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Large-scale Acceleration of Slow Slip Before the 2015 Mw 8.4 Illapel, Chile Earthquake

Hui Huang, & Lingsen Meng

Published August 12, 2017, SCEC Contribution #7527, 2017 SCEC Annual Meeting Poster #064

Foreshocks and/or slow slip were observed to accelerate before some recent large earthquakes, e.g., the 2011 M 9.0 Tohoku-Oki and the 2014 M 8.2 Iquique earthquake. However, it is still controversial regarding the universality of precursory signals and their value in hazard assessment or mitigation. On 16 September 2015, the Mw 8.4 Illapel earthquake ruptured a section of the subduction thrust on the west coast of central Chile. Small earthquakes are important in resolving possible precursors but are often incomplete in routine catalogs. Here, we employ the matched filter technique to recover the undocumented small events in an ~4-years period before the Illapel mainshock. We augment the template dataset from Chilean Seismological Center (CSN) with previously found new repeating aftershocks in the study area. We detect a total of 17644 events in the 4-years period before the mainshock, ~6.3 times more than the CSN catalog. The magnitudes of detected events are determined according to different magnitude-amplitude relations estimated at different stations. Among the enhanced catalog, 183 repeating earthquakes are identified before the mainshock. Repeating earthquakes are located at the rim of highly locked zones, both to the north and south of the principal coseismic slip zone. The seismicity and repeater-inferred aseismic slip progressively accelerate in a small low-coupling area around the epicenter starting from ~140 days before the mainshock. The acceleration leads to a low-angle thrust M 5.3 event ~36 days before the mainshock, then followed by a relative quiescence in both seismicity and slow slip until the mainshock. This quiescence may correspond to a slow aseismic nucleation phase after the slow-slip transient ends. In addition, to the north of the mainshock rupture area, the last aseismic-slip episode occurs within ~175-95 days before the mainshock and accumulates the largest amount of slip in the observation period. The simultaneous occurrence of slow slip over a large area indicates a large-scale unloading process preceding the mainshock. In contrast, in a region ~70-150 km south of the mainshock, the aseismic-slip rate is relatively steady and mostly reflects the decelerating afterslip. Our results highlight the importance of continuously monitoring seismicity and repeating earthquakes at the transition from low to high coupling areas where large earthquake ruptures may initiate.

Huang, H., & Meng, L. (2017, 08). Large-scale Acceleration of Slow Slip Before the 2015 Mw 8.4 Illapel, Chile Earthquake. Poster Presentation at 2017 SCEC Annual Meeting.

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