of φ to a more nearly trench parallel direction. The splitting parameters for above-slab paths, determined from events around 200 km deep to western stations, yield small delay times (≤0.3 s) and N-S fast polarization directions. Assuming the anisotropy is limited to the top 400 km of the mantle (olivine stability field), these data suggest the following spatial distribution of anisotropy. For the above-slab component, as one goes from east (where (*)KS reflects the above-slab component) to west, φ changes from E-W to N-S, and delay times are substantially reduced. This change may mark the transition from the Brazilian craton to actively deforming (E-W shortening) Andean mantle. We see no evidence for the strain field expected for either corner flow or shear in the mantle wedge associated with relative plate motion. The small delay times for above-slab paths in the west require the existence of significant, spatially varying below-slab anisotropy to explain the (*)KS results. The implied anisotropic pattern below the slab is not easily explained by a simple model of slab-entrained shear flow beneath the plate. Instead, flow induced by the retrograde motion of the slab, in combination with local structural variations, may provide a better explanation.
• Swenson, J. L., Beck, S. L., & Zandt, G. (2000). Crustal structure of the Altiplano from broadband regional waveform modeling: Implications for the composition of thick continental crust. Journal of Geophysical Research B: Solid Earth, 105(B1), 607-621.
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  Abstract: We have modeled the full waveforms from six intermediate-depth and two shallow earthquakes recorded at regional distances by the BANJO Broadband Andean Joint Experiment (BANJO) and Seismic Exploration of the Deep Altiplano (SEDA) portable seismic networks in the central Andes. In this study we utilize data from those BANJO and SEDA stations located within the Altiplano and Eastern Cordillera. We used reflectivity synthetic seismograms and a grid search to constrain four parameters of the Altiplano-Eastern Cordillera lithosphere: crustal thickness, average crustal velocity (Vp) and crustal and upper mantle Poisson's ratios (σcrust and σmantle). Using our grid search, we investigated the crustal and upper mantle structure along 36 individual event station paths and applied forward modeling to 56 event station paths. Robust models for the Altiplano that provide the best overall fit between the data and synthetic seismograms are characterized by an average Vp of 5.75-6.25 km/s, crustal thicknesses of 60-65 km, σcrust = 0.25, and σmantle = 0.27-0.29. We find a north-south variation in the structure of the Altiplano, with the crust south of the BANJO transect characterized by either lower than average crustal P wave velocities or a slightly higher σcrust relative to crust north of the BANJO transect. These results are consistent with a model of crustal thickening caused predominantly by tectonic shortening of felsic crust, rather than by underplating or magmatic intrusion from the mantle. Copyright 2000 by the American Geophysical Union.
• Velasco, A. A., Ammon, C. J., & Beck, S. L. (2000). Broadband source modeling of the November 8, 1997, Tibet (M_w = 7.5) earthquake and its tectonic implications. Journal of Geophysical Research B: Solid Earth, 105(B12), 28065-28080.
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  Abstract: We studied the source process of a large (Ms = 7.9) intraplate earthquake that occurred on November 8, 1997, at 1002 UT in a remote region of northern Tibet. We used four distinct methods to investigate the broadband source process and thereby better understand the tectonic implications of this event. We relocated aftershocks using a master event technique and found that the distribution of aftershocks covers a region of 200 km in lateral extent. We also employed a surface wave spectral inversion technique to estimate the mainshock moment, depth, centroid location, and centroid time and utilized an empirical Green's function technique to extract rupture directivity information and a detailed source time function from observed seismograms. We also inverted body waves to estimate the moment release along the fault and the source time function. The 1997 earthquake ruptured a strike-slip fault that appears to be an extension or splay of the Kun Lun fault system. This fault is one of the most seismically active strike-slip faults within the Tibetan plateau and has had events with surface wave magnitudes of 6.1, 7.4, and 7.9 in this region since 1973. The rupture released most of the energy within the first 20 s and propagated bilaterally initially, with the later rupture propagating westward for 20-30 s. The absence of large aftershocks suggests that the earthquake efficiently released the stored strain. Comparing mainshock to the largest aftershock energy ratios for this event and other large strike-slip events shows that faulting within the plateau has the characteristics of weak faults (e.g., fracture zone faulting). Copyright 2000 by the American Geophysical Union.
• Yuan, X., Sobolev, S. V., Kind, R., Oncken, O., Bock, G., Asch, G., Schurr, B., Graeber, F., Rudloff, A., Hanka, W., Wylegalla, K., Tibi, R., Haberland, C., Rietbrock, A., Giese, P., Wigger, P., Röwer, P., Zandt, G., Beck, S., , Wallace, T., et al. (2000). Subduction and collision processes in the Central Andes constrained by converted seismic phases. Nature, 408(6815), 958-961.
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  PMID: 11140679;Abstract: The Central Andes are the Earth's highest mountain belt formed by ocean-continent collision 1,2. Most of this uplift is thought to have occurred in the past 20 Myr, owing mainly to thickening of the continental crust 2-6, dominated by tectonic shortening 7-10. Here we use P-to-S (compressional-to-shear) converted teleseismic waves observed on several temporary networks in the Central Andes to image the deep structure associated with these tectonic processes. We find that the Moho (the Mohorovičić discontinuity - generally thought to separate crust from mantle) ranges from a depth of 75 km under the Altiplano plateau to 50 km beneath the 4-km-high Puna plateau. This relatively thin crust below such a high-elevation region indicates that thinning of the lithospheric mantle may have contributed to the uplift of the Puna plateau. We have also imaged the subducted crust of the Nazca oceanic plate down to 120 km depth, where it becomes invisible to converted teleseismic waves, probably owing to completion of the gabbro-eclogite transformation; this is direct evidence for the presence of kinetically delayed metamorphic reactions in subducting plates. Most of the intermediate-depth seismicity in the subducting plate stops at 120 km depth as well, suggesting a relation with this transformation. We see an intracrustal low-velocity zone, 10-20 km thick, below the entire Altiplano and Puna plateaux, which we interpret as a zone of continuing metamorphism and partial melting that decouples upper-crustal imbrication from lower-crustal thickening.
• Baumont, D., Paul, A., Beck, S., & Zandt, G. (1999). Strong crustal heterogeneity in the Bolivian Altiplano as suggested by attenuation of Lg waves. Journal of Geophysical Research B: Solid Earth, 104(B9), 20287-20305.
