足柄平野の複雑な地下構造に起因する地震動の後続位相

山梨県東部の地震記録の解析

抄録

Successive later arrivals forming long duration of strong motion have been a great interest in the field of engineering seismology, since the 1985 Michoacan, Mexico earthquake. We analyze the strong motion data set obtained at Ashigara valley from two earthquakes (M_J 5.3 and M_J 4.5) of east Yamanashi prefecture. The data set consists of ground motion records obtained at seven rock sites and twenty-one sediment sites. Focusing on frequency range between 0.2 and 0.5Hz, direct S-wave and two significant later arrivals are discussed. The one of the later arrivals predominates in horizontal component and the other is apparent in vertical component. These later arrivals are identified at different time and their dominant frequencies are not the same. These later arrivals are significant at sediment sites in southern part of the valley, but they are unclear at the northern part of the valley and rock sites. To clarify the nature of those later arrivals is our major concern in this study. In order to determine the apparent propagation velocity and azimuth of the later arrivals, a semblance analysis is applied to small array data at the southern part of the valley. The later arrivals have relatively low velocity, and the propagation azimuths does not necessarily coincide with the propagation azimuth of direct S-wave. The propagation velocities match with the ones of Love and Rayleigh waves. The results of seismic refraction surveys suggest that the depth to the basement is not uniform in central area of the valley and the depth at southern part is deeper than that at northern part of the valley. The later arrivals are supposed to be basin-induced surface wave generated by oblique basement structure at central area of the valley. The location does not coincide with the surface geological boundary or basin edge (Kannawa fault) at the north of Ashigara Valley.