Regional Probabilistic Tsunami Hazard Assessment for The Sea of Japan

Abstract:

The Sea of Japan is known to host large tsunamis generated by earthquakes (M>7) on several active fault systems: the 1993 South-west Hokkaido (M 7.8), 1983 Japan Sea (M 7.7), 1964 Niigata (M 7.5), 1940 Shakotan-oki (M7.5), and 1833 Shonai-oki (M 7.5). The aforementioned tsunamis were responsible for severe destructions and many casualties in the region. As part of disaster countermeasures, we conducted a probabilistic tsunami hazard assessment (PTHA) at 154 municipalities along the west coast of Japan. The result is expected to provide a general overview on the characteristic of tsunami hazard around the study area.

Here we analyzed the regional tsunami hazard associated with 60 active faults beneath the eastern margin of the Sea of Japan, which were identified based on seismic reflection surveys (MLIT, 2014). In our PTHA study, we set a minimum magnitude of Mw 6.5 at all faults, while a maximum magnitude at each fault varies from Mw 6.8 to 7.9 according to the estimated fault sizes. We then discretized the faults into a 10 km × 10 km subfault size to facilitate the slip heterogeneity and Green’s function summations. To account for the epistemic uncertainty, we generated a large number of stochastic slip realizations using a Monte Carlo approach at each fault, and simulated the corresponding tsunamis. In addition to the observed seismicity used to estimate the earthquake recurrence rate, we also used measured tsunami run-up heights of historical events in the region to define the aleatory uncertainty primarily attributed to the modeling capability and limitation.

Our probabilistic analysis indicates that the tsunami hazard generally increases from southwest to northeast, which is consistent with the number and type of the identified fault systems, i.e. many reverse faults exist in northeastern parts whereas strike-slip faults dominate the southwest part. Using nearshore tsunami heights at the 50 m isobath and an amplification factor by the Green’s law, the highest maximum coastal tsunamis at a single site in the region are expected to be approximately 3.7, 7.7, and 11.5 m for the return periods of 100-, 400-, and 1000-year, respectively. Furthermore, the aggregation of hazard suggests that tsunamis in the northeast are predominated by local sources, while the southwest parts are likely affected by several regional sources.