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  Abstract: Using the seismological data of four temporary networks installed across the Andes of Bolivia and Chile between 1990 and 1995, we study the amplitudes of the Lg phase for 45 regional events located at crustal depths. This analysis reveals that Lg is strongly attenuated for most of the ray paths crossing the Altiplano independent of the azimuth of propagation. To map the anomalies and evaluate their frequency dependence, we conduct a regionalization of the crustal average Qs factor using a damped least squares inversion technique in the frequency range 0.6-5 Hz. The resulting maps delineate regions with contrasting values of apparent attenuation. The Altiplano is characterized by strong attenuation with an average Qs of 100 at 1 Hz, whereas the Cordilleras are regions of weaker attenuation. The frequency dependence of Qs is investigated in an attempt to separate the source of the apparent attenuation into scattering and anelastic absorption. Within the hypothesis of a weak frequency dependence of the intrinsic Q, we show that at 1 Hz, scattering plays a major part in the attenuation of Lg waves whereas at 4 Hz, both mechanisms are of equal importance. These results indicate that the attenuation of Lg waves in the Altiplano is due more to scattering by small-scale heterogeneities than to a high percentage (> 1%) of partial melt involving a large part of the crust. Copyright 1999 by the American Geophysical Union.
• Nettles, M., Wallace, T. C., & Beck, S. L. (1999). The March 25, 1998 Antarctic plate earthquake. Geophysical Research Letters, 26(14), 2097-2100.
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  Abstract: The March 25, 1998, Antarctic plate earthquake (M w=8.1) occurred ∼250 km from the nearest plate boundary, in oceanic lithosphere with an age of 35-55 my. Analysis of aftershock patterns shows that the earthquake ruptured a fault, or series of strike-slip fault segments, nearly 300 km long. The strike of the fault(s) is nearly perpendicular to the north-south trending fossil fracture zones which are the most marked bathymetric features of this region. Moment release during the mainshock was concentrated in two large subevents, clearly visible in the teleseismic body wave waveforms. Modeling of these body waves using a finite fault source places the first of the two subevents near the point of rupture initiation, on the eastern end of the fault, with the second large subevent occurring 220-280 km to the west. The two pulses of moment release are found to be separated in time by ∼65 s. Comparison of the relative S wave amplitudes of the first and second pulses suggests that a rotation of the focal mechanism by ∼10° occurred between the two subevents. Copyright 1999 by the American Geophysical Union.
• Swenson, J. L., & Beck, S. L. (1999). Source characteristics of the 12 November 1996 M(w) 7.7 Peru subduction zone earthquake. Pure and Applied Geophysics, 154(3-4), 731-751.
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  Abstract: The 12 November 1996 M(w) 7.7 Peru subduction zone earthquake occurred off the coast of southern Peru, near the intersection of the South American trench and the highest topographical point of the subducting Nazca Ridge. We model the broadband teleseismic P-waveforms from stations in the Global Seismic Network to constrain the source characteristics of this subduction zone earthquake. We have analyzed the vertical component P-waves for this earthquake to constrain the depth, source complexity, seismic moment and rupture characteristics. The seismic moment determined from the nondiffracted P-waves is 3-5 x 1020 N·m, corresponding to a moment magnitude M(w) of 7.6-7.7 The source time function for the 1996 Peru event has three pulses of seismic moment release with a total duration of approximately 45-50 seconds. The largest moment release occurs at approximately 35-40 seconds and is located ~90 km southeast of the rupture initiation. Approximately 70% of the seismic moment was released in the third pulse. We find that the 1996 event reruptured part of the rupture area of the previous event in 1942. The location of the 1996 earthquake corresponds to a region along the Peru coast with the highest uplift rates of marine terraces. This suggests that the uplift may be due to repeated earthquakes such as the 1996 and 1942 events.
• Swenson, J. L., Beck, S. L., & Zandt, G. (1999). Regional distance shear-coupled PL propagation within the northern Altiplano, central Andes. Geophysical Journal International, 139(3), 743-753.
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  Abstract: Properties of the shear-coupled P wavetrain (SPL) from regional earthquakes provide important information about the structure of the crust and upper mantle. We investigate broad-band seismic data from intermediate-depth earthquakes and develop a grid search technique using synthetic seismograms to study the sensitivity of SPL and to model the crustal structure of the northern Altiplano, central Andes. Waveforms from an earthquake that occurred on 1994 December 12 within the Nazca slab beneath the Altiplano display a clear SPL wavetrain at the temporary stations deployed during the BANJO and SEDA experiments. We relocate this event and determine the moment tensor by inverting the complete long-period waveforms. With these source parameters fixed, we perform sensitivity analyses using a reflectivity technique to compute synthetic seismograms at a distance of 313 km (BANJO staion 2, SALI). We find that, at this distance, the long-period SPL wavetrain is sensitive to the following model parameters, in order of decreasing sensitivity: crustal V(P)/V(S), mantle V(P)/V(S), average crustal velocity, crustal thickness, focal depth, distance (location), crustal Q(α) and Q(β), and mantle velocity. We develop a grid search method to investigate the four parameters of the crust/upper mantle model to which the synthetic seismograms are most sensitive at SALI (crustal V(P)/V(S), mantle V(P)/V(S), average crustal velocity, crustal thickness). Trade-offs exist among all four of the model parameters, resulting in a range of acceptable crustal models that provide excellent fits between the data and synthetic seismograms in the passband of 15-100 s at a single station. However, by using data at a range of distances (150-450 km) we find that the model that provides the best overall fit between the data and synthetic seismograms, and thus best approximates the average characteristics of the crust and upper mantle structure of the northern Altiplano, is characterized by an average crustal velocity of 6.0 km s-1, a crustal Poisson's ratio (σ(crust)) of 0.25 and a mantle Poisson's ratio (σ(mantle)) of 0.27. The resulting models confirm previous estimates of low Poisson's ratios, low average crustal velocity and thick crust in the Altiplano.
• Beck, S., Barrientos, S., Kausel, E., & Reyes, M. (1998). Source characteristics of historic earthquakes along the central Chile subduction zone. Journal of South American Earth Sciences, 11(2), 115-129.
