S. Beck’s research while affiliated with University of Arizona and other places

Costa Rica is located near the southern end of the Middle American Trench (MAT) in a complicated tectonic setting controlled by the interaction of the Cocos, Caribbean, and Nazca plates. The oceanic Cocos plate subducts to the northeast underneath the Caribbean plate creating a volcanic arc located 150 km away from MAT. In Northern Costa Rica the arc basement is represented by part of Caribbean Plateau that includes flood basalts, mafic oceanic rocks, serpentinized peridotites, and silicic sediments. For this study, P and PP wave receiver functions have been calculated using teleseismic earthquakes recorded in Northern Costa Rica by broadband stations of the CRSEIZE, Pocosol, and Corisubmod experiments, and stations JTS and HDC from the Global Seismology Network and the Geoscope Project, respectively. The goal of this work is to constrain the major boundaries such as the base of the continental crust and the top of the subducting Cocos slab, as well as Vp/Vs ratios to estimate the composition and physical state of the lithosphere. These calculations are relevant as they provide a velocity structure that directly improves earthquake locations, gives insights into the tectonic evolution of the region, and are useful to describe the extent of the serpentinized forearc mantle wedge. Receiver functions are computed using an iterative pulse stripping time domain deconvolution technique. The depth and average Vp/Vs ratio to the discontinuities are estimated using a stacking algorithm that sums receiver function amplitudes of direct Ps and its multiples. Our results show a thick crust of 41 km underneath the volcanic arc and a thinner crust underneath the backarc and forearc, where the Moho discontinuity is visible at depths of 33-38 km. Moho is observed as a weak signal beneath stations located in the forearc region, which is consistent with previous studies that suggested serpentinization of the mantle wedge. The descending Cocos slab is observed at depths from 20 to 40 km beneath the Nicoya Peninsula in good agreement with the contours of the top of the Cocos slab from previous studies in this region and between 50-78 km underneath forearc and arc. The average Vp/Vs to Moho is 1.90 underneath the volcanic arc and varies from 1.72 to 1.88 in the forearc and backarc regions.

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.

Crustal seismicity in the Andean backarc between 30°S and 33°S is responsible for the largest damaging earthquakes in Argentina. This region is characterized by the presence of a northeast elongated flat subduction of the Nazca plate at about 100-km depth, a shut off of the volcanic arc and the basement cored uplifts of the Sierras Pampeanas. The overriding South American plate is composed of accreted terranes, sometimes reactivated by Paleozoic-Mesozoic extensional processes. We have used regional broadband data recorded during the CHilean ARgentinean Geophysical Experiment (CHARGE) to characterize the crustal structure and crustal seismicity by modeling regional broadband waveforms. Our study shows differences in the backarc terrane crustal seismic parameters that may have implications for the nucleation of present-day crustal seismicity. The Cuyania terrane has a more mafic composition, thicker crust with possible partial eclogitization at lower crustal levels and exhibits large numbers of earthquakes of higher magnitudes from historic to modern times. In contrast, the Pampia terrane to the east has thinner more quartz-rich crust and is comparatively seismically quiet for moderate-sized events.

The history of Argentina has repeated occurrences of damaging crustal earthquakes with examples like the 1944 (Mw 7.0) San Juan earthquake, considered the largest natural disaster. These large earthquakes occur in the continental Andean backarc crust as far as 600 to 800 km east from the Trench. Of high significance is the correlation of this large-sized continental seismicity with the horizontal position of the subducted Nazca plate at about 100-km depth. In addition, lateral variations of the crustal structure are expected since several terranes have been accreted to western South America since the Paleozoic. Given the high seismic potential of this region, understanding of these seismotectonic processes and the crustal structure is essential for the assessment of seismic hazards and the mitigation of their effects. In this presentation we show our work based on an integrated research effort that combines permanent and temporal seismic networks from the Argentinean National Institute for Seismic Disaster Mitigation (INPRES) and IRIS- Passcal arrays. This international collaboration started in 2000 and involves researchers, technicians and students from the University of Arizona (USA), the National University of San Juan (Argentina) and INPRES (Argentina).

Characterization of the highly seismically active Andean backarc is crucial for assessment of earthquake hazards in western Argentina. Moderate-to-large crustal earthquakes have caused several deaths, damage and drastic economic consequences in Argentinean history. We have studied the Andean backarc crust between 30°S and 36°S using seismic broadband data available from a previous ("the CHARGE") IRIS-PASSCAL experiment. We collected more than 12 terabytes of continuous seismic data from 22 broadband instruments deployed across Chile and Argentina during 1.5 years. Using free software we modeled full regional broadband waveforms and obtained seismic moment tensor inversions of crustal earthquakes testing for the best focal depth for each event. We also mapped differences in the Andean backarc crustal structure and found a clear correlation with different types of crustal seismicity (i.e. focal depths, focal mechanisms, magnitudes and frequencies of occurrence) and previously mapped terrane boundaries. We now plan to use the same methodology to study other regions in Argentina using near-real time broadband data available from the national seismic (INPRES) network and global seismic networks operating in the region. We will re-design the national seismic network to optimize short-period and broadband seismic station coverage for different network purposes. This work is an international effort that involves researchers and students from universities and national government agencies with the goal of providing more information about earthquake hazards in western Argentina.

