Control Factors of Spatial Variation of Long-Period Strong Ground Motions in the Yufutsu Sedimentary Basin, Hokkaido, during the Mw 8.0 2003 Tokachi-oki, Japan, Earthquake

We successfully reproduced important features of long-period (7 to 8 sec), large-amplitude ground motions observed in the Yufutsu basin, Hokkaido, during the M w 8.0 2003 Tokachi-oki, Japan, earthquake through numerical simulation of wave propagation. Recordings from the dense nationwide strong ground motion networks in Japan demonstrated significant spatial variation of long-period strong ground motions within the Yufutsu basin and revealed that the long-period shaking in the Tomakomai western port was the strongest of all the sites within the basin. In interpreting or predicting long-period strong ground motions in sedimentary basins, the importance of deep basin structures, such as the depth of bedrock with an S -wave velocity over 3 km/sec has been emphasized. However, the characteristics of the underground structure model of the Yufutsu basin that explain the observations indicate that thickness of near-surface (