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  Abstract: We have analyzed four large to great historic earthquakes that occurred along the central Chile subduction zone from north to south on November 11, 1922 (Ms=8.3), April 6, 1943 (Ms=7.9), December 1, 1928 (Ms=8.0) and january 25, 1939 (Ms=7.8). Waveform modeling and P-wave first motions indicate that the 1922, 1928 and 1943 earthquakes are shallow and consistent with underthrusting of the Nazca Plate beneath the South American plate. in contrast, the 1939 earthquake is not an underthrusting event but rather a normal fault event within the down-going slab. The 1922 earthquake is by far the largest event with a complex source time function showing three pulses of moment release and a duration of 75 s. The 1943 earthquyake has a simple source time function with one pulse of moment release and a duration of 24 s. This event had a local tsunami of 4 m and a far-field tsunami height in Japan of 10-30 cm. The 1928 earthquake also has a simple source time function with a duration of 28 s. The aftershocks and highiest intensities are south of the epicenter indicating a southward rupture with most of the seismic moment release occurring 50-80 km south of the 1928 epicenter but still north of the adjacent 1939 earthquake region. The 1939 Chillan earthquake was not an underthrusting but rather a complex normal fault earthquake. Our preferred model is a normal fault mechanism at a depth of 80 to 100 km with two pulses of moment release and a total duration of approximately 60 s. The high intensities, lack of tsunami, and inland location associated with the 1939 event are all consistent with an intraplate event within the down-going slab. The 1939 earthquake was clearly more destructive than the other similar size or larger events. This may in part be due to the intraplate nature of the event but also due to high amplification of the sites in the Central Valley of south central Chile.
• Myers, S. C., Beck, S., Zandt, G., & Wallace, T. (1998). Lithospheric-scale structure across the Bolivian Andes from tomographic images of velocity and attenuation for P and S waves. Journal of Geophysical Research B: Solid Earth, 103(9), 21233-21252.
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  Abstract: We have developed a three-dimensional, lithospheric-scale model across the Bolivian Andes at ∼20°S, based on tomographic images of velocity and attenuation for both P and S waves. Observations of travel time and attenuation for this study are from regional, mantle earthquakes in the subducted Nazca plate recorded on a portable, broadband seismic array (Broadband Andean Joint Experiment and Seismic Exploration of the Deep Andes) in Bolivia and Chile. The shallow mantle under the Altiplano from ∼18°S to ∼21°S is high-velocity and moderately high Q (Vp≈8.3,Vs≈4.7, Qp≈500, and Qs≈200), suggesting lithospheric mantle. High-velocity material in the Altiplano extends to a depth of ∼125-150 km. The shallow mantle of the Western Cordillera is characterized by high Vp/Vs (∼1.83), suggesting a correlation between Vp/Vs and arc volcanism. Seismic velocity in the Western Cordillera mantle is, on average, only slightly reduced from global averages; however, velocity and attenuation anomalies are locally strong (Vp≈7.8,Vs≈4.3, Qp≈200, and Qs≈100), consistent with partial melt conditions. Under the Los Frailes volcanic field, in the Eastern Cordillera, shallow mantle velocity and Q decrease drastically from the neighboring Altiplano (Vp≈7.8, Vs≈4.3, Qp≈300, Qs≈100); however, high Vp/Vs is not as pervasive as it is in the Western Cordillera. We believe that slab-derived water, and perhaps other volatiles, strongly influence the Western Cordillera, while the Eastern Cordillera low-velocity region is more affected by partial melt and/or compositional changes. Average velocity and Q in the shallow mantle across the Bolivian Andes, where the tomographic images are best resolved, are significantly higher than in most mantle wedge environments where corresponding images are available. This is likely the result of a compressional "back arc" setting in the Andes. This implies that lithospheric shortening and thickening associated with the formation of the Andes has profoundly influenced the shallow mantle structure across the range. Shallow mantle structure is locally influenced by the subduction processes, particularly under the Western Cordillera; however, the differing volcanism and seismic character under the two Cordilleras suggest that the volcanic process in the Eastern Cordillera may be distinct from arc volcanism. Tertiary volcanism in the Eastern Cordillera is located in the region where mantle shortening is suspected to be greatest. Both the timing and location of volcanism are consistent with upward migration of mantle wedge asthenosphere following the removal of over thickened lithosphere.
• Tinker, M. A., Beck, S. L., Jiao, W., & Wallace, T. C. (1998). Mainshock and aftershock analysis of the June 17, 1996, deep Flores Sea earthquake sequence: Implications for the mechanism of deep earthquakes and the tectonics of the Banda Sea. Journal of Geophysical Research B: Solid Earth, 103(5), 9987-10001.
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  Abstract: One of the largest (Mw = 7.8) deep earthquakes (595 km) in recorded history occurred beneath the Flores Sea on June 17, 1996. We analyze broadband body waveforms to determine the seismic source characteristics of this important earthquake. The source time function has a small initial subevent (5 s duration) followed by three major subevents with a total source duration of 29 s. Directivity analysis indicates an unilateral rupture to the east-southeast with a subhorizontal rupture length of 75 km. A time-independent moment tensor (TIMT) inversion gives a total seismic moment of 5.4 × 1020 N m (Mw = 7.8). A time-dependent moment tensor (TDMT) inversion indicates rupture on a near-vertical fault (strike of 130°, dip of 80°SW, rake of -166°) during the first 5 s, releasing 8% of the seismic moment. The major part of the moment was released along a shallower dipping plane (strike of ∼100°, dip of ∼55°, rake of ∼-45°). This change of faulting geometry may imply that the rupture initiated at a structural, thermal, or phase boundary of high stress concentration and then triggered the major moment release along a regional weak zone. The mainshock produced at least 23 (mb > 2.9) aftershocks within the first 3 days. Seventeen of the largest aftershocks were located using the Joint Hypocenter Determination algorithm. The aftershocks define a plane dipping antithetically to the dip of the subducting slab with lateral and depth dimensions of 145 km and 76 km, respectively. The aftershock locations correlate with the south-east striking nodal plane of the moment tensor mechanism.
• Beck, S. L., Zandt, G., Myers, S. C., Wallace, T. C., Silver, P. G., & Drake, L. (1996). Crustal-thickness variations in the central Andes. Geology, 24(5), 407-410.
