Rupture history of the 2016 Mw 7.0 Kumamoto earthquake constrained by the local strong motion, teleseismic body and surface waves

The 2016 Kumamoto earthquake sequence occurred near the south end of Median Tectonic line (MTL), where the MTL bifurcates into Hinagu Fault (orienting N205oE) and Futagawa Fault (orienting N235oE). According to JMA, this sequence started as Mw 6.2 foreshock on April 14. The Mw 7.0 mainshock occurred a day later. We have selected 14 3-component strong motion observations, accompanying with waveforms of teleseismic broadband body waves and long period surface waves of the 2016 Kumamoto mainshock to constrain its temporal and spatial distribution of slip. Our result reveals that the Komamoto mainshock had a complicated rupture scenario. Our preferred model is composed of Hinagu (dipping 73o northwest) and Futagawa (dipping 60o) fault segments. To test how rupture across fault interaction, we let Futagawa segment overlap with Hinaga segment and initiate nearly simultaneously. However, the inverted model has negligible fault slip on the overlap portion, illustrating the data resolution. Rupture initiated at JMA hypocenter on the Hinagu segment in dominant right-lateral strike-slip motion. The significant rupture on the Futagawa segment occurred 4-5 s later at a depth about 10 km. The rupture on Futagawa fault segment initiates as pure strike-slip motion but normal fault component increases as the rupture propagates close to the Aso Volcano. The total rupture duration is 15 s. The cumulative seismic moment on the Hinagu segment is 1.40×1019 Nm (Mw 6.7) while that of Futagawa segment is 2.94×1019 Nm (Mw 6.9). Their summation in tensor field yields a total seismic moment of 3.9×1019 Nm (Mw 7.0). The best double couple solution of this cumulative moment tensor has strike, dip, rake angles of 224o, 64o and -152o, respectively, agreeing remarkably with the long period best double couple solutions of USGS W-Phase solution (224o, 66o, -152o) and the GCMT project (222o, 77o, -163o). However, the CLVD component of our solution is negligible, consistent with USGS solution. It is noteworthy that unlike Hinagu segment, the focal mechanisms of Mw>5 foreshocks all have sub-vertical nodal planes and slightly dipping to the southeast, suggesting the Hinagu surface trace associate with two fault planes in depth. This scenario is closely analogous with the relation of 1989 Loma Prieta, earthquake and San Andreas fault.