Abstract

To investigate the relationship between subduction of the Nazca plate and features inside and over it, we deployed a seismic network of 83 short-period stations between 26.25° and 30.75°S, that is the region known as the Chilean “Pampean flat-slab segment”. From the seismic cycle point of view, our study region is located in the rupture length of the Mw8.5 Atacama 1922 earthquake and in the northern part of the rupture length of the Mw8.4 Illapel 2015 earthquake. This region has two Low Coupling Zones (LCZ) limiting the Atacama High Coupling segment: Barranquillas LCZ at 28°S and La Serena LCZ at 30°S. According to Metois et al. (2016), most low coupling zones in Chile are correlated with the subduction of ridges or fracture zones in the Nazca Plate, and all of them are associated with morphological features on the coast as bays and peninsulas. In the study area the Barranquillas LCZ correlates with the subduction of the Copiapo Ridge and the La Serena LCZ correlates with the subduction of the Challenger Fracture Zone. So, If both LCZs in our study region respond to geomorphological asperities in the Nazca Plate: Why did the rupture of the Mw8.5 Atacama 1922 earthquake cross Barranquilla LCZ and not La Serena LCZ? Why is there a big bay close to La Serena LCZ in comparison to close to Barranquilla LCZ, where the bay is interrupted by basement highs ? Could there be a relationship between rupture behavior, local slab features and the large-scale geomorphology of the coast? To answer these questions, we determined a 3D body wave velocity model with the data registered from December 2018 to January 2020. Our main results show that: (1) there is a double seismic zone in the Nazca plate at 30-90 km depth, with two parallel planes separated by 20 km depth and (2) there are differences in the subduction wedge size along strike that coincides with morphological features in the coast and LCZs. These variations along strike could be related to features in the subducted oceanic lithosphere as seamounts and fractured zones which can store fluids and influence the tectonic erosion process at the Chilean margin and the downward of these features along with the subducted slab could release volatiles that generates hydrated minerals at different depths, which could explain the latitudinal differences in the subduction wedge size and in the seismic behavior.