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  Abstract: We estimated the crustal thickness along an east-west transect across the Andes at lat 20°S and along a north-south transect along the eastern edge of the Altiplano from data recorded on two arrays of portable broadband seismic stations (BANJO and SEDA). Wave" forms of deep regional events in the downgoing Nazca slab and teleseismic earthquakes were processed to isolate the P-to-S converted phases from the Moho in order to compute the crustal thickness. We found crustal-thickness variations of nearly 40 km across the Andes. Maximum crustal thicknesses of 70-74 km under the Western Cordillera and the Eastern Cordillera thin to 32-38 km 200 km east of the Andes in the Chaco Plain. The central Altiplano at 20°S has crustal thicknesses of 60 to 65 km. The crust also appears to thicken from north (16°S, 55-60 km) to south (20°S, 70-74 km) along the Eastern Cordillera. The Subandean zone crust has intermediate thicknesses of 43 to 47 km. Crustal-thickness predictions for the Andes based on Airy-type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation. In contrast, at the boundary between the Eastern Cordillera and the Subandean zone and in the Chaco Plain, the crust is thinner than predicted, suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere. With additional constraints, we conclude that the observation of Airy-type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting Nazca plate and the cratonic lithosphere of the Brazilian craton.
• Fan, G., Wallace, T. C., Beck, S. L., & Chase, C. G. (1996). Gravity anomaly and flexural model: Constraints on the structure beneath the Peruvian Andes. Tectonophysics, 255(1-2), 99-109.
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  Abstract: Bouguer gravity data along the Nazca profile of Fukao et al. (1989), NE-SW striking across the Peruvian Andes, are used to investigate the flexure of the Brazilian Shield. Modeling of the observed gravity data indicates that the Brazilian Shield lithosphere is flexed downward and may extend beneath the Andes as far as 150 km. Assuming that the Brazilian Shield behaves elastically, flexural analysis shows that the isostatic gravity inequilibrium in the eastern Andes can be explained by deflection of the Moho due to the bending of an elastic plate beneath the Sub-Andes. The elastic thickness of the plate is estimated to be between 25 and 55 km, with corresponding flexural rigidity between 0.1 and 1.7 × 1024 Nm. Many small to moderate-sized earthquakes that have focal depths of tens of kilometers are distributed over a broad area from the Eastern Cordillera to the Brazilian Shield. These events are located well above the subducted flat Nazca plate and very likely are associated with the underthrusting of the Brazilian Shield. A step-like gravity anomaly of 150 mGal on the eastern margin of the Eastern Cordillera cannot be fully explained by the flexure model alone and requires a sharp-edged mass deficit or discontinuous Moho beneath the Eastern Cordillera. Our gravity modeling shows that Bouguer gravity anomalies along the Nazca profile are best explained by a flexed Moho, which is deflected by the underthrusting of the Brazilian Shield beneath the Andes along with a wedge-shaped body of low-density material directly beneath the Eastern Cordillera.
• Swenson, J. L., & Beck, S. L. (1996). Historical 1942 Ecuador and 1942 Peru subduction earthquakes, and earthquake cycles along Colombia-Ecuador and Peru subduction segments. Pure and Applied Geophysics, 146(1), X4-101.
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  Abstract: Two large shallow earthquakes occurred in 1942 along the South American subduction zone in close proximity to subducting oceanic ridges. The 14 May event occurred near the subducting Carnegie ridge off the coast of Ecuador, and the 24 August event occurred off the coast of southwestern Peru near the southern flank of the subducting Nazca ridge. Source parameters for these two historic events have been determined using long-period P waveforms, P-wave first motions, intensities and local tsunami data. We have analyzed the P waves for these two earthquakes to constrain the focal mechanism, depth, source complexity and seismic moment. Modeling of the P waveform for both events yields a range of acceptable focal mechanisms and depths, all of which are consistent with underthrusting of the Nazca plate beneath the South American plate. The source time function for the 1942 Ecuador event has one simple pulse of moment release with a duration of 22 seconds, suggesting that most of the moment release occurred near the epicenter. The seismic moment determined from the P waves is 6-8 × 1020N·m, corresponding to a moment magnitude of 7.8-7.9. The reported location of the maximum intensities (IX) for this event is south of the main shock epicenter. The relocated aftershocks are in an area that is approximately 200 km by 90 km (elongated parallel to the trench) with the majority of aftershocks north of the epicenter. In contrast, the 1942 Peru event has a much longer duration and higher degree of complexity than the Ecuador earthquake, suggesting a heterogeneous rupture. Seismic moment is released in three distinct pulses over approximately 74 seconds; the largest moment release occurs 32 seconds after rupture initiation. The seismic moment as determined from the P waves for the 1942 Peru event is 10-25 × 1020N m, corresponding to a moment magnitude of 7.9-8.2. Aftershock locations reported by the ISS occur over a broad area surrounding the main shock. The reported locations of the maximum intensities (IX) are concentrated south of the epicenter, suggesting that at least part of the rupture was to the south. We have also examined great historic earthquakes along the Colombia-Ecuador and Peru segments of the South American subduction zone. We find that the size and rupture length of the underthrusting earthquakes vary between successive earthquake cycles. This suggests that the segmentation of the plate boundary as defined by earthquakes this century is not constant.
• Tinker, M. A., Wallace, T. C., Beck, S. L., Myers, S., & Papanikolas, A. (1996). Geometry and state of stress of the Nazca plate beneath Bolivia and its implication for the evolution of the Bolivian orocline. Geology, 24(5), 387-390.
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  Abstract: A series of deep earthquakes beneath Bolivia in 1994 provides valuable constraints on the geometry and state of stress of the subducting Nazca plate. These earthquakes occurred northeast of the Bolivian orocline, which is bracketed between two regions of a "flattened" Wadati-Benioff zone. Furthermore, these events occurred in an area that experienced no seismicity in at least the past 30 years. North and south of this zone, the deep seismicity is abundant and indicates down-dip compression. Collectively, the seismicity defines the geometry of the deep Nazca slab, which is continuous and bent about a sharp arc. This bending is a function of the dynamic evolution of the South America-Nazca plate system. Since the Tertiary, South America has incurred a maximum shortening along the Bolivian orocline. North and south of the orocline, the South American crust has shortened less; the differential shortening occurs in conjunction with the flattened and bent Wadati-Benioff zone. The upper Nazca slab has undergone a tremendous amount of horizontal deformation that is controlled by the convergence direction relative to the trend of the trench. In response, the lower slab has been slowly bent about an axis parallel to the dip orientation of the deep Nazca plate.
• Zandt, G., Beck, S. L., Ruppert, S. R., Ammon, C. J., Rock, D., Minaya, E., Wallace, T. C., & Silver, P. G. (1996). Anomalous Crust of the Bolivian Altiplano, Central Andes: Constraints from Broadband Regional Seismic Waveforms. Geophysical Research Letters, 23(10), 1159-1162.
