Analysis of the Seismic Activity During the Preparatory Phase of the Mw 8.2 Iquique Earthquake, Chile 2014

The 2014 Iquique seismic crisis, culminating with the main Mw 8.2 Iquique earthquake (Chile), 1st of April 2014, and the largest Mw 7.7 aftershock, 3rd of April, highlighted a complex unlocking of the North Chile subduction interface. Indeed, during many months preceding this event, at least three large seismic clusters have been observed, in July 2013, in January and in March 2014. Their location and final migration towards the mainshock rupture area represents the main motivation of this work. We built a new, more complete catalogue for the period over December 2013 to March 2014 in Northern Chile, using a new automated array method for earthquake detection and location [Poiata et al. 2015] with some additional process in order to ensure the location. Using the data-set provided by the IPOC and ILN networks, we detected a total of 36000 events, forty times more than the catalogue produced by Centro Sismologico National del Chile. The new catalogue decreases the magnitude of completeness by more than two units, from 3.3 to 1.2. We observe the appearance of two shallow clusters offshore of the cities of Iquique and Pisagua in January 2014, only the Pisagua cluster remains active until the Iquique earthquake, helped by the upper-plate Mw 6.7 major foreshock of the 16th March. A spatial-temporal- statistical analysis of those activities allows us to better characterize the whole preparatory phase. We interpret our results in light of the location, timing and energy of several aseismic slip events, evidenced by Boudin et al. [AGU 2014], which coincide with the seismic clusters. We propose that the preparatory phase of the Iquique earthquake consists of a complex interplay of seismic and aseismic slip along the subduction surface. Furthermore, our analysis raises new questions regarding the complex slip during the Mw 7.7 aftershock, and the spatial variation of the effective coupling along the subduction interface, imaged by GPS studies, suggesting new research direction that will be outlined.