Three‐dimensional finite difference simulation of long‐period ground motions for the 2003 Tokachi‐oki, Japan, earthquake

To explore the generation and propagation mechanisms of long‐period ground motions, we used a 3‐D finite difference method to simulate wave propagation during the 2003 M w 8.3 Tokachi‐oki, Japan, earthquake. A large‐scale computation with an inverted slip model and a geological‐ and geophysical‐based velocity structure model successfully reproduced the overall characteristics of substantial observed velocity seismograms for a period range of 3.3 to 25 s. Significant long‐period ground motions of extended duration were observed in the Yufutsu basin, where oil tanks were damaged at Tomakomai because of sloshing associated with the ground motions. Our simulation revealed that the combination of the large and shallow earthquake and the deep extent of the sedimentary basin generated long‐period ground motions with a duration of several hundred seconds. The Yufutsu basin contains an internal subbasin that deepens from east to west, in contrast with the geometry of the overall basin that deepens from west to east. The long‐period ground motions were amplified in both the eastern and western parts of the basin, with the durations being greatly extended in the western part. A series of computations with and without each of the velocity layers within the basin demonstrated that the basin response amplifies long‐period ground motions as expected and that the subbasin response is effective in amplifying and extending long‐period ground motions. Our quantitative 3‐D finite difference simulations clarify the generation of disastrous long‐period ground motions that occurred in the Yufutsu basin during the 2003 Tokachi‐oki earthquake.