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Focal Mechanism Inversion on the Aftershock Sequence of the October 25th, 2010 Mw 7.8 Mentawai Earthquake: A Preliminary Result a) Master Program of Earth Sciences, Faculty of Earth Sciences and Technology, Bandung Institute of Technology, Bandung, Indonesia Abstract A Mw 7.8 tsunami earthquake ruptured the shallow part of the Mentawai megathrust on 25 October 2010 and generated large tsunami that hit the Mentawai Islands. The earthquake continued a partial rupture sequence of the 2007 Mw 8.5 and 7.9 Bengkulu earthquake and the 2008 Mw 7.2 North Pagai earthquake. The aftershock activity gives the opportunity to investigate the geometry of the subduction interface which is critical to understand the propagation and termination of a large earthquake. However, it is necessary to have a precise location and mechanism of the aftershock distribution to clearly image the fault geometry. In this preliminary study, we have applied the generalized Cut-and-Paste (gCAP) focal mechanism inversion method to 20 aftershocks (M > 4.9) of the 2010 Mentawai earthquake. The seismic waveforms recorded by the BMKG broadband seismic network on the regional distance were used in the inversion and successfully obtained 10 focal mechanisms more than that provided in the GCMT catalog. The gCAP inversion by using teleseismic stations was then performed to validate the focal mechanism solution and its centroid depth. The result exhibits a distinctly different thrust and normal faulting aftershocks activity. The thrust faulting aftershocks are mainly distributed in the downdip of the Mentawai 2010 mainshock rupture, indicating the stresses that have not been released during the mainshock. In the shallow region near the trench, the aftershocks are dominated by a normal faulting mechanism which shows the tensional stress caused by the bending of the subducted plate. Keywords: 2010 Mentawai earthquake, aftershock, focal mechanism, fault geometry Topic: Earth and Planetary Sciences |
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