Study on prediction of long period ground motions on sedimentary basins in Southwest Japan for huge subduction-zone

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We studied long-period ground motions (>2seconds) in large sedimentary basins of South-Western Japan during huge subduction earthquakes. In the first year, we analyzed nation-wide strong motion network data to find sites that have long-period ground motion characteristics. In the South-Western Japan area, the Osaka and Oita basins could generate long-period ground motions. For the Osaka basin, there are plural underground velocity structure models for ground motion prediction. We validated the models by simulating ground motions for the 2004 Kii-Hanto-Toho-Oki event. Peak periods of observed ground motions can be explained by the simulations within 10%. For the Oita basin, there are few underground information and we conducted microtremor observations to estimate basin structures. We constructed underground velocity structure model for the Oita basin and confirmed the model by reproducing ground motions at Oita station for the 2000 Tottori earthquake.Based on those analyses, we conducted long-period ground motion prediction for next hypothetical Nankai earthquake and compared the long-period ground motion characteristics. We pointed out that large long-period ground motions could attack both basin area during the next Nankai earthquake, especially for the Oita basin for the first time. We also developed the effect of source rupture scenarios of the events. For the Oita basin, ground motion levels would be changed between the source model of the East hypocenter model and that of the West hypocenter model.