Multiple point source inversion with regional broadband and local waveform data -a case study of the 2016 Mw7. 1 Kumamoto, Japan earthquake

Moderate-size (M6 - 7) earthquakes possess great threats to human society, as they occur much more frequent than M8 + earthquakes and many take place inland. The rupture processes and fault geometry of this kind of earthquake are usually very complex. Precise determination of their kinematic rupture parameters is a fundamental task in seismology, which is critical to understanding the earthquake physics as well as to better prepare for seismic hazard. As an earthquake source model with a precision between point source and finite fault model, Multiple Point Source (MPS) solution of large earthquakes provide reliable fault geometry and first order rupture evolution of earthquakes, which is extremely helpful for more detailed finite fault inversion and modeling. Here we generalize a Markov-Chain-Monte-Carlo (MCMC) based MPS inversion algorithm to the regional broadband waveform inversion and apply to the 2016 M(w)7. 1 Kumamoto mainshock. We first calibrate the path and select the frequency ranges for the strong motion (<100 km) and regional (<1000 km) broadband stations through the point source modeling of the M-W 5.4 and M-W 6.0 foreshocks, which results in up to 0.3 Hz for Pnl waves and 0.2 Hz for surface waves. The inversion results from regional, strong motion and high-rate GPS datasets show highly consistent solutions, validating the generalization of our MPS inversion method. Our preferred model show that the earthquake is composed of at least four sub-events. The first (M-W 6.7) sub-event is a high-dip-angle purely strike-slip event, taking place near the hypocenter at the depth of 12 km. The strike of the first sub-event is well consistent with the strike of the Hinagu fault. The second (M-W 6.7) and the third (M-W 6.7) sub-events all took place at the depth of 7 km, with strike more consistent with the Futagawa fault and with slight normal slip component. The last sub-event (M,6. 6) is the shallowest (1 2 km) and its focal mechanism is composed of almost equal percentage of strike-slip and normal-slip components. Our five point source inversion with the closest high-rate GPS station (0701) shows that the last stage of the rupture bifurcated onto a secondly fault, probably playing an important role in stopping the rupture.