Revisiting the 2015 Mw=8.3 Illapel Earthquake: Unveiling Complex Fault Slip Properties Using Bayesian Inversion.

SUMMARY The 2015 moment magnitude Mw = 8.3 Illapel earthquake is the largest mega-thrust earthquake that has been recorded along the Chilean subduction zone since the 2010 Mw = 8.8 Maule earthquake. Previous studies indicate a rupture propagation from the hypocentre to shallower parts of the fault, with a maximum slip varying from 10 to 16 m. The amount of shallow slip differs dramatically between rupture models with some results showing almost no slip at the trench and other models with significant slip at shallow depth. In this work, we revisit this event by combining a comprehensive data set including continuous and survey GNSS data corrected for post-seismic and aftershock signals, ascending and descending InSAR images of the Sentinel-1A satellite, tsunami data along with high-rate GPS, and doubly integrated strong-motion waveforms. We follow a Bayesian approach, in which the solution is an ensemble of models. The kinematic inversion is done using the cascading capability of the AlTar algorithm, allowing us to first get a static solution before integrating seismic data in a joint model. In addition, we explore a new approach to account for forward problem uncertainties using a second-order perturbation approach. Results show a rupture with two main slip patches, with significant slip at shallow depth. During the rupture propagation, we observe two regions that are encircled by the rupture, with no significant slip, westward of the hypocentre. These encircling effects have been previously suggested by back-projection results but have not been observed in finite-fault slip models. We propose that the encircled regions correspond to zones where the yield stress largely exceeds the initial stress or where fracture energy is too large to be ruptured during the Illapel earthquake. These asperities may potentially break in the future and probably already broke in the past.