Rupture process of the 2016 Kumamoto earthquake revealed by backprojection of local strong motion records.

Abstract:

The 2016 Kumamoto earthquake (M7.3) occurred on 16^th April, 2016 at depth 12.5 km and is an inland earthquake surrounded by an extensive strong-motion observation network, and plenty of seismic records were obtained. Understandings of the accurate earthquake rupture process leads to improvement of the fault model used in strong ground motion prediction, which realize strong motion prediction with high accuracy. In this study, the rupture process of the 2016 Kumamoto earthquake was investigated by the backprojection method using local strong ground motion records.In this study, in order to estimate the rupture process with high accuracy, we used empirical travel time data obtained by Matsubara et al. (2017) by hypocenter relocation. We used KiK-net (Aoi et al., 2004) underground strong-motion records within 120 km of the epicenter and paid attention not to contaminate the waveforms by refracted waves from the Moho boundary. We did not used the records obtained at the station where local site effects seem to be significant, so as not to deteriorate the rupture image on the fault plane obtained by backprojection analysis. To further mitigate the local site effects on resultant rupture image, we used only underground stations of KiK-net. Backprojection was performed using the velocity waveforms obtained by integrating the acceleration records of KiK-net. In this study, we also used only the records with high correlation among nearby station records so that we can get clear rupture image.

In backprojection analysis, although it is not necessary to specify the fault planes in advance, we constructed a fault model comprised of 5 fault planes based on Yoshida et al. (2016) and Himematsu and Furuya (2016). In this fault model, the Hinagu fault consists of one plane, and the Futagawa fault consists of four planes. We performed backprojection and gained a spatiotemporal distribution of seismic radiation intensity (Oshima et al., 2016) on the 5 fault planes.

As a result of the backprojection analysis, it was found that seismic waves with a period of 1 s or more may be emitted mainly from the area shallower than the hypocenter. It was also found that the rupture progressed along both the Hinagu fault and the Futagawa fault. We also gained the rupture image on the normal fault nearby the Aso volcano.