Abstract
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.
Highlights
The M7.8 Hokkaido Nansei-Oki earthquake that occurred on 12 July, 1993 at 22:17 local time, produced a devastating tsunami wave that quickly hit Okushiri Island [1,2,3,4,5,6].Approximately 4 min after the mainshock, a tsunami that was reported to be approximately 10 m high struck and swept away much of the urban area of the island’s Aonae Cape
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
When the first tsunami wave was reflected in Aonae Bay due to the lens effect between the Aonae Cape south shallows and the Okushiri Spur [3], people who had evacuated to the nearby hills of the Aonae District witnessed the five fishing boats moored at the harbor catching fire and burning
Summary
The M7.8 Hokkaido Nansei-Oki earthquake that occurred on 12 July, 1993 at 22:17 local time, produced a devastating tsunami wave that quickly hit Okushiri Island [1,2,3,4,5,6] (see Figure 1a–f).Approximately 4 min after the mainshock, a tsunami that was reported to be approximately 10 m high struck and swept away much of the urban area of the island’s Aonae Cape. When the first tsunami wave was reflected in Aonae Bay due to the lens effect between the Aonae Cape south shallows and the Okushiri Spur [3] (see Figure 1b), people who had evacuated to the nearby hills of the Aonae District witnessed the five fishing boats moored at the harbor catching fire and burning. Spur [3] (see Figure1b), people who had evacuated to the nearby hills of the Aonae District witnessed the five fishing boats moored at the harbor catching fire and burning simultaneously (see Figure 2a). The boat fires were spread to the shore by the gusting winds that accompanied the second tsunami wave, where they ignited propane gas released from winds that accompanied the second tsunami wave, where they ignited propane gas released from storage cylinders, gasoline from the cars, and kerosene from home-heating fuel tanks. A survey team team of the Japan Society of Civil Engineers (JSCE) investigated the damage caused by the of the Japan Society of Civil Engineers (JSCE) investigated the damage caused by the tsunami-related tsunami-related fires [7] but were unable to conclusively determine their origins
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