Aftershock Activity at Intermediate-Depth Earthquakes in Northern Chile Controlled by Plate Hydration

We investigate the differences of the seismic source and aftershock activity using kinematic inversions and template matching respectively, for the six largest intraslab intermediate-depth earthquakes occurred in northern Chile (Mw ~6.3) since 2010 at depths between 90 and 130 km and recorded by dense strong-motion and broad-band seismic networks. In addition, we developed a thermal model using the finite element method in the study region with the aim of analyze the impact of temperature on seismic behavior as the oceanic plate subducts. Our results show that geometries of rupture zones are similar, with semi-axis for an elliptical patch approach about 5 km, and stress drop values between 7 and 30 MPa. On the other hand, the number of aftershocks exhibits clear differences, and their amount decreases with increasing the depth within the slab bounded by the 450 ºC isotherm, which represents a limit between a high-hydrated and a dry or low-hydrated region. Furthermore, mainshocks occur at distances from the top of the slab from 7 to 40 km, and all of them exhibit normal focal mechanisms suggesting that the extensional regimen deepens within the slab to the 700-750 ºC isotherm-depth. We suggest that in northern Chile the abrupt decrease of aftershocks in the lower part of the extensional regimen is caused by the absence of a hydrated slab at those depths.