Seismic Amplification Formula using Average Vs in Equivalent Surface Layer established by Vertical Array Strong Motion Records

摘要

Site amplification essential for seismic zonation between ground surface and a base layer was investigated using vertical array records in a number of KiK-net arrays in Japan during recent destructive earthquakes. Based on soil investigation data available for individual downhole arrays, theoretical amplifications for surface arrays consistent with the vertical array records were calculated. 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, V_s for different sites and different earthquakes. The value of V_s was evaluated from fundamental mode frequency and a thickness of equivalent surface layer in which peak amplification was exerted. The conventional parameter V_(S30) often used in current practice, showed poorer correlation than V_s with the obtained amplifications. It is suggested that V_s may be determined from microtremor measurements.