Abnormal tsunamis caused by the June 13, 1984, Torishima, Japan, earthquake

Abstract

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 4×10^(13) cm^3, and the potential energy is calculated as 2×10^(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.