史料にみる地震津波発光

抄録

One of the intriguing events accompanying large tsunamis, especially those that strike the Pacific coasts of Japan, is the luminous phenomenon; many historical Japanese documents have described how fire balls or pillars of fire seemed to come out from the sea when tsunami approached. Among 17 events of large tsunamis from 684 to 1946, where the surface wave magnitude was greater than 8 or the tsunami magnitude was greater than 3, nine tsunami events accompanied luminous phenomena. In spite the event's high probability, few explanations have been offered as to the source mechanism except luminescent planktonic organisms, which is hardly plausible because luminous tsunamis have been eye-witnessed even in the winter season when such planktonic organisms are less active, and even in the daytime when the intensity of light emitted from planktonic organisms is unlikely to exceed the day-time brightness.
Most tsunami earthquakes are thought to be associated with sediments in the accretionary prism. One recent important finding is that large volumes of stable methane hydrate are present within ocean-floor sediments at water depths exceeding about 500m at lower temperatures. When the equilibrium conditions of coupled low temperature and moderate hydrostatic pressure are disturbed by an earthquake, the hydrate abruptly decomposes. Conversely, a breakdown of hydrate may cause a further mass movement, and a cascading chain of events may occur. Some eye-witness reports in historical documents strongly suggest that luminous phenomena associated with tsunami are attributable to methane hydrate disruption, not others causes such as luminescent planktonic organisms.