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  Abstract: A one-year deployment of broadband seismographs in the Bolivian Altiplano recorded numerous intermediate-depth earthquakes at near-regional distances. We modeled the associated broadband waveforms of two earthquakes to estimate an average crustal structure for the Altiplano. The resulting model is characterized by an anomalously low mean P velocity of 6.0 km/s, a low Poisson's ratio of 0.25, and a crustal thickness of 65 km. The combination of the low mean velocity and low Poisson's ratio can be explained only by a predominantly quartz-rich, felsic bulk composition. This constraint precludes significant volumes of magmatic addition from the mantle contributing to the great thickness of the Altiplano crust, but is consistent with thickening by compressive shortening concentrated in a weak felsic layer. Copyright 1996 by the American Geophysical Union.
• Beck, S. L., Silver, P., Wallace, T. C., & James, D. (1995). Directivity analysis of the deep Bolivian earthquake of June 9, 1994. Geophysical Research Letters, 22(16), 2257-2260.
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  Abstract: The June 9, 1994 deep Bolivian earthquake (MW = 8.3, dw = 636 km) occurred on a nearly horizontal rupture plane with an approximate area of 2000 km2. The rupture plane is constrained by directivity analysis of the P wave displacement pulses recorded at local, regional, and teleseismic distances. The moment release had at least four distinct episodes, or subevents. The best overall explanation for the subevents is rupture which initiates on the western margin of the fault plane, propagates to the east, and then continues bilaterally along a 350° azimuth, with most of the moment release occurring to the north. The apparent rupture velocity between subevents is very low, approximately 1-2 km/s. -from Authors
• Clarke, T. J., Silver, P. G., Yeh, Y. -., James, D. E., Wallace, T. C., & Beck, S. L. (1995). Close in ScS and sScS reverberations from the 9 June 1994 Bolivian earthquake. Geophysical Research Letters, 22(16), 2313-2316.
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  Abstract: Recordings of the Bolivian earthquake from two portable experiments were analyzed for ScS and sScS reverberations. These stations span the distance range 6°-22°, sampling the mantle beneath South America along an EW line from the central Andes to the Brazilian Craton. We used the CORE method to obtain path-averaged locations of the Moho and 400 and 660 km discontinuities. The average depth for D400 is 410km for the BANJO stations and 395km beneath Brazil, and the value for D660 is 700km for BANJO and 675km for BLSP, while the difference D660-D400 ≡ ΔD is 290km and 280km respectively. These values are much larger than the global average and suggest 200°-300° colder temperatures in the region sampled. A rupture model is presented. -from Authors
• Jiao, W., Wallace, T. C., Beck, S. L., Silver, P. G., & Zandt, G. (1995). Evidence for static displacements from the June 9, 1994 deep Bolivian earthquake. Geophysical Research Letters, 22(16), 2285-2288.
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  Abstract: The 1994 Bolivian Earthquake (MW = 8.3) was recorded on-scale by 8 very-broadband stations deployed approximately 600 km south of the epicenter. When these seismograms are carefully converted to ground displacement, there appears to be a static offset which is on order of 2 cm. The waveforms have been modelled with a synthetic waveform method that contains near-, intermediate- and far-field terms. Although the synthetics are only calculated for an elastic half-space, they compare favorably with the observed waveform in terms of shape. However, the theoretical amplitude is 1/2 of the observed static offset, which probably reflects the limitations of a homogeneous half-space model and/or the instabilities in deconvolution. -Authors
• Jun, W. u., Wallace, T., & Beck, S. (1995). A very broadband study of the 1994 deep Bolivia earthquake sequence. Geophysical Research Letters, 22(16), 2237-2240.
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  Abstract: The source process of the June 9, 1994 Bolivia earthquake has been examined in the period range of body waves to free oscillations. A series of moment tensor inversions were performed on various teleseismic wave types and frequency bands to infer the source parameters and to map the temporal character of the source processes. Also modeled were the P-waves for the January 10, 1994 event, the strong aftershock of June 9, 1994, and the August 8, 1994 earthquake. Rotation of the P axes for events after the mainshock may suggest a possible stress adjustment. -from Authors
• Silver, P. G., Beck, S. L., Wallace, T. C., Meade, C., Myers, S. C., James, D. E., & Kuehnel, R. (1995). Rupture characteristics of the deep Bolivian earthquake of 9 June 1994 and the mechanism of deep-focus earthquakes. Science, 268(5207), 69-73.
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  PMID: 17755232;Abstract: The Mw = 8.3 deep (636 kilometers) Bolivian earthquake of 9 June 1994 was the largest deep-focus earthquake ever recorded. Seismic data from permanent stations plus portable instruments in South America show that rupture occurred on a horizontal plane and extended at least 30 by 50 kilometers. Rupture proceeded at 1 to 3 kilometers per second along the down-dip azimuth of the slab and penetrated through more than a third of the slab thickness. This extent is more than three times that expected for a metastable wedge of olivine at the core of the slab, and thus appears to be incompatible with an origin by transformational faulting. These large events may instead represent slip on preserved zones of weakness established in oceanic lithosphere at the Earth's surface.
• Tinker, M. A., & Beck, S. L. (1995). Inversion of regional surface-wave spectra for source parameters of aftershocks from the 1992 Petrolia earthquake sequence. Bulletin - Seismological Society of America, 85(3), 705-715.
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  Abstract: Regional distance surface waves are used to study the source parameters for moderate-size aftershocks of the 25 April 1992 Petrolia earthquake sequence. The Cascadia subduction zone had been relatively seismically inactive until the onset of the mainshock (Ms = 7.1). This underthrusting event establishes that the southern end of the North America-Gorda plate boundary is seismogenic. It was followed by two separate and distinct large aftershocks (Ms = 6.6 for both) occurring on 26 April, as well as thousands of other small aftershocks. Many of the aftershocks following the second large aftershock had magnitudes in the range of 4.0 to 5.5. Using intermediate-period surface-wave spectra, focal mechanisms and depths for one foreshock and six of the larger aftershocks (Md = 4.0 to 5.5) are estimated. These seven events can be separated into two groups based on temporal, spatial, and principal stress orientation characteristics. -from Authors
• Tinker, M. A., Wallace, T. C., Beck, S. L., Silver, P. G., & Zandt, G. (1995). Aftershock source mechanisms from the June 9, 1994, deep Bolivian earthquake. Geophysical Research Letters, 22(16), 2273-2276.
