1970年秋田県南東部地震の発震機構 (その2)

Abstract

The faulting mechanism of the Southeastern Akita Earthquake of October 16, 1970, was investigated by using both the seismic data and geodetic ones. The focal mechanism solution for the earthquake derived from the initial motions of P waves and that derived from the polarization angles of S waves indicate a reverse faulting with the pressure axis in the direction of northeast or east-northeast. The hypocentral distribution of the aftershocks which occurred just after the main shock was studied in order to discriminate the fault plane from two nodal planes. The nodal plane of S wave solution dipping to the direction of east-northeast is considered to be the fault plane from the obtained hypocentral distribution.Rayleigh waves generated by the main shock were analysed, and the observed azimuthal distribution of amplitude spectral density is consistent with that expected from the above-stated mechanism solution. Further, the fault parameters such as fault dimensions, seismic moment, average dislocation and stress drop are estimated. The field observations of vertical tectonic movements along a near-by leveling route, strain-steps observed at the observation stations in the Tohoku District, and ground motions in the focal region are all in accordance with the faulting mechanism obtained here.