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An MCMC multiple point sources inversion scheme and its application to the 2016 Kumamoto MW 6.2 earthquake

Event Type: 
Event Date: 
08 Sep 2017
EOS Seminar Room - N2-01B-28
Shi Qibin
About the speaker: 

SHI Qibin is the graduate student at EOS and ASE; he obtained his bachelor degree in Geophysics in 2016 from Nanjing University. His research focuses on waveform seismology, kinematic and dynamic modeling of earthquake and faulting behavior, and better understanding of tectonics using geophysical, geodetic and geological observations and methods.

About the event: 

While single point source is an oversimplified representation of medium to large earthquakes, finite fault models in many cases over parameterize the inversion due to the lack of sufficient near field data. Multiple point source solutions can fill in the gap in-between these two representations. Here we propose a Markov-Chain-Monte-Carlo (MCMC) multiple point source inversion scheme, in combination with the advantage of the Cut-And-Paste technique. We apply the approach to the MW 6.2 foreshock in the 2016 Kumamoto earthquake sequence by using the strong-motion observations within 100km. With the path calibration of the Mw5.4 aftershock, we are able to perform the inversion at relatively high- frequency ranges (0.02-0.3 Hz for Pnl and 0.02-0.15 Hz for surface waves) with confidence on the velocity structure. Our results show that the earthquake is mainly composed of three sub-events that have ruptured on an anti-dipping fault system around the intersection of the Futagawa and Hinagu faults. 2.0 s and 5.5 s after the first sub-event, the second and the third sub-event took place to the north and southwest of the first event, respectively. The rupture lasted for about 12s with a total moment magnitude of Mw6.2. The fault geometry shows remarkable consistency with the relocated the aftershocks, which indicates a SE-dipping fault around the first sub-event and two NW-dipping faults to the north and south, respectively, corresponding to the second and the third sub-event. The combination of sub-events that have different orientations and rake angles leads to strong (~60%) CLVD component. With a local 1D crustal model, a full-moment-tensor inversion using the long-period broadband data also detects strong CLVD component of this earthquake. In contrast, using the PREM model results in an almost pure double-couple solution. In short, we precisely resolve the rupture process, the intricate fault geometry, and the strong CLVD component with seismological data. This highlights the importance of extracting the relatively high-frequency information from the observations and using accurate velocity model in seismic source analyses of large earthquakes.  The successful application of the new inversion scheme indicates that we can generalize the study to the global earthquakes, including the Mw9.2 Sumatran earthquake.