Abstract
In earthquake resistant designs, for example, design specifications of highway bridges and building standard law, the engineering bedrock, where the S-wave velocity is about 400 m/sec, is used as the bedrock to specify the design spectrum. However, the seismic bedrock should be defined at the most upper part of the earth crust where S-wave velocities are about 3 km/sec. Therefore it is difficult to evaluate long-period ground motions excited by the deep underground structure from the design spectrum specified on the engineering bedrock. To evaluate the amplification due to the deep underground structure, the spectral ratio of ground motions on the engineering bedrock with respect to the seismic bedrock is computed by using the strong motion records in the Kanto Plain. In case of the shallow sediments, the amplification tends to be large in short periods. In case of the deep sediments, the amplification is large in both short and long periods. These results suggest that the ground motion spectrum on the engineering bedrock depends on the thickness of the sediments. Large variation in the amplifications for the sites with the same thickness is also observed and may be explained by the effects of the angle of incidence.