The central Andes area classic convergent margin orogenic belt with an active volcanic arc, a high broad plateau, and a major fold and thrust belt. Geophysically, an important distinction must be made between the arc terrane and the back arc terrane, which comprises the bulk of the tectonically shortened mountain belt. The high elevations and thick crust (60-75km) of the backarc region are related to the shortening of the weak western edge of South America between the strong lithospheres of the subducting Nazca plate and the underthrusting Brazilian shield. In the backarc the seismic velocities indicate a weak, overall felsic quartz-rich crust with a heterogeneous upper mantle consistent with piecemeal removal of parts of the lithosphere. The northern part of the Altiplano and the Eastern Cordillera show low seismic velocities beneath the crust consistent with lithospheric removal. In contrast, seismic velocities beneath the central Altiplano are consistent with lithospheric mantle and/or ecologitized lower crust still attached to the crust. Oxygen isotope studies in the northern Altiplano suggest rapid surface uplift between 10 and 6.8 Ma that would require lithospheric removal (Garzione et al., 2006). This locality occurs above a transition between fast and slow P- wave velocities in the upper mantle beneath the northern Altiplano. Overall, the large amount of shortening and the lack of a high velocity lower crust suggests that lithospheric material has been recycled into the mantle as the Brazilian craton has subducted beneath the Eastern Cordillera. Variations in the lithospheric structure both along strike and perpendicular to Andean plateau are indications of different stages in the process of lithospheric removal and "felsification" of the crust.

We combine the results from a range of seismic studies, which used data collected during the Chile Argentina Geophysical Experiment (CHARGE) to present an up to date model of the crustal and upper mantle structure in central Chile and Argentina. This area of central Chile and Argentina is of particular interest due to the presence of the central Chilean flat slab, a region in which the downgoing Nazca plate descends normally to a depth of 100 km, and then flattens, traveling horizontally for several hundred kilometers before resuming its descent into the mantle. The flat slab region is characterized by an absence of modern arc volcanism and the presence of the inland basement cored uplifts of the Sierras Pampeanas, which have been considered to be modern analogues to the Laramide uplifts in the western United States (Allmendinger et al. (1986)). The combined results of these studies, that include tomography, receiver functions, SKS and local S-wave splitting, local earthquake relocations and mechanisms, and regional waveform modeling, indicate an unusually cold and brittle crust underlain by a cold, dry, and depleted sliver of lithosphere, which is trapped between the crust and the horizontally traveling flat slab. The western Sierras Pampeanas has thick crust (55km) with a high velocity, high density lower crust that explains the relatively low average elevation. In contrast, the eastern Sierras Pampeanas crust is much thinner (35-40 km) and has lower P-wave velocities and a lower Vp/Vs ratio. The change in crustal character correlates with the Cuyania terrane composed of mafic basement and the Pampian terrane composed of more quartz-rich basement.

Earthquake hypocenter locations and focal mechanisms for intermediate-depth earthquakes between 30° and 36°S in South America reveal subducting slab geometry and deformation that is slightly different from past studies. The CHARGE broadband seismic network was arranged in two transects across the Andes in this region between 2000-2002. During this period, we recorded 1098 intermediate-depth earthquakes with enough P- and S-wave arrivals for obtaining reliable locations. We use the multiple-event location algorithm GMEL to refine the hypocenter locations of these events which yields tighter clustering of events within the slab compared to hypocenters produced by the ISC and single-event hypocenters calculated with our data. In addition, station corrections produced by this method are consistent with independently-determined crustal thickness variations across the network and local changes in uppermost mantle velocity. The Wadati-Benioff zone in the region between 30° and 33°S has been shown by previous earthquake location studies to dip eastward into the mantle to a depth of 100 km and then extend another 300 km with a flat geometry before dipping again into the mantle. To the south of 33°S the slab dips into the mantle at a consistent 30° angle. Using our new hypocenter locations, we produce contours of the top of the Wadati-Benioff zone which exhibits the shallowest depths coinciding with the projected extent of the Juan Fernandez aseismic ridge (JFR) within the subducting slab. All other parts of the slab to the north, east and south dip away from this zone, though more steeply to the east and south than to the north. We estimate a conservative confidence interval on the depths of our events to be between 7-12 km, which means the small variation in depth (15-20 km) between the JFR and the slab to the north near 30°S is close to the edge of our resolution. However, previous studies have also noted deeper events north of the JFR, which supports our depth determinations. We also determine focal mechanism solutions from first motions for many of the located intermediate-depth events, the results of which show normal faulting dominating brittle slab deformation. The orientation of the sub-horizontal T-axes is perpendicular to local slab strike almost everywhere in the slab, given our new slab contours, which is consistent with slab-pull acting throughout the slab. There is an exception to this trend within a small portion of the slab near the slab-dip transition (near 33°S) at 150 km depth, where T-axes are rotated away from the slab dip direction. The implication is that there may be a gap in the slab at this depth. The absence of a double seismic zone below 90 km depth along the JFR and tomography results that show a dry mantle wedge immediately above the JFR suggests that dehydration embrittlement may not be the root cause of these events. We suggest an alternative hypothesis that the JFR, because it is the shallowest portion of the flat slab, may be a locus of bending of the slab, hence increased seismic activity compared to other portions of the slab.