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[MIS13-01] Identifying wave source of the event layers using sediment transport modeling in the Hidaka coast, Hokkaido, Japan
Keywords:Sediment transport modeling, Hidaka, Hokkaido
Geological surveys along the Pacific coast of Hokkaido suggest the occurrence of >M8 earthquakes in the Kurile Trench (Nanayama et al., 2003; Sawai, 2020). In numerical simulations of tsunamis caused by these earthquakes, the parameters of source faults have been estimated using the tsunami deposit data along the coast of eastern Hokkaido, but it is necessary to use data from regions far from the trench to constrain the extent of the rupture zone in more detail. Nakanishi et al. (2022) investigated the Shizunai area in the central part of the Hidaka coast, 250 km away from the trench, and reported the seven candidates layers for tsunami deposits. However, because the distribution area is limited to the coastal area, it is difficult to determine whether they are tsunami or storm deposits based on sedimentological features only. Sediment transport modeling is expected to be a useful tool for identifying whether event layers are deposited by tsunamis or storms and their wave sources (Watanabe et al., 2018, 2021; Sugawara et al., 2019). However, there are challenges in applying sediment transport modeling to prehistoric events because of the uncertainties associated with paleogeography.
In this study, to estimate the wave sources of the event layers observed in the Shizunai area, realistic computational parameters were set based on detailed geological data, and sediment transport modelings for tsunami, storm surge and storm wave were performed using Delft-3D. The results of the sediment transport modelings show that the coastal evolution process, especially the shape of the beach ridge, has a significant impact on the formation of tsunami deposits by slowing down the return flow and obstructing the overflow. In Delft-3D, the sand source can be set with multiple grain sizes, which makes it possible to compare the observed sedimentary structures with the simulation results even when the layer thickness distribution alone is very similar. The sedimentary structures and the layer thickness distributions inferred from the sediment transport modelings for each source model were compared with the observed results, and the results were interpreted as tsunami deposits except for some event layers. Therefore, sediment transport modeling is an effective tool to distinguish between tsunami and storm deposits even short run-up events in the area far from the wave source. The sediment stransport modeling can estimate the origin of prehistoric event layers by constraining the numerical parameters by the detailed geological survey.
In this study, to estimate the wave sources of the event layers observed in the Shizunai area, realistic computational parameters were set based on detailed geological data, and sediment transport modelings for tsunami, storm surge and storm wave were performed using Delft-3D. The results of the sediment transport modelings show that the coastal evolution process, especially the shape of the beach ridge, has a significant impact on the formation of tsunami deposits by slowing down the return flow and obstructing the overflow. In Delft-3D, the sand source can be set with multiple grain sizes, which makes it possible to compare the observed sedimentary structures with the simulation results even when the layer thickness distribution alone is very similar. The sedimentary structures and the layer thickness distributions inferred from the sediment transport modelings for each source model were compared with the observed results, and the results were interpreted as tsunami deposits except for some event layers. Therefore, sediment transport modeling is an effective tool to distinguish between tsunami and storm deposits even short run-up events in the area far from the wave source. The sediment stransport modeling can estimate the origin of prehistoric event layers by constraining the numerical parameters by the detailed geological survey.