Long-Period Ground Motion: 3-D Finite Difference Simulation of the 2003 Tokachi-oki, Japan, Earthquake (Mw 8.0)

During the 2003 Tokachi-oki earthquake (2003/09/26,04:50, 41.7797N, 144.0785E, 42km; JMA, Mw 8.0), which is an interplate earthquake of Kuril Trench, the long-period ground motion with long duration was observed. At Tomakomai located more than 200 km from epicenter, large oil tanks were damaged by sloshing and heavy fires broke out. It was pointed out that the cause of these damages was the long-period ground motion due to the combination of the large earthquake and the deep sedimentary plain. We performed a simulation of the wave propagation of this earthquake with 3D finite-difference method (FDM) to examine this phenomenon. We constructed a crustal model for FD simulation based on seismic velocity profiles by travel time analysis (Iwasaki et al., 1991). On the top of the model, taking into consideration the result of the refraction surveys, reflection surveys, downhole measurements and the geological information, we assumed sedimentary layer model consisting of five layers. There are several plains that have deep sediments such as Yuhutsu, Tokachi, Ishikari and Konsen plains. The Yufutsu plain where Tomakomai is located has especially thick sediment whose maximum thickness is near 10 km. We used a rupture process model of this earthquake by Honda et al. (2003) for FD simulation. It was estimated by the multi-time window linear waveform inversion analysis. Our 3D FD simulation (0.04-0.3 Hz) successfully reproduced observed waveforms, spectra, PGV's, and duration-time of later phases in a wide area of Hokkaido. This implies the validity of our source model and underground structures. The incident waves are amplified by the soft sediment and successfully reproduced the long-duration waves in the Yufutsu plain. Waves are trapped in the soft sediment and continue for several hundred seconds by propagating back and forth in the plain. The mechanism of wave trapping is complicated. Velocity gradient of the sediment as well as the basin topography affect both amplitude and duration of the ground motions.