Slow Unlocking Processes Preceding the 2015 Mw 8.4 Illapel, Chile, Earthquake

On 16 September 2015, the Mw 8.4 Illapel earthquake occurred in central Chile with no intense foreshock sequences documented in the regional earthquake catalog. Here we employ the matched-filter technique based on an enhanced template data set of previously catalogued events. We perform a continuous search over an similar to 4-year period before the Illapel mainshock to recover the uncatalogued small events and repeating earthquakes. Repeating earthquakes are found both to the north and south of the mainshock rupture zone. To the south of the rupture zone, the seismicity and repeater-inferred aseismic slip progressively accelerate around the Illapel epicenter starting from similar to 140days before the mainshock. This may indicate an unlocking process involving the interplay of seismic and aseismic slip. The acceleration culminates in a M 5.3 event of low-angle thrust mechanism, which occurred similar to 36days before the Mw 8.4 mainshock. It is then followed by a relative quiescence in seismicity until the mainshock occurred. This quiescence might correspond to an intermediate period of stable slip before rupture initiation. In addition, to the north of the mainshock rupture area, the last aseismic-slip episode occurs within similar to 175-95days before the mainshock and accumulates the largest amount of slip in the observation period. The simultaneous occurrence of aseismic-slip transients over a large area is consistent with large-scale slow unlocking processes preceding the Illapel mainshock. Plain Language Summary The universality of precursory foreshock accelerations before large earthquakes is still controversial. We employ the template matching method to detect the small earthquakes and aseismic-slip transients before the 2015 Mw 8.4 Illapel earthquake. Our results show the large-scale aseismic-slip transients surrounding the impending Illapel rupture zone, suggesting a slow unlocking process. Our results have important implications on monitoring and time-dependent hazard assessment.