Gradual unlocking of a plate boundary controlled the April 2014 M8.1 Iquique, Northern Chile megathrust earthquake

Abstract:

On April 1^st, 2014, Northern Chile, was struck by a magnitude 8.1 earthquake near the city of Iquique following a protracted series of foreshocks. The earthquake occurred within a seismic gap left behind by two great earthquakes devastating the northern Chilean and southern Peruvian coast about 140 years ago in 1868 and 1877. This segment, about 500 km long, was the only one along the Chilean subduction zone that has not ruptured within the last century. The Integrated Plate boundary Observatory Chile (IPOC) monitored the entire sequence of events, providing unprecedented resolution of the build-up to the main event and its rupture evolution.

We analyzed the entire seismicity in this section of the subduction zone since 2007. The offshore rupture region of the Iquique event has been more or less continuously seismically active within our observation period. This is in contrast to the segments to the north and south, which are still unruptured and seismically quiet. Starting in July 2013, three foreshock clusters with increasingly larger magnitudes occurred within the future rupture area. The largest Mw 6.7 foreshock, two weeks before the mainshock, had a source mechanism distinctively different from the mainshock and with a centroid depth of only 9 km probably occurred in the upper plate. The Iquique mainshock initiated then at the northern end of the foreshock zone, inside a region of intermediate interseismic locking. Comparing the foreshock distribution to the long term deformation history of the margin, we find that the area exhibits a high gradient of locking from weakly locked updip to fully locked downdip.

Mapping the b-value of the foreshocks indicates significantly lower b-values in the source area compared to all other regions where the b-value can be resolved. Importantly, a gradual drop of the b-value from about 0.75 to below 0.6 is observed in the source region within the three years before the Iquique earthquake. This has only been reversed within the last days of the foreshock sequence. We conclude that gradual weakening of the central part of the seismic gap accentuated by the foreshock activity in a zone of intermediate seismic coupling was instrumental in causing final failure.