Abstract

On February 27, 2010, a megathrust subduction earthquake (Mw 8.8) occurred in Chile, followed by a sequence of crustal earthquakes in the Central Chile margin with normal focal mechanisms. This activity lasted several months, with the two largest events (Mw = 6.9 and Mw = 7.0) occurring on 11 March 2010. An initial study of this sequence analyzed a data set of about 630 earthquakes recorded locally by a network of 8 short-period seismic stations. This sequence was associated with the activation of the Pichilemu Fault System, but without corroboration from visible surface faulting excepting for ca. 1 m of subsidence. A network of 20 seismic stations operated between the towns of Navidad and Iloca during a two-month period in 2017, from which we generated a dataset of 16029 P and 9463 S wave arrival times from 1493 earthquakes. We also recovered an additional 9961 P and 9714 S wave arrival times from 2137 previously undetected earthquakes recorded by the 2010 network. With this combined dataset, and using a joint inversion methodology, we inverted the body wave arrival times to generate a 3D tomographic image of compressional wavespeed (Vp), shear wavespeed (Vs) and Vp/Vs ratio from the surface to about 70 km depth. We find that low-velocities mostly correlate with fracture zones associated with the Pichilemu fault, and a high contrast in the Vp/Vs ratio correlates with zones of the forearc crust that are likely weakened by the presence of fluids. The orientation of anomalies is mainly NNW, coinciding with structures of Paleozoic to Mesozoic age that are related to the development of an accretionary wedge. We infer that the orientations of inherited structures and crustal blocks play a key role in faulting related to subduction earthquakes.