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  Abstract: The Mw 8.3 Bolivia earthquake occurred on June 9, 1994, at a depth of 636 km. This is the largest deep event in recorded history and ruptured a portion of the down-going Nazca slab unknown to have ruptured previously. The main shock and aftershocks were recorded on the BANJO and SEDA portable, broadband seismic arrays in Bolivia. A grid search technique was used to determine focal mechanisms for 12 of 36 recorded aftershocks ranging in magnitude from 2.7 to 5.3. The aftershock focal mechanisms indicate a rotation of the P-axis within the slab from down-dip compression prior to the main shock to a near-vertical direction afterwards. This observation is consistent with the release of shear stress on the near-horizontal rupture plane and the subsequent rotation of the maximum compressive stress to a fault-normal orientation. -from Authors
• Christensen, D. H., & Beck, S. L. (1994). The rupture process and tectonic implications of the great 1964 Prince William Sound earthquake. Pure and Applied Geophysics PAGEOPH, 142(1), 29-53.
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  Abstract: We have determined the rupture history of the March 28, 1964, Prince Williams Sound earthquake (Mw=9.2) from long-period WWSSN P-wave seismograms. Source time functions determined from the long-period P waves indicate two major pulses of moment release. The first and largest moment pulse has a duration of approximately 100 seconds with a relatively smooth onset which reaches a peak moment release rate at about 75 seconds into the rupture. The second smaller pulse of moment release starts at approximately 160 seconds after the origin time and has a duration of roughly 40 seconds. Because of the large size of this event and thus a deficiency of on-scale, digitizable P-wave seismograms, it is impossible to uniquely invert for the location of moment release. However, if we assume a rupture direction based on the aftershock distribution and the results of surface wave directivity studies we are able to locate the spatial distribution of moment along the length of the fault. The first moment pulse most likely initiated near the epicenter at the northeastern down-dip edge of the aftershock area and then spread over the fault surface in a semi-circular fashion until the full width of the fault was activated. The rupture then extended toward the southwest approximately 300 km (Ruff and Kanamori, 1983). The second moment pulse was located in the vicinity of Kodiak Island, starting at ∼500 km southwest of the epicenter and extending to about 600 km. Although the aftershock area extends southwest past the second moment pulse by at least 100 km, the moment release remained low. We interpret the 1964 Prince William Sound earthquake as a multiple asperity rupture with a very large dominant asperity in the epicentral region and a second major, but smaller, asperity in the Kodiak Island region. The zone that ruptured in the 1964 earthquake is segmented into two regions corresponding to the two regions of concentrated moment release. Historical earthquake data suggest that these segments behaved independently during previous events. The Kodiak Island region appears to rupture more frequently with previous events occurring in 1900, 1854, 1844, and 1792. In contrast, the Prince William Sound region has much longer recurrence intervals on the order of 400-1000 years. © 1994 Birkhäuser Verlag.
• Myers, S. C., & Beck, S. L. (1994). Evidence for a local crustal root beneath the Santa Catalina metamorphic core complex, Arizona. Geology, 22(3), 223-226.
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  Abstract: Data from the Tucson, Arizona, broad-band seismic station are used to analyze crustal and mantle structure under and adjacent to the Catalina metamorphic core complex. The data can be modeled with a simple two-layer crust having an average Moho depth of ~30km and seismic velocities consistent with a felsic composition. The lower crust thickens under the Catalina Mountains, depressing the Moho by ~4.2km. The crustal root can isostatically compensate the average topography of the Catalina Mountains, indicating that the depth of isostatic compensation is at or below the Moho and that the Catalina Mountains act as a coherent block bounded by high-angle normal faults. The presence of a root favours a model of metamorphic core complex development described by crustal thickening in shortening episodes (crustal-welt model) followed by collapse of the crustal welt through extension. -from Authors
• Zandt, G., Velasco, A. A., & Beck, S. L. (1994). Composition and thickness of the southern Altiplano crust, Bolivia. Geology, 22(11), 1003-1006.
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  Abstract: Slant stacking of broadband seismograms recorded in the western US for two 1993 intermediate-depth earthquakes that occurred near the Bolivia-Argentina-Chile borders reveals small but clear precursors to both the wave and S wave depth phases. These precursors represent underside reflections from the thickened Andean Altiplano crust. The best-fit model has an average crustal velocity of 5.9-6.0 km/s, a crustal Vp/Vs of 1.6, a crustal thickness of 75-80 km, and a high-velocity, high-Vp/Vs mantle wedge. The finding of a thick felsic crust overlying a high-velocity mantle supports models of Altiplano uplift due predominantly to crustal shortening as opposed to mafic magmatic addition and is inconsistent with recent mantle delamination. -from Authors
• Fan, G., Beck, S. L., & Wallace, T. C. (1993). The seismic source parameters of the 1991 Costa Rica aftershock sequence: evidence for a transcurrent plate boundary. Journal of Geophysical Research, 98(B9), 15,759-15,778.
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  Abstract: The April 22, 1991, Valle de la Estrella, Costa Rica earthquake (Ms = 7.6) was a back-arc thrusting event associated with the underthrusting of the Caribbean plate beneath Central America. A network of three PASSCAL-type, portable instruments was deployed to monitor the aftershock activity in southern Costa Rica 2 to 6 weeks after the main shock. The focal mechanisms are generally in good agreement with P wave first-motion fault plane solutions determined from a local short-period network. The aftershocks show a clear spatial segmentation based on focal mechanism type. Most aftershocks near or southeast of the main shock were thrusting events with focal mechanisms similar to the main shock. In contrast, a cluster of aftershocks northwest of the main shock showed dominantly left-lateral, strike-slip motion on a northeasterly striking nodal plane. -from Authors
• Houston, H., Anderson, H., Beck, S. L., Zhang, J., & Schwartz, S. (1993). The 1986 Kermadec earthquake and its relation to plate segmentation. Pure and Applied Geophysics PAGEOPH, 140(2), 331-364.
