Transient behavior and role of barriers in the North Chile - South Peru seismic gap

Abstract

The aim of this Ph.D. thesis is to have a better comprehension of the interactions between coupling, slow slip events (SSEs) and the seismic ruptures in subduction zones. This work focuses on the North Chile - South Peru subduction zone that has been recognized as a mature seismic gap. Thus, the region has been the target of an important international effort in geophysical instrumentation (GPS and seismological stations), since the mid-2000s. The region has been affected by several well-registered earthquakes, that makes it a good case to study the earthquake preparation phase and the relationship between coupling, SSE and seismic rupture. The 65 stations available in the region have been processed in double differences using the GAMIT-GLOBK software in the period 2000 - 2014. The GPS displacement time series have been analyzed and the associated displacements to the different stages of the seismic cycle as well as seasonal signals have been modeled. The analysis of the tendencies in the GPS time-series evidences a change in the velocity field before and after the Tarapaca slab-pull earthquake occurrence (Mw 7.8) in June 2005, in the range of latitude affected by Iquique earthquake (Mw 8.1) in 2014. This velocity change is associated with a change in the shallow (z < 40 km) and deep (z > 80 km) seismicity rates. The analysis of the declustered catalog shows that the velocity change observed affects the background seismicity as well, that often seen as a proxy for the tectonic loading. Finally, we find interactions between shallow and deep seismicity, that may play an important role in the interface earthquakes preparation phase. At a shorter time scale, the time series show another change in velocity 8 months before Iquique earthquake. Models indicate that the velocity change corresponds to an SSE Mw 6.5, mainly aseismic, corresponding to the long preparation phase of the earthquake. The short-term GPS velocity variations have also been analyzed on residual signal. It allows identifying, thanks to a matched-filter, 41 small SSEs during the interseismic period. These events are localized mostly in the deeper part of the seismogenic zone, in areas where the coupling is low (38% of SSEs where φ < 0.25) or with intermediate values (27% of SSEs where 0.25 < φ < 0.75). It suggests that the slip in these regions is produced in burst way. Some of those events are correlated with peaks of seismic activity, especially at intermediate depths. Finally, the kinematic rupture process of Iquique earthquake process and its biggest aftershock are studied, employing a combination of high-rate GPS and strong motion data. The static displacements are inverted to characterize the slip. This static solution is used as apriori information ii for a sequence of kinematic inversions in the frequency domain. The results show that both events have a bimodal slip distribution, segmented along dip. The lateral extension of the mainshock is centered on a forearc offshore basin associated with a gravity anomaly, and may be controlled by tectonic structures of the upper crust. The aftershock is located in an area with strong Coulomb Stress Changes induced by the mainshock, suggesting that it was triggered by the mainshock. Thanks to the combination of geodetic and seismological data, this thesis provides a detailed vision of the processes involved during the Iquique earthquake and the previous decades. The research prospects raised by this work are numerous, particularly on the possibilities of refined observations of the phenomena associated with the seismic cycle and the preparation of large subduction earthquakes.