ABNORMAL TSUNAMIS CAUSED BY THE JUNE 13, 1984, TORISHIMA, JAPAN, EARTHQUAKE

An earthquake with an M(S) = 5.6 (m(b) = 5.5) which occurred near Torishima, Japan, on June 13, 1984, generated abnormally large tsunamis (tsunami magnitude M(t) = 7.3) for its relatively small earthquake magnitude. The maximum amplitude of the tsunamis is not a simple function of distance as the magnitude formula implies. In order to quantify the abnormal tsunamis, we modeled the tsunami source using a finite difference computation on the actual bathymetry. The source of these abnormal tsunamis is modeled as a water surface displacement, with a radius of 12 km and maximum water height of 13 cm. The displaced water volume is 4 x 10(13) cm3, and the potential energy is calculated as 2 x 10(17) ergs. If this water displacement is due to a fault motion at 5 km depth, a moment magnitude M(w) is estimated to be 6.3. This value is larger than M(w) estimated from seismic waves by 0.7, but smaller than M(t) by 1.0. The former difference suggests that the earthquake source is very different from an ordinary faulting. If we assume the source is a horizontally lying tensile crack at shallow depth (about 2 km), that explains both the water surface displacement and the seismic moment tensor. The tensile crack is interpreted as an injection of material, as suggested by a seismological study. The discrepancy between M(w) and M(t) indicates that the propagation path is responsible for the observed large tsunamis. The numerical computation shows a focusing of energy during the propagation of the tsunami along the shallow Izu-Bonin ridge system. Thus the abnormally large tsunamis from the Torishima earthquake are a result of both source and path effects.