State of Stress in the Interseismic in the Salar Grande Area and its Relationship with the Foreshock Sequence of Mw8.1 Pisagua Earthquake

Abstract:

We have installed short period network in order to monitor the crustal seismicity in the Tarapacá region. The main task is to try to understand the stress transference from the interplate contact to the upper crustal. The network consisted in a total of 21 short period stations, which were installed in the Coastal Cordillera (around Salar Grande) and the Precordillera (around Quebrada Blanca). The processing of the records in this network permitted the analysis of ~5000 events occurred during 2005-2012, each were localized using the NonLinLoc software. A part of this catalog, specifically the data collected during 2010, were processed in order to obtain some parameters associated to the stress as stress tensor, b-value and the maximum horizontal stress (σ_HMAX). The results of the σ_HMAX and the stress tensor show that the direction of shortening in the upper crust is NS, which is clearly deviated from the direction of convergence. This pattern, found in the Salar Grande, is practically the same that we observed in the Mw6.7 earthquake occurred on March 16^th -event that triggered the sequence of foreshock associated to the Mw8.1 Pisagua earthquake (ca. 170 km to the NW of the Salar Grande area). The NS direction of shortening is also seen in several upper crustal events occurred in the first days, before Mw8.1 Pisagua earthquake. On the other hand, the b-value > 0.9, calculated for the Salar Grande area, coincides with places where the seismic activity is associated to EW reverse faults mapped in the surface by geological studies -these structures are a common feature in the Atacama Fault System at these latitudes. They are also an indication of a NS direction of shortening, being in agreement with our seismological observations. The NS direction of shortening is not restricted to shallow depth, but it is extended to the interplate contact between the deeper part of the couple zone and the transitional zone. The last indicates that the NS shortening is a characteristic pattern associated to entire crustal plate in the zone of the Salar Grande and its possible origin would be linked to the northward gradient in the parallel to the trench component of the convergence velocity, which is zero when the convergence is perpendicular to the trench. Thus, as we observed in the Mw8.1 Pisagua sequence, these forces are able to trigger a Mega-thrust event.