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  Abstract: To evaluate the tectonic significance of the October 20, 1986 Kermadec earthquake (Mw=7.7), we performed a comprehensive analysis of source parameters using surface waves, body waves, and relocated aftershocks. Amplitude and phase spectra from up to 93 Rayleigh waves were inverted for centroid time, depth, and moment tensor in a two-step algorithm. In some of the inversions, the time function was parameterized to include information from the body-wave time function. The resulting source parameters were stable with respect to variations in the velocity and attenuation models assumed, the parameterization of the time function, and the set of Rayleigh waves included. The surface wave focal mechanism derived (φ{symbol}=275°, δ=61°, λ=156°) is an oblique-compressional mechanism that is not easy to interpret in terms of subduction tectonics. A seismic moment of 4.5×1020 N-m, a centroid depth of 45±5 km, and a centroid time of 13±3 s were obtained. Directivity was not resolvable from the surface waves. The short source duration is in significant contrast to many large earthquakes. We performed a simultaneous inversion of P and SH body waves for focal mechanism and time function. The focal mechanism agreed roughly with the surface wave mechanism. Multiple focal mechanisms remain a possibility, but could not be resolved. The body waves indicate a short duration of slip (15 to 20 s), with secondary moment release 60s later. Seismically radiated energy was computed from the body-wave source spectrum. The stress drop computed from the seismic energy is about 30 bars. Sixty aftershocks that occurred within three months of the mainshock were relocated using the method of Joint Hypocentral Determination (JHD). Most of the aftershocks have underthrusting focal mechanisms and appear to represent triggered slip on the main thrust interface. The depth, relatively high stress drop, short duration of slip, and paucity of true aftershocks are consistent with intraplate faulting within the downgoing plate. Although it is not clear on which nodal plane slip occurred, several factors favor the roughly E-W trending plane. The event occurred near a major segmentation in the downgoing plate at depth, near a bend in the trench, and near a right-lateral offset of the volcanic are by 80 km along an E-W direction. Also, all events in the region from 1977 to 1991 with CMT focal mechanisms similar to that of the Mainshock occurred near the mainshock epicenter, rather than forming an elongate zone parallel to the trench as did the aftershock activity. We interpret this event as part of the process of segmentation or tearing of the subducting slab. This segmentation appears to be related to the subduction of the Louisville Ridge, which may act as an obstacle to subduction through its buoyancy. © 1993 Birkhäuser Verlag.
• Beck, S. L., & Christensen, D. H. (1991). Rupture process of the February 4, 1965, Rat Islands earthquake. Journal of Geophysical Research, 96(B2), 2205-2221.
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  Abstract: The great Rat Islands underthrusting earthquake (Mw=8.7), of February 4, 1965, represents subduction of the Pacific plate beneath the North American plate along a 600-km segment of the western end of the Aleutian Islands. Body wave inversion techniques are used to determine the spatial and temporal heterogeneities associated with the Rat Islands earthquake. World-Wide Standard Seismograph Network long-period teleseismic P wave seismograms were deconvolved to obtain source time functions. Directivity associated with the three major pulses of moment release in the source time functions indicates a total source duration of 160 s, unilateral rupture in the direction 300°, fault length of 420 km, and average rupture velocity of 2.5 km/s. The three pulses of moment release are located along the fault, and these regions of high moment release are interpreted as asperities. The P wave seismic moment release of the Rat Islands earthquake is controlled by the lateral segmentation of the overriding plate. -from Authors
• Schwartz, S. Y., Lay, T., & Beck, S. L. (1991). Shear wave travel time, amplitude, and waveform analysis for earthquakes in the Kurile slab: constraints on deep slab structure and mantle heterogeneity. Journal of Geophysical Research, 96(B9), 14,445-14,460.
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  Abstract: Shear wave travel times, amplitudes and broadband waveforms from 16 events in the Kurile slab are analyzed to place further constraints on the seismic velocity heterogeneity asociated with the subducted oceanic lithosphere. The absence of a strong deep slab effect on the shear waves indicated that deep slab heterogeneity is less pronounced for shear waves than expected on the basis of existing P wave models derived from travel time residual sphere analysis. Various interpretations include: estimates of δVs/δT and compositional effects previously used to predict shear velocity structure of the deep Kurile slab are in error, with actual lower mantle slab anomalies being much weaker than proposed; the Kurile slab is so distorted in the lower mantle that no source depth dependent trends are produced for our limited focal sphere coverage; or the Kurile slab does not penetrate into the lower mantle. -from Authors
• Beck, S. L., & Nishenko, S. P. (1990). Variations in the mode of great earthquake rupture along the central Peru subduction zone. Geophysical Research Letters, 17(11), 1969-1972.
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  Abstract: The historic record for the central Peru subduction zone suggests significant variations in the earthquake size during the last 400yr. During this century there have been 4 great underthrusting earthquakes along the central Peru seismic zone. Modified Mercalli intensity data and tsunami observations for the earthquakes in this century are compared with the 29 October 1746 and 20 October 1687 earthquakes. We find that the 1746 event occurred along the segment that includes both 1940 and 1966 earthquakes. The 1687 earthquake probably ruptured the 1974 segment as well as the adjacent segment to the S where there is at present a gap between the 1942 and 1974 rupture zones. In contrast to the simple, single asperity nature of the 20th century earthquakes, these older and larger events may represent multiple-asperity ruptures along the Peru subduction zone. -from Authors
• Beck, S. L., & Ruff, L. J. (1989). Great earthquakes and subduction along the Peru trench. Physics of the Earth and Planetary Interiors, 57(3-4), 199-224.
