Reconstruction of Tsunami Occurrence on Okushiri Island, Southwestern Hokkaido, Japan

The eastern margin of the Japan Sea is located along an active convergent boundary between the North American and Eurasian tectonic plates. Okushiri Island, which is situated off the southwest coast of Hokkaido, is located in an active tectonic zone where many active submarine faults are distributed. Studying the records of past tsunamis on Okushiri Island is important for reconstructing the history and frequency of fault activity in this region, as well as the history of tsunamis in the northern part of the eastern margin of the Japan Sea. Five tsunami deposit horizons have been identified previously on Okushiri Island, including that of the 1741 tsunami, which are interbedded in the coastal lowlands and Holocene terraces. However, these known tsunami deposits date back only ~3,000 years. A much longer record of tsunami occurrence is required to consider the frequency of submarine fault activity. In this study, we cored from 7 to 25 m depth in the Wasabiyachi lowland on the southern part of Okushiri Island, where previous studies have confirmed the presence of multiple tsunami deposits on peat layer surfaces. The results indicate that the Wasabiyachi lowland comprises an area that was obstructed by coastal barriers between the lowland and the coast at ~8.5 ka and consists of muddy sediment and peat layers formed in lagoons and floodplains, respectively. In addition, event deposits and 15 tsunami horizons were observed among the turbidites and peat layers, dating back as far as 3,000 years. Combined with previous findings, Okushiri Island has sustained 20 tsunami events between ~7.5 ka and the present. These findings are critical for investigating the activities of submarine faults off the southwestern coast of Hokkaido, as well as for determining tsunami risks along the coast of the Japan Sea between North Tohoku and Hokkaido.

Possible Mechanism for the Tsunami-Related Fires That Occurred at Aonae Harbor on Okushiri Island in the 1993 Hokkaido Nansei-Oki Earthquake

In this paper, we investigate the mysterious tsunami fires that occurred at Aonae Harbor on Okushiri Island during the 1993 Hokkaido Nansei-Oki earthquake. Specifically, five fishing boats moored separately from each other in the harbor suddenly caught fire and burned nearly simultaneously with the arrival of the first tsunami wave. However, the ignition mechanism of those fires has, until now, remained largely unknown. At the time the earthquake occurred, an NHK (Japan Broadcasting Corporation, Tokyo, Japan) crew that was on the island to report on its scenic natural attractions just happened to capture video footage of those tsunami-related fires. Using that NHK video footage in combination with eyewitness accounts, this study investigates the spatio-temporal process leading to those tsunami-related fires. For example, one witness said, "There was whitish bubbling in the offshore area and I saw five burning fishing boats moored on the seawall being blown about by the strong winds. The burning boats were swept ashore with the tsunami and ignited the gasoline of a car that was rolling in the waves. The fire eventually spread to the center of the Aonae District." The NHK video footage confirmed flames arising from the five fishing boats almost simultaneously and the shimmering white color of the tsunami waters striking the seawall, which were consistent with the eyewitness testimony. Based on these spatio-temporal data, we propose the following hypothetical model for the origin of tsunami fires. Combustible methane gas released from the seabed by the earthquake rose toward the surface, where it became diffused into the seawater and took the form of whitish bubbles. The tsunami strike on the Aonae Harbor seawall resulted in the generation of large electrical potential differences within the seawater mist, which quickly developed sufficient electrical energy to ignite the methane electrostatically. The burning methane bubbles accumulated on the boat decks, which then burned violently.

The role of diffraction effects in extreme run-up inundation at Okushiri Island due to 1993 tsunami

The tsunami generated on 12 July 1993 by the Hokkaido–Nansei–Oki earthquake (Mw = 7.8) brought about a maximum wave run-up of 31.7 m, the highest recorded in Japan during the 20th century, near the Monai Valley on the west coast of Okushiri Island (Hokkaido Tsunami Survey Group, 1993). To reproduce the extreme run-up height, the three-dimensional non-hydrostatic model (Flow Science, 2012), referred to here as the NH-model, has been locally applied with open boundary conditions supplied in an offline manner by the three-dimensional hydrostatic model (Ribeiro et al., 2011), referred to here as the H-model. The area of the H-model is sufficiently large to cover the entire fault region with one-way nested multiple domains. For the initial water deformation, Okada's fault model (1985) using the sub-fault parameters is applied. Three NH-model experiments have been performed, namely without islands, with one island and with two islands. The experiments with one island and with two islands give rise to values close to the observation with maximum run-up heights of about 32.3 and 30.8 m, respectively, while the experiment without islands gives rise to about 25.2 m. The diffraction of the tsunami wave primarily by Muen Island, located in the south, and the southward topographic guiding of the tsunami run-up at the coast are, as in the laboratory simulation (Yoneyama et al., 2002), found to result in the extreme run-up height near Monai Valley. The presence of Hira Island enhances the diffraction of tsunami waves but its contribution to the extreme run-up height is marginal.

The role of diffraction effects in extreme runup inundation at Okushiri Island due to 1993 tsunami

The tsunami generated on 12 July 1993 by Hokkaido-Nansei-Oki earthquake (Mw = 7.8) has brought about the maximum wave run-up of 31.7 m, the highest record in Japan of 20th century, near the Monai Valley on the west coast of the Okushiri island (Hokkaido Tsunami Survey Group, 1993). To reproduce the extreme run-up height the three-dimensional non-hydrostatic model (Flow Science, 2012) denoted by NH-model has been locally applied with open boundary conditions supplied in an offline manner by the three-dimensional hydrostatic model (Ribeiro et al., 2011) denoted by H-model which is sufficiently large to cover the entire fault region with one-way nested multiple domains. For the initial water deformation Okada's fault model (1985) using the 3 sub-fault parameters is applied. Three non-hydrostatic model experiments have been performed, namely experiment without island, with one island and with two islands. The experiments with one island and with two islands give rise to values close to the observation with maximum run-up heights of about 32.3 and 30.8 m, respectively, while the experiment without islands gives rise to about 25.2 m. The diffraction of tsunami wave primarily by Muen Island located at the South and the southward topographic guiding of tsunami run-up at the coast are as in the laboratory simulation (Yoneyama et al., 2002) found to result in the extreme run-up height near the Monai Valley. The presence of Hira Island enhances the diffraction of tsunami waves but its contribution to the extreme run-up height is marginal.

DAMAGE AND RECONSTRUCTION OF LIFELINES IN PHANG NGA PROVINCE, THAILAND AFTER THE 2004 INDIAN OCEAN EARTHQUAKE AND TSUNAMI

The Indian Ocean earthquake and tsunami occurred in December 2004 caused destructive damage to Phang Nga Province, Thailand. We carried out two times of interview surveys about 1 and 3 years after the event to administrative bodies and lifeline companies for getting the information on lifeline damage, restoration and reconstruction situation, and summarized the basic concept of reconstruction plan of tsunami suffered towns considering lifeline restoration. On the other hand, as for the comparison of reconstruction problems, the lifelines recovery is reviewed at Aonae district in Okushiri Island after the 1993 Hokkaido-Nansei-oki earthquake. As the result, the difference of the process of reconstruction of town and lifelines has been revealed and the importance of preparing of the reconstruction plan before the event under the consideration of a long-term city planning is pointed out.