Crustal fault regime and stress field in northern Chile: Insights on partition from the subduction interface
Abstract
The Nazca subduction zone in northern Chile features significant seismicity on the plate interface, and within both the subducting Nazca plate and the overriding South America plate. To date, the first two types of seismicity have been studied widely in terms of rupture locations, properties, and triggering mechanisms. Conversely, seismicity within the overriding plate, which generally features smaller earthquakes, has been studied much less.
In this work, we use high-precision earthquake locations within the South America plate from two recently-pusblished catalogs based on dense temporary and permanent seismic networks in the region. These catalogs report significant upper-plate seismicity beneath the Coastal Cordillera, while seismicity beneath the Andean Precordillera is found to be less frequent. From these catalogs, we invert ~350 well-constrained focal mechanisms from P-wave polarities and S/P amplitude ratios. This focal mechanism catalog is used as input to invert for the crustal tectonic stress field and its principal stress components. Our results indicate that crustal events beneath the Coastal Cordillera feature a clear N-S compression between latitudes 19° S and 22° S. Focal mechanisms in this area show a predominantly reverse-fault regime with E-W strikes. This occurs throughout the crust and is found to be approximately stationary in time for over a decade of observations (2007 to 2018), resulting in a stress tensor with a well-defined maximum compressive stress that is nearly horizontal and oriented N-S. The data set for the Andean Precordillera is considerably smaller and limited between latitudes 20.5° S and 21.5° S, featuring more strike-slip events, suggesting a predominantly strike-slip regime with a maximum compressive stress oriented NE-SW. As the main subduction process should primarily create an approximate E-W compression, the observed crustal N-S compression in the Coastal Cordillera could be the result of deformation partitioning due to the oblique and concave subduction, where the Gephart symmetry plane intersects the coastline at around 20.5° S, roughly in the middle of these data, indicating that the N-S compression could be related to processes along the inner arc of the Bolivian Orocline.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMS038.0001H
- Keywords:
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- 7299 General or miscellaneous;
- SEISMOLOGY