The Crustal Seismicity of the Western Andean Thrust (Central Chile, 33°–34° S): Implications for Regional Tectonics and Seismic Hazard in the Santiago Area

Abstract Most of the recorded seismicity in central Chile can be linked to the subduction of the Nazca plate. To the east, a much smaller fraction is observed at 0–30 km depths beneath the western Andean thrust. Paleoseismic studies evidenced the occurrence of at least two major earthquakes (M>7) over the past 17 ka, associated with the San Ramón fault (SRF): an important tectonic feature characterizing the west Andean thrust, close the Santiago metropolitan area. To better constrain the crustal seismicity in this area, the Chilean Seismological Center (CSN) extended its permanent seismic network with seven new broadband seismometers deployed around the scarp of the SRF and farther east. The improved azimuthal distribution and reduced station spacing allowed to complete the CSN catalog with more than 900 smaller magnitude earthquakes (ML<2.5) detected and located within the study region. The use of a 3D velocity model derived from P- and S-wave travel-time tomography considerably lowered the uncertainties associated with hypocentral locations. Our results show an important seismicity beneath the Principal Cordillera located at a depth of ∼10 km, and a deeper seismicity (~15 km) aligned with the main Andean thrust more to the west, parallel to the scarp of the SRF. Regional stress inversion results suggest that the seismicity of the west Andean thrust is accommodating northeast–southwest compressional stress, consistent with the convergence of the Nazca plate. Based on our improved crustal seismicity, combined with observations from previous studies, we have been able to refine the scenario of an Mw 7.5 earthquake rupturing the SRF. Ground-motion prediction results show peak ground accelerations of ∼0.8g close to the fault scarp.