A Silent Earthquake of December 9, 1989, in the Tokyo Bay, as Revealed by the Continuous Observation of Crustal Movements in the Southern Kanto District, Central Japan

Abstract

Continuous records of crustal tilts and strains in the southern Kanto district, central Japan, during ten years from 1985 to 1994 are analyzed. Applying the Bayesian tidal analysis method, the original records of a sampling interval of an hour are decomposed into tide, atmospheric pressure effect and trend components. In the trend component, synchronous ramp-type steps of an order of 10^-8 radian are found with a time constant of about one day on December 9, 1989. A grid search by the inversion of the steps is made to obtain fault parameters. The optimal model is a low-angle thrusting toward the north with a slip direction of hanging wall of S60°E at the depth of around 24km beneath the central part of the Tokyo Bay along the subduction interface between the Philippine Sea and the overriding north American plates. Thus, it is concluded that a silent earthquake of a time constant of about one day on December 9, 1989, released an interplate moment of around 0.75×10¹⁸Nm, which is equivalent to that of M_w5.9 seismic event, beneath the Tokyo Bay near the transition zone between the coupled and the uncoupled regions on the subduction interface.