Partial breaking of the Northern Chile seismic gap by the 2014 Pisagua-Iquique earthquake as a consequence of seismic slip transients

Abstract:

The Northern portion of the Chilean margin is considered to be a mature seismic gap based on the magnitude and time of the last great earthquake (Mw~8.8 in 1877), and the assumed long-term slip deficit accumulation rate (67mm/yr). The central fraction of the gap was affected by the April 1st 2014 Pisagua-Iquique earthquake (Mw=8.1), which was preceded by a long-lasting series of foreshocks with increasing magnitudes. The seismic gap has been extensively monitored by the Integrated Plate Boundary Observatory Chile (IPOC) with various geophysical and geodetic techniques . The excellent temporal and spatial coverage of the IPOC GPS network along the entire gap enable us to analyze the kinematics of the plate interface leading up to the mainshock with unprecedented resolution.

We use Finite-Element Modelling (FEM) to investigate the subduction zone mechanisms that are responsible for the observed GPS deformation field during the interseismic, coseismic and early postseismic periods. Furthermore, we separate the relative contributions of aseismic and seismic plate interface slip to the short-term elastic deformation leading up to and following the mainshock. GPS time-series show a trenchward acceleration between the March 16th Mw 6.7 foreshock and the mainshock. The cumulative continuous-GPS transient signals can be explained by the elastic displacement of the foreshocks suggesting the dominance of seismic slip leading up to the mainshock.

Both the slip distribution of the Pisagua-Iquique event, and its largest aftershock, correlate well with areas that were previously highly locked, and both were enclosed by creeping interface zones. Our model suggests that the plate interface geometry varies the fault normal stress distribution, which influences stress concentration and release in the central part of the gap. The first three months of postseismic ground surface displacement is greatest along the rupture area and in the northern adjacent segment with cumulative displacements of about 10 cm diminishing to the south of the rupture. The early postseismic displacement only accounts for less than 20 % of the total coseismic displacement. Slip deficit calculation shows that the unbroken locked regions of the remaining seismic gap could release an earthquake with a magnitude of 8.5.