Surface-to-base amplification evaluated from KiK-net vertical array strong motion records

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 Vs¯, for different sites and different earthquakes. The value of Vs¯ was evaluated from fundamental mode frequency and the thickness of an equivalent surface layer in which peak amplification is exerted. The conventional parameter Vs30Vs30; 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 Vs¯ may be determined not only from Vs-logging data but also from microtremor measurements.