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Assessment of Site Effects on Seismic Motion in Ashigara Valley, Japan

¹ Seismic Design Group, R&D Center
The Tokyo Electric Power Company, Inc.
4-1, Egasaki-cho, Tsurumi-ku
Yokohama 230-8510, Japan
 (T.U.)

² Earthquake Research Institute
The University of Tokyo
1-1-1, Yayoi, Bunkyo-ku
Tokyo 113-0032, Japan
 (K.K.)

We compared site amplifications at rock sites and sediment sites of Ashigara Valley, Japan, using ground-motion data from five remote (>700 km) large (>M 7) events. The use of remote large events is advantageous to estimating site factors because the source and path effects are considered to be common with a sufficient accuracy and the ground motions will cover a wide-frequency band. Ground motions at both sediment and rock sites were coherent in frequencies lower than 0.1 Hz. This means that the wavelength in these frequencies is longer than the size of the valley (12 km long and 5 km wide). Site amplification factors were determined by taking spectral ratios with reference to one rock outcrop site. The amplification factors of sediment sites deviated 2–10 times with respect to the rock site in the frequency range higher than 0.1 Hz, in which significant peaks at about 1–2 Hz were found at most sites. These dominant amplifications in sedimentary basin are most essential for assessing earthquake hazard in the region. For sediment sites, the peak frequencies of spectral ratios to the rock sites were stable for different events and coincided with those of horizontal to vertical spectral ratios for the S-wave portion and those of relative site factors estimated separately by the generalized inversion method using local small-events data in the frequency range higher than 2 Hz. Although spectral ratios for frequencies lower than about 1 Hz should be affected by 3D basin structure, 1D S-wave responses represent the amplification of ground motion in the sediment sites for frequencies higher than 2 Hz.