Three-dimensional local earthquake tomography of pre-Cenozoic structures in the coastal margin of central Chile: Pichilemu fault system

Abstract

Following the Mw 8.8 Maule megathrust earthquake that occurred on February 27, 2010, in central Chile, a sequence of normal faulting crustal earthquakes occurred close to the city of Pichilemu. This activity lasted several months and included two large events (Mw = 6.9 and Mw = 7.0) on March 11, 2010. An initial investigation of this activity analyzed a data set of about 630 earthquakes recorded locally by 8 short-period seismic stations and attributed this seismicity to the activation of the “Pichilemu fault system.” While there is no visible surface faulting associated with this system, it is inferred from discontinuities in lithofacies in the metamorphic complex of Pichilemu and from morphological breaks attributed to post-Pliocene neotectonic activity. There are no recorded shallow crustal earthquakes in the Pichilemu area prior to the 2010 events. In this study, we combine locally recorded earthquake data from the 2010 seismic deployment and data from 20 seismic stations (short period, three components, continuous recording) deployed in 2017 around Pichilemu, Chile, to create a more detailed characterization of the Pichilemu fault system through local earthquake tomography (LET). The combined data set composed of P- and S-wave arrival times from 3691 events was inverted to generate a 3D elastic wave speed model from the surface to about 50-km depth. One hundred twenty-two focal mechanisms for relocated earthquakes with M ≥ 1 were also generated. Relocated hypocenters show that most of the recorded seismicity is associated with the Pichilemu fault system; its main structure is oriented N145°E, and it is seismically active along about 50 km long. Normal faulting mechanisms predominate for events with M≥2, being similar to the mechanisms of the Mw 7.0 March 11, 2010, Pichilemu earthquakes. Low-velocity anomalies correlate with fracture zones associated with the Pichilemu fault, and a high contrast in Vp/Vs coincides with known structures of Paleozoic to Mesozoic age. A high Vp/Vs ratio is observed where a projection of the fault reaches the interplate contact, suggesting that this zone of the forearc crust is likely weakened by the presence of fluids from the slab. A high Vp anomaly is contiguous with the fault system and appears to be related to the presence of granitic rocks which belong to the Coastal Batholith within the Cordillera de la Costa. We suggest that the location of the Pichilemu fault system is governed by rheological contrasts inherited from the evolution of the subduction complex represented by the current Cordillera de la Costa, and we infer that the orientations of those structures, the possible hydration of them from the interplate contact and associated crustal blocks, play a key role in fault activation following the stressing by a great subduction earthquake.