Statical and dynamical study on faulting mechanism of the 1923 Kanto earthquake.

Abstract

An inverse method to find optimum fault parameters from geodetic data with random errors is extended so as to be applicable to a case of the data including a systematic error caused by movements of reference points in triangulation. Application of the new inverse method to static displacement data associated with the Kanto earthquake of 1923 yields a dislocation source model which adequately explains both the seismological and the geodetic data.From the geodetic data, it is found that the fault motion of the Kanto earthquake is a reverse, right-lateral slip of 4.8m with a slip-angle of 140° on a plane which dips 25° towards N24°E, where the slip-angle is measured counterclockwise from a strike on the fault plane. The fault length, width, and the depth to the upper fault margin are determined as 95km, 54km, and 1.5km respectively. The seismic moment and stress drop of this earthquake are estimated to be 8.4×1027 dyne·cm and 45 bars, respectively.Taking the static fault solution as the basic model, the dynamic process of the fracture is investigated on the basis of the long-period seismograms recorded at Hongo, Tokyo. The result shows that the rupture starts from a relocated hypocenter, 35.41°N, 139.22°E and 13.5km (depth), and extends outwards on the fault plane with a propagating velocity of the rupture front of 2.0km/sec. The rise time of the source time function is assumed to be 5.0sec. The maximum amplitude of acceleration for a frequency range of 0.0-3.3Hz at Tokyo is estimated to be about 280gal for the horizontal component and to be 60gal for the vertical component, by applying an empirical formula to the calculated ground displacements.