Abstract

We present results from a borehole observation network composed of 12 borehole sites deployed in a 20 × 20 km area in the Sendai region, Japan. The subsurface Quaternary layers vary with each site, but a Pliocene layer is commonly underlying at a depth of 0 to 80 m throughout the Sendai region. We define this Pliocene layer, whose S-wave velocity is larger than 500 m/sec and whose N value of the standard penetration test is greater than 50, as an engineering bedrock in the region. Once we characterize ground motions at the engineering bedrock, we can either use these motions directly in the response analysis of whole soil-building systems or use them in the nonlinear analysis of soft soil layers as an input. The purpose of this study is to evaluate the local site effects due to surface layers overlying the engineering bedrock and to remove them by using one-dimensional (1D) soil models whose properties are determined by weak-motion records of 18 events. First we identify S-wave velocities and frequency-dependent quality factors Q from amplification factors between surface records and borehole records observed at 10 sites at depths of several tens of meters. The identified S-wave velocities are very close to S-wave logging values. The identified Q values show strong frequency dependence, proportional to f(0.46 to 1.16), where f is frequency. We confirm that the observed amplification factors in the frequency range from about 0.1 to 20 Hz can be explained well by the theoretical ones based on the 1D wave propagation theory with the identified S-wave velocities and Q. Then we estimate the so-called engineering bedrock waves, which are supposed to be observed on the outcrop of the engineering bedrock, from borehole records by using the 1D models with these identified soil constants. We confirm that local site effects due to surface layers overlying the engineering bedrock are properly removed so that we have similar characteristics in the resultant engineering bedrock waves from records at different sites.

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