5:15 PM - 7:15 PM
[HDS10-P06] Investigating characteristics of the source model for the 12th century southwest off-Hokkaido earthquake using tsunami simulations
Keywords:earthquake, tsunami, Hokkaido Nansei-oki, 12th century, Green's function
An earthquake source model with a moment magnitude of 7.9 was proposed by a previous study for an earthquake that occurred along the eastern margin of the Sea of Japan in the 12th century (referred to as the 12th century earthquake). The average slip value for one of the source segments was estimated to be 18 m or larger to produce a tsunami capable of inundating areas where tsunami deposits from this event were identified. In contrast, the 18 m slip was much larger than that of other historical earthquakes in the Sea of Japan and the expected value based on scaling laws for the same source area. Furthermore, it exceeded the average slip value assumed by the Japanese Government for the future worst-case scenario in the Sea of Japan. The ultimate objective of this study was to investigate the source model characteristics of the 12th-century earthquake to determine whether the large slip of 18 m was necessary for the source model.
We first simulated tsunami propagation and inundation using the proposed source model to roughly grasp the tsunami characteristics. The tsunami deposits were identified in several coastal areas, among which those on Okushiri Island and in Hiyama, Hokkaido, were distributed relatively inland from the shore. Thus, the inundation characteristics in these two areas were likely key to understanding the event. The simulation results showed that the tsunami peaked during the subsequent waves at a nearshore site of Okushiri Island and during the primary wave in Hiyama. These results suggest that considering both the primary wave and subsequent waves is essential for constructing a source model of the 12th-century earthquake.
We assigned unit sources of sea-surface displacement to an area covering the tsunami source area and simulated their propagation based on the linear longwave theory to obtain Green’s functions at locations near the area where the tsunami deposit were found. Each unit source had a two-dimensional Gaussian shape and its height was determined at +1 m at the central location. We plotted the value of each waveform at the time of interest onto the central location of the unit source to investigate the effects of source locations. The peak times of the subsequent waves for Okushiri Island and the primary wave at Hiyama were determined as the times of interest.Extracting the waveform values from each unit wave source at the timing when the maximum water surface level in the previous study appeared, we could evaluate the contribution of each subregion to the maximum water surface level at the observation point. The results show that the tsunami inundation on Okushiri Island can be significantly influenced by a local source in the tsunami source area, while it moderately impact on Hiyama. The local source area was located in the northern part of the source area assumed in the previous study. Based on these results, for the tsunami propagation and inundation simulation, we tested several slip distributions within the assumed source area, with a large slip assigned to the local area. However, average slip values of our tested models were all smaller than 18 m. We found that the tsunamis simulated using the source models could inundate the areas where the tsunami deposits were distributed. These results suggest that the average slip value of 18 m may not necessarily be needed for the source model of the 12th-century earthquake. However, the average slips in our tested scenarios were still greater than those for future worst-case earthquakes in the Sea of Japan. We should further investigate the characteristics of the 12th-century earthquake in detail, focusing on earthquake physics and tsunami dynamics, to better understand the event and estimate the future disaster potential in the Sea of Japan.
We first simulated tsunami propagation and inundation using the proposed source model to roughly grasp the tsunami characteristics. The tsunami deposits were identified in several coastal areas, among which those on Okushiri Island and in Hiyama, Hokkaido, were distributed relatively inland from the shore. Thus, the inundation characteristics in these two areas were likely key to understanding the event. The simulation results showed that the tsunami peaked during the subsequent waves at a nearshore site of Okushiri Island and during the primary wave in Hiyama. These results suggest that considering both the primary wave and subsequent waves is essential for constructing a source model of the 12th-century earthquake.
We assigned unit sources of sea-surface displacement to an area covering the tsunami source area and simulated their propagation based on the linear longwave theory to obtain Green’s functions at locations near the area where the tsunami deposit were found. Each unit source had a two-dimensional Gaussian shape and its height was determined at +1 m at the central location. We plotted the value of each waveform at the time of interest onto the central location of the unit source to investigate the effects of source locations. The peak times of the subsequent waves for Okushiri Island and the primary wave at Hiyama were determined as the times of interest.Extracting the waveform values from each unit wave source at the timing when the maximum water surface level in the previous study appeared, we could evaluate the contribution of each subregion to the maximum water surface level at the observation point. The results show that the tsunami inundation on Okushiri Island can be significantly influenced by a local source in the tsunami source area, while it moderately impact on Hiyama. The local source area was located in the northern part of the source area assumed in the previous study. Based on these results, for the tsunami propagation and inundation simulation, we tested several slip distributions within the assumed source area, with a large slip assigned to the local area. However, average slip values of our tested models were all smaller than 18 m. We found that the tsunamis simulated using the source models could inundate the areas where the tsunami deposits were distributed. These results suggest that the average slip value of 18 m may not necessarily be needed for the source model of the 12th-century earthquake. However, the average slips in our tested scenarios were still greater than those for future worst-case earthquakes in the Sea of Japan. We should further investigate the characteristics of the 12th-century earthquake in detail, focusing on earthquake physics and tsunami dynamics, to better understand the event and estimate the future disaster potential in the Sea of Japan.