Study of mechanical properties and transient deformation at subduction zones from analysis of seismic activity, the case of Chile.

Subduction zones are the most seismically active regions in the world. They are also the seat of megathrust-earthquakes such the chilean earthquake of 1960, magnitude 9.5. The last major ruptures in Chile have revealed complex seismic-aseismic interactions. In order to study these interactions, we investigated two seismic crises: the seismic swarm of April 2017 and the preparatory phase of the Iquique earthquake (M w 8.1) from April 1 st, 2014. The seismic swarm took place near the city of Valparaiso in an area which is known to have experienced mega-earthquakes in the past (in 1730 and in 1906). In order to study the dynamics of this swarm, we built a rich catalog of more than 2000 earthquakes composing the sequence. An intense seismic activity began on April 22 nd , two days before the main earthquake of the sequence M w 6.9. This seismic activity was accompanied by a gradual slip along the interface that we observed both in the GPS data and by detecting repeating-earthquakes. Our analysis suggests that the swarm was driven by aseismic slip. The second study concerns an earthquake of magnitude 8.2 which occurred on April 1 st 2014 near the city of Iquique. This event broke one-third of the seismic gap in northern Chile. It was preceded by a sequence of precursor seismic swarms that appeared to have been accompanied by stable slip in the subduction interface. By building a more complete earthquake catalog (>30000 events), we were able to thoroughly analyse the preparatory phase of the Iquique earthquake. Following a statistical approach, we observe the occurrence of a large-scale seismic quiescence in the region of the mainshock before its nucleation. We link this quiescence to a downdip transient-slip potentially related to fluid channeling along the subduction interface.