The 2014 Iquique Chile earthquake: Preparatory breaking processes of a locked asperity and natural constraints for fluid migration along the plate interface

Abstract:

Variations in pore fluid pressure are of paramount importance in controlling fault stability. Recent studies have focused on investigating the underlying causes of pore fluid pressure, but natural constraints for lateral fluid migration have not yet been described. Here, we investigate the evolution of the plate interface kinematics and fluid pressure migration leading up to the Mw 8.1 Iquique earthquake. We present the deformation evolution of the last decade from a combination of ~40 continuous and ~70 survey-type GPS. The slip distribution of Iquique event covered areas that were previously intermediately to highly locked and enclosed by creeping interface zones. The seismicity recorded in the years before the mainshock tended to encircle the central locked patch (~40 km length asperity), which itself exhibited a low amount of seismicity. This earthquake was preceded by a series of foreshocks and transient deformation that was initiated by an Mw=6.7 event and which lasted for 16 days. The observed transient GPS signals can be explained by deformation due to foreshocks, indicating that slow slip was not a dominant factor in the stress build up that led to the mainshock. Foreshocks initiated at the edge of the asperity and migrated along the plate interface towards the mainshock hypocentre. The spatial and temporal proximity to the mainshock, as well as the migration pattern of events suggest that the foreshocks and mainshock were mechanically coupled. We suggest that the Mw 6.7 (that likely occurred on a splay fault) is responsible for a transient pore fluid pressure increase in the plate interface region, hence diminishing the effective normal stress on the interface. This segment of the plate interface, presumably mechanically weakened by near-lithostatic pore fluid pressures, was subsequently subjected to a propagation of foreshocks. We propose that the 16-days seismicity swarm could represent the lateral propagation of a fluid front through the place interface, possibly as a porosity-wave mechanism. This fluid migration could enhance the stress concentration around the nearest intact large asperity, which finally broke and concentrated the highest slip during the mainshock. The implications of our study are far-reaching, establishing natural constraints for fluid migration prior to large earthquakes.