Site Amplification Formula for Seismic Zonation Based on Downhole Array Records During Strong Earthquakes

Abstract eng:
Site amplification essential for seismic zonation is defined here as a peak value of spectrum ratio between ground surface and a base layer. The amplification was investigated using surface and base accelerations recorded in a number of KiK-net downhole arrays in Japan during three recent destructive earthquakes. An important task was to determine the spectral amplifications relative to outcropping motions. Based on soil investigation data available for individual arrays, theoretical amplifications consistent with the peak amplifications of the array records were calculated corresponding to the outcropping motions. A good and unique correlation was found between the peak amplification thus obtained and S-wave velocity ratio, defined as S-wave velocity in a base layer divided by average S-wave velocity, Vs for different sites and different earthquakes. The value of Vs was evaluated from fundamental mode frequency and a thickness of equivalent surface layer in which peak amplification was exerted. The conventional parameter Vs30; averaged shear wave velocity in the top 30m often used in current practice, showed poorer correlation than Vs with the obtained amplifications. It is suggested that Vs may be determined not only from Vs-logging data but also from microtremor measurements.

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