Rupture process of the two main 2014 Pisagua earthquakes (Mw 8.1 and 7.6) from strong motion, geodetic and global seismic data

Wednesday, 17 December 2014
Martin Vallee1, Raphael Grandin1, Sergio Ruiz2, Bertrand Delouis3, Christophe Vigny4, Efrain Rivera5, El-Madani Aissaoui6, Sebastien Allgeyer7, Claudio Satriano1, Natalia Poiata1, Pascal Bernard6, Jean-Pierre Vilotte1 and Bernd Schurr8, (1)Institut de Physique du Globe de Paris, Paris, France, (2)Universidad de Chile, Departamento de Geofísica, Facultad de Ciencias Físicas y Matemáticas, Santiago, Chile, (3)Geoazur - CNRS, Valbonne, France, (4)ENS/CNRS, Paris, France, (5)Universidad de Chile, Departamento de Geofísica, Facultad de Ciencias Físicas y Matemáticas, Santiago, Chile, (6)Institut de Physique du Globe, Paris, France, (7)Australian National University, Canberra, Australia, (8)GeoForschungsZemtrum Potsdam, Potsdam, Germany
The 2014 seismic sequence in Northern Chile culminated with the Mw8.1 Pisagua earthquake (2014/04/01), rapidly followed by a large aftershock close to Iquique (Mw=7.6, 2014/04/03). A detailed analysis of these two earthquakes is required to better relate these major events with their preparatory phase, and in particular with the intense activity of the area between March 16th and April 1st. This area has been densely instrumented by several international and Chilean projects, including strong motion and broadband sensors as well as high-rate GPS stations. In this study, we make an extensive use of the seismo-geodetic data collected by the IPOC/CSN (http://www.ipoc-network.org/) and ONEMI networks. In a first step, we model several of the foreshocks with magnitudes in the range [5.5 6.5] in order to determine a seismic velocity model able to reproduce the observations up to 0.1Hz. This velocity model is then used to analyze the rupture process of the two main shocks. Besides the use of the local displacement waveforms (coming from strong motion sensors and high-rate GPS), the inversion procedure also includes the static GPS displacements and the teleseismic P and SH waves. The source models are finally validated with other geophysical information not included in the inversion, such as InSAR or surface waves analysis through an empirical Green function approach. These data consistently show that most of the coseismic slip of the 1st April main shock is located in a 50km long zone (along strike), close and South from the hypocenter. In the time domain, this large slip patch has been activated 15-20s after origin time and lasted about 30s. Before this activation, seismic moment release was low and rupture remained in the hypocentral region. Differences and relations between the two shocks, as well as their interpretation in the 2014 seismic sequence, will be discussed.