Active Forearc Faults in the Central Part of the Mature Northern Chile Seismic Gap: Insights from Seismological and Neotectonic Detailed Analysis

Abstract:

The Andean margin is one of the archetypical venues for studying processes related to subduction of oceanic lithosphere and the evolution of continental lithosphere. The abundance of seismic activity both in the subducting Nazca plate and in the overriding South American plate allow us to better understand this margin through the generation of high resolution subsurface images of the subducting slab and the upper crust. A regional body wave tomographic 3D model was obtained using 23.300 locally recorded earthquakes coming from different deployments done in northern Chile between 1990 and the 2014 Mw=8.2 sequence, including the dense 2013 short period FONDECYT 1130071 around Pisagua. Through this way, the 3D architecture of the major forearc faults in the study area was determined for the first time using a geological and geophysical approach. Previous studies in the northern Chile seismic gap, between the coast and the Altiplano, established the presence of abundant microseismic activity in the upper part of the interplate contact, corresponding to continental intraplate seismicity. These geometrical arrangements, from the surface down to 40 km depth, are almost perpendicular to the subducting slab, and are well correlated with low Vp/Vs ratio, indicating the presence of a fractured zone and well correlated with the Neogene. Kilometer-long faults existing in the northern Chilean region, indicating that the outer forearc absorb permanent deformation through active faulting. It is interesting to note that some low-magnitude earthquakes associated with the 2014 aftershock sequence are found along the mentioned faults, indicating their ability to be activated upon the occurrence of a large earthquake