Surface-to-base amplification evaluated from KiK-net vertical array strong motion records |
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| Authors: | Takaji Kokusho Katsuharu Sato |
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| Institution: | aFaculty of Science and Engineering, Chuo University, Tokyo, Japan |
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| Abstract: | Site amplification defined as the peak value of spectrum ratio was investigated using surface and base accelerations recorded in a number of KiK-net down-hole arrays in Japan during three major earthquakes. An important task was to determine the spectral amplifications relative to outcropping motions with the aid of the down-hole array records. Based on soil data available for individual arrays, theoretical amplifications were calculated and adjusted to coincide with the peak amplifications of the array records. A good and unique correlation was found between the peak amplifications thus obtained and S-wave velocity ratios, defined by S-wave velocity in base layer divided by average S-wave velocity  , for different sites and different earthquakes. The value of  was evaluated from fundamental mode frequency and the thickness of an equivalent surface layer in which peak amplification is exerted. The conventional parameter Vs30; averaged shear wave velocity in the top 30 m used in current design codes, did not correlate well with the obtained amplifications. It is suggested that  may be determined not only from Vs-logging data but also from microtremor measurements. |
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| Keywords: | Seismic amplification S-wave velocity ratio Multi-reflection theory Seismic zonation Base layer |
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