Anomalous seismicity in the lower crust of the Santiago Basin, Chile

In central Chile, intraplate, crustal seismicity is mostly constrained to the upper 20 km; nevertheless, sparse observations of earthquakes have been reported deeper, without a definitive explanation. In the lower crust of the Santiago Basin, located within a compressional, subduction environment, we have observed a series of events organized in two clusters. This work presents a detailed study of these earthquakes, recorded from November 2002 to April 2005, with depths from 28 to 36 km and magnitudes ranging from 2.8 to 4.3. Using a local network (composed of vertical short-periods, broadbands, and accelerometers) we performed a time and spectral waveform analysis to determine their exact location and certain source parameters: seismic moment, corner frequency, and seismic energy. We found that each cluster presents extremely well correlated waveforms, indicating that each event in the cluster ruptures the same patch in the same fashion. Even more, we determined that representative events show a large apparent stress drop, releasing six times more energy than interplate events, for a given seismic moment. The most plausible interpretation seems to be a decollement between the crust and the mantle, where few (but strong) asperities are located in a broader region that continuously creeps. We believe that this work will help improve our understanding of the processes involved in subduction zones, as well as the assessment of the Seismic Hazard for the Santiago Metropolitan Area.