Estimation of the detailed structure of tsunami source based on survey results of submarine active aults

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Submarine cored sediments from the Asamigawa Fault in the Oita port, Beppu Bay, and the seismic profile records show that the fault is one of the causative faults of the 1596 Keicho-Bungo earthquake. The single-channel high-resolution records illustrate not only seismic offsets of sediment layers, but also submarine avalanche caused by the 1596 event. The mass of avalanche-derived sediment is 2-3m high and roughly 100m wide. The observation appears to be consistent with the legend of sunken Uryujima island and historically documented landslides of Takasakiyama mountain.The seismic profiting records in the entire area of Beppu Bay show two apparent reflecting layers in most parts, namely the Akahoya ash layer (about 6700 yrs B.P.) and Yufu ash layer (about 2500 yrs B.P). The observed cumulative vertical offsets shown by the Akahoya ash are found to be proportional to that of the Yufu ash, indicating the similarity of fault offset distribution for each seismic event. On the basis of the … More similarity, fault offsets at the time of the 1596 Keicho-Bungo earthquake is estimated and tsunami source model is constructed, which shows very complicated pattern, contrasting to much simpler conventional source models used for tsunami hazard estimate. For a comparison, a simple tsunami source model is also constructed by assuming the central Beppu Bay fault is the sole causative fault of the 1596 earthquake. Numerical simulation shows that the southern coast suffers the highest tsunami for the both source model. However, radiation in the other directions is very different between the two models. It suggests that a simple conventional source model may predict some major feature of tsunami, but not in details. This indicates a limit of tsunami forecast ; tsunami measure dependent on fine feature of simulated tsunami may be fatal. Furthermore, the result of this study suggests some disagreement in estimated fault heterogeneities between seismic and tsunami observations may be due to a complicated feature of actual submarine faults. Less