2014, Mw=8.2 Pisagua, Northern Chile Foreshocks: Not Limited to the Interplate Contact

Abstract:

The magnitude and the rupture area of the Mw=8.2 April 1^st, 2014 Pisagua earthquake suggests that northern Chile is still a mature seismic gap reaching the end of its megathrust cycle. The Pisagua earthquake exhibited foreshock activity with four Mw~6.4 earthquakes and an associated increase of seismicity offshore of Pisagua, that started on March 16^th (Mw=6.7). The Pisagua area exhibits some tectonic singularities: (1) the crustal forearc deformation is seismically active; (2) it coincides with the influence zone related to the buoyant Iquique Ridge; and (3) it is also located at the maximum curvature of the Bolivian Orocline. Previous studies established the presence of abundant microseismicity in the upper part of the interplate contact, along some geometrical arrangements suggesting forearc fault-related seismicity. We analyzed the 2014 foreshock sequence using a 3D body wave velocity model obtained from the dense short period FONDECYT 1130071 and the broadband permanent IPOC networks. Although there is limited azimuthal coverage for the foreshock activity, it seems clear that most of them are located along the interplate contact. However, there were foreshocks also located along structures parallel to those that were previously identified eastward, almost perpendicular to the slab. Additionally, we invert regional moment tensors from full broadband waveforms to compute the best centroid depth of the foreshocks (Mw>4.5) recorded by the IPOC network. The centroid depth and focal mechanism agrees rather well with the spatial distribution of the located seismicity. Besides thrust faulting events occurred along the plate interface, shallow crustal events were also triggered within the overriding South America plate. After the 2010 Mw=8.8 Maule and 2011 Mw=9.3 Tohoku megathrust earthquakes, the activation of coastal forearc faults associated with intraplate shallow earthquakes (Mw~7) occurred, evidencing a relationship between the large earthquakes and the forearc faulting process. Considering that the 2014 Mw=8.2 earthquake did not fill the northern Chile seismic gap, and that there were foreshocks located in the offshore upper crust, we believe that there is an urgent need to understand not only the issues related to forearc-subduction deformation processes, but also to seismic hazard evaluations.