Abstract
The typical characteristics of earthquake ground motion in the Osaka Plain are the later arrivals with large amplitudes that are considered to be generated at the edges of the basin structure. We found that the amplitudes of later arrivals have the azimuth dependency according to the location of the epicenters; the amplitudes of the later arrivals generated by the southern earthquakes are larger than those generated by the northern earthquakes. It is, found that the azimuth dependency is not caused by the source effects but by the effects of the basement structure. The basin structure of the Osaka Plain in the N-S direction was obtained through refraction seismic surveys. The survey lines were extended in both north and south directions from the explosion point at a northern area of the Osaka Port; each line had a length of 20-30 km.
The modified Aki and Lamer (A-L) method was applied to study the azimuth dependency of the amplitudes of the later arrivals using the surveyed N-S section. From the calculation, the later arrival had a group velocity of 600 m/s which corresponded well to the observed velocities. The azimuth dependency, however, was not well simulated. It is supposed that the incongruity was caused by insufficient information on the edge structure of the sedimentary basin and the structure of shallow sediments.
