A closer look to the San Ramón fault seismicity in the west Andean thrust front of Santiago, Chile

In Central Chile, most of the recorded seismicity is linked with the subduction of the Nazca plate and associated intra-slab deformation, under the South American plate. More to the east, a much smaller portion of the regional seismicity ( 5%) is related to thrusting deformation of the western cordilleran front of the south Central Andes. Although large earthquakes are far less frequent on this shallow region than on the plate boundary, paleoseismic studies evidenced the occurrence of at least 2 major episodes (Mw 7-7.5) over the past 17 kyr, associated with the San Ramón fault; the most obvious manifestation of the west Andean thrust system. The Santiago basin, located in the central valley at the western flank of the Andes, hosts close to 50% of the total Chilean population and thus receives particular attention. In order to better understand the seismicity of the San Ramón fault, the Chilean National Seismological Center (CSN) improved its permanent network with seven new broadband seismic stations, located around and over this geological structure. Because the number of events recorded is very large, we use a multiband array detection and seismic source location method to differentiate events associated with the San Ramón's system and those coming from other seismogenic zones. Our results show that the San Ramón fault and the west Andean thrust system in general, present seismic activity characterized by low magnitude events (Ml < 2.5). Preliminary seismic locations show focal depths around 8-12 km, in good agreement with the estimated depth of the west-vergent main décollement level associated to the western cordilleran front. In the future, event locations, magnitude and focal mechanism estimations will be improved in order to present a more complete catalog for a better assessment of seismic hazard in this region, together with the general tectonic evolution of the western cordilleran front of the south Central Andes.