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The aseismic slip acceleration before the 2015 Illapel Mw 8.4 event from repeating earthquake observations

Hui Huang, & Lingsen Meng

Published August 24, 2016, SCEC Contribution #6802, 2016 SCEC Annual Meeting Poster #160

Recent studies provide evidence of accelerating short-term or long-term seismicity possibly accompanied by aseismic slip preceding large interplate earthquakes, such as the 2011 Tohoku-Oki Mw 9.0 earthquake and the 2014 Iquique Mw 8.2 earthquake. However, the generality of such precursory patterns is still under heavy debate. Two underlying mechanisms are proposed to explain the foreshock sequences: cascade triggering of a mainshock-aftershock sequence following Omori’s Law and the large-scale aseismic slip or other transient processes that load the foreshocks. Although acceleration of seismicity is observed before some large earthquakes, the evidence of slow slip is often unclear due to the weak signals or low resolution of geodetic observations. Repeating earthquake analysis is an alternative way to indirectly measure aseismic slip on the plate interface. The 2015 Illapel Mw 8.4 event ruptured at the central part of the Chilean subduction zone. A recent study shows that the seismicity rate within 50 km from the Illapel mainshock elevated ~60 days before the mainshock. However, it is not clear whether slow slip developed on the plate interface prior to its occurrence. In this study, we investigate the repeating earthquakes from 2011 to 2016 along the Chilean margin (18-38°S) to infer the aseismic slip evolution along the different segments: the 2014 Iquique rupture zone in northern Chile, the 2015 Illapel rupture zone in central Chile and the 2010 Maule rupture zone in southern Chile. We find that the aseismic slip around the Illapel rupture zone starts to accelerate ~150 days before the mainshock. In contrast, around the Maule and Iquique rupture zones, the aseismic slip rate tends to be steadily decreasing in the years before the Illapel event. This suggests that the area around the Illapel rupture zone experiences accelerating slow slip during its late interseismic period. In contrast, the decaying slow slip rate around the Maule and Iquique rupture zones is consistent with the afterslip model. The deformation transient around the Illapel rupture zone may promote the unstable slip during the nucleation process of the Illapel mainshock. The unique aseismic slip acceleration found around the Illapel rupture zone among the whole Chilean margin highlights the importance of monitoring slow slip for possible hazard assessment in the locked portion of the megathrust.

Huang, H., & Meng, L. (2016, 08). The aseismic slip acceleration before the 2015 Illapel Mw 8.4 event from repeating earthquake observations. Poster Presentation at 2016 SCEC Annual Meeting.

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Tectonic Geodesy