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  Abstract: Subduction along the Peru trench, between 9 and 15° S, involves both large interplate underthrusting earthquakes and intraplate normal-fault earthquakes. The four largest earthquakes along the Peru trench are, from north to south, the 1970 (Mw = 7.9) intraplate normal-fault earthquake, and the interplate underthrusting earthquakes in 1966 (Mw = 8.0), 1940 (M = 8) and 1974 (Mw = 8.0). We have studied the rupture process of these earthquakes and can locate spatial concentrations of moment release through directivity analysis of source-time functions deconvolved from long-period P-wave seismograms. The 1966 earthquake has a source duration of 45 s with most of the moment release concentrated near the epicenter. Two intraplate normal-fault events occurred in 1963 (Ms = 6.7 and 7.0), at the down-dip edge of the 1966 dominant asperity. The 1940 earthquake is an underthrusting event with a simple source time function of 30 s duration that represents the rupture of a single asperity near the epicenter. The 1974 earthquake has a source duration of 45-50 s and two pulses of moment release. This earthquake has a bilateral rupture with the first pulse of moment release located northwest of the epicenter and the second pulse of moment release located southeast of the epicenter. Both pulses of moment release occur on the northern half of the aftershock area. The 1970 earthquake is one of the largest intraplate normal-fault earthquakes to occur in a subduction zone and has a moment release comparable with many large underthrusting events. The aftershocks for the 1970 earthquake form two distinct clusters, the smaller cluster near the epicenter has focal mechanisms characterized by down-dip tension but the second aftershock cluster, located 80 km southeast of the epicenter, has focal mechanisms characterized by down-dip compression. The P-waves for the main shock can be modeled as two sources with different focal mechanisms and depths similar to the two clusters of aftershocks. The first event has a down-dip tensional focal mechanism and is followed 40 s later by a distinct second event located 80 km southeast of the epicenter with a down-dip compressional focal mechanism and a somewhat shallower depth than the first event. The observable directivity indicates that the second source is located at the second cluster of aftershocks that have down-dip compressional focal mechanisms. The occurrence of both down-dip tensional and compressional focal mechanisms may be explained by extreme 'unbending' stresses associated with the anomalous slab geometry. The unusually large size of the 1970 earthquake may also be related to the subduction of the Mendaña fracture zone. The historic earthquake record along the Peru trench indicates that the previous event in 1746 was much larger than any of the three underthrusting earthquakes this century. The 1746 earthquake may have ruptured the entire segment in a multiple asperity earthquake. Thus, the mode of rupture along the Peru coast has changed between successive earthquake cycles. © 1989.
• Ruff, L. J., & Beck, S. L. (1987). Rupture process of the Great 1963 Kurile Islands Earthquake Sequence: Asperity interaction and multiple event rupture. Journal of Geophysical Research, 92(B13), 14123-14138. doi:10.1029/jb092ib13p14123
• Beck, S. L., & Lay, T. (1986). TEST OF THE LOWER MANTLE SLAB PENETRATION HYPOTHESIS USING BROADBAND S WAVES.. Geophysical Research Letters, 13(10), 1007-1010.
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  Abstract: The digital data used in previous studies have a very restricted azimuthal coverage. We have analyzed a greatly expanded data set of 70 pairs of transverse component S and ScS displacement pulses from deep-focus earthquakes in the Sea of Okhotsk region from stations with relatively good azimuthal coverage in order to further document the waveform behavior. We attempt to constrain the location of the associated lower mantle heterogeneity, and assess the possibility of a deep slab extension in the lower mantle beneath the source region.
• Beck, S. L., & Ruff, L. J. (1984). RUPTURE PROCESS OF THE GREAT 1979 COLOMBIA EARTHQUAKE: EVIDENCE FOR THE ASPERITY MODEL.. Journal of Geophysical Research, 89(B11), 9281-9291.
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  Abstract: The asperity distribution in the segment of the Ecuador-Colombia subduction zone that was ruptured by the 1979 (M//w equals 8. 2) and 1958 (M//w equals 7. 7) earthquakes has been determined. By combining results from both teleseismic P-wave and surface wave directivity, three regions have been defined, namely: from 0 to 56 km northeast of the epicenter, with low moment release and a small seismic displacement; the asperity, from 56 to 116 km northeast of the epicenter, with most of the moment release and a displacement of approximately 6m; and a small-displacement region from 116 to 180-240 km northeast of the epicenter. Thus the 1979 rupture zone is composed of an asperity 60 km in length, bounded by two weak (small displacement) regions. The displacement at the asperity is equal to the cumulative tectonic displacement between 1979 and 1906. The 1958 earthquake is characterized by the rupture of a small asperity (length scale approximately 25 km) in the epicentral region. The 1958 earthquake is characterized by the rupture of a small asperity (length scale approximately 25 km) in the epicentral region. The 1958 event continued to rupture past this small epicentral asperity and stopped near the epicenter of the following 1979 event. This detailed study of the spatial variation in moment release offers strong support for the asperity model of large earthquake occurrence.
• Picard, M. D., Bruhn, R. L., & Beck, S. L. (1983). Mesozoic and Early Tertiary Paleostructure and Sedimentology of Central Wasatch Mountains, Uinta Mountains, and Uinta Basin: ABSTRACT. AAPG Bulletin, 67(8), 1351-1352. doi:10.1306/03b5b95c-16d1-11d7-8645000102c1865d
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  During latest Cretaceous-Eocene time, 5,000 m (16,000 ft) of beds were deposited in central and northeast Utah. In the Late Cretaceous, sediment derived from the Sevier-Laramide thrust belt was transported to the east and southeast. Southerly paleocurrent directions in the base of the Currant Creek Formation (Maestrichtian) raise the possibility that uplift of the Uintas may have begun by then. The thrust belt continued as a major highland during the early Paleocene, and major uplift of the Uintas occurred. By the middle Paleocene there was an extensive lake which regressed during the late Paleocene as uplift of the Uintas continued. Lake Uinta reached its maximum size during the middle Eocene. During the late Eocene, Lake Uinta regressed and, near the end of the epoch, t e lake expired. Major sediment influx was from the east and southeast. Lower (early Duchesnean) and upper (Late Duchesnean) conglomeratic intervals record major episodes of uplift in the Uintas during latest Eocene. Structurally, the Wasatch Mountains are part of a marginal foreland fold and thrust belt. In the northern Wasatch Mountains, pre-Late Cretaceous thrust fault plates were folded in part of a large, ramp-anticline that is cored by allochthonous, crystalline basement. Foreland thrust belt structures in the central Wasatch Mountains were folded about the east-trending Uinta axis as the Uinta Mountains formed. Eastward movement on the Hogsback thrust during the Paleocene was transferred onto the adjacent Uinta axis and Uinta Mountains structure, causing about 20 km (12 mi) of sinistral slip in the western Uinta Mountains. Deformation in the Uinta Mountains continued following cessation of movement on the Hogsback thrust system. A south-dipping fault ramp was located beneath the Uinta Mount ins and extended to depths of 15 to 20 km (9 to 12 mi). End_Page 1351------------------------------ Oblique-slip on this ramp probably resulted in about 20 km (12 mi) of crustal shortening perpendicular to the trend of the mountains. End_of_Article - Last_Page 1352------------
• Guth, L. R., Bruhn, R. L., & Beck, S. L. (1981). Fault and joint geometry at Raft River geothermal area, Idaho ( USA).. Array.
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  Abstract: Raft River geothermal reservoir is formed by fractures in sedimentary strata of the Miocene and Pliocene salt lake formation. The fracturing is most intense at the base of the salt lake formation, along a decollement that dips E at