Intermediate depth events within Nazca plate: Insights from recent earthquakes in Northern and Central Chile

Intermediate depth earthquakes are the most destructive events in Chile. The 24 January 1939 earthquake of magnitude 7.7 destroyed the Chillán city and caused around 20,000 fatalities. Recent deployments of seismological stations along the Chilean margin allowed high precision studies (e.g. 3D location) of these events. Taking advantage of this new available data set, we perform a P and S waves 3D Bayesian tomography in Northern to South-Central Chile. The results highlight a clear double seismic zone (DSZ) in the Nazca plate. Then, we characterized the seismic rupture parameters and analyzed the aftershocks sequence for events of magnitude larger than 6 occurred after 2010. The largest event for this period was Coquimbo 2019 earthquake of magnitude Mw 6.7. This earthquake was a slab-pull, located at 70 km depth, around the 600°C isotherm within the Nazca slab. The aftershocks were located shallower than the mainshock event, around the 450°C isotherm. Accelerometer data recorded in the Coquimbo region, allowed to estimate a 3s rupture duration. This duration agrees with a rupture radius smaller than 7 Km, and a stress drop of 7.5 MPa. We further report how aftershocks are not located on the same fault plane than the mainshock. Comparison of these results with the ones we obtained from events in northern Chile, we can confirm the similarity of the rupture parameters (e.g. stress drop, rupture velocity) for events close to the higher isotherm, and shallower ones occurring in the Nazca subducted crust. Nevertheless, aftershocks productivity varies, with larger productivity for shallower events.