Three-Dimensional Local Earthquake Tomography of Pre-Cenozoic Structures in the coastal central Chile margin: The Pichilemu Fault System
Abstract
We combine locally recorded earthquake data from two individual seismic deployments (short period, three components, continuous recording) in the area around Pichilemu, Chile, to create a more detailed characterization of the Pichilemu fault system through Local Earthquake Tomography (LET). The first network was deployed in 2010 with 8 stations (Farias et al., 2011), and the second was deployed in 2017 with 20 stations. The combined dataset is composed of P and S wave arrival times from 3691 events. These arrival times were estimated using the automated REST algorithm (Comte et a.l, 2019), and used to generate a 3D tomographic model of compressional wavespeed (Vp), shear wavespeed (Vs) and Vp/Vs ratio from the surface down to 50 km depth. The POLEST algorithm (Comte et al., 2019) was used in combination with FOCMEC (Snoke., 2003) to generate 122 focal mechanisms for earthquakes with M ≥ 1. Checkerboard and reconstruction tests show good resolution in the study area. Relocation of hypocenters in the final 3D model shows that most of the recorded seismicity is associated with the Pichilemu fault system. The main structure of the fault system is oriented N145°E and is approximately 50 km long. Normal faulting mechanisms predominate for events with M≥2 and are similar to the mechanisms of the Mw 7.0 2010 March 11th 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 Carboniferous intrusives and Jurassic magmatic activity within the coastal range. 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 and associated crustal blocks play a key role in faulting related to subduction earthquakes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMS061.0022C
- Keywords:
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- 7299 General or miscellaneous;
- SEISMOLOGY