Vs(30), NEHRP SITE CLASSES AND SITE AMPLIFICATION FACTOR ESTIMATIONS IN THE KANTO REGION, CENTRAL JAPAN

Abstract

Data from K-NET and KiK-net, together with shallow/deep borehole recordings obtained for the pre-Tertiary basement, were used to evaluate regional site effects in the Kanto region. The average velocity to 30 m depth (Vs(30)) was calculated, so as to classify sites to predict their potential for amplifying seismic shaking, depending on the NEHRP Site Class System. In many cases, however, shallow shear-wave velocity data for K-NET, from which Vs(30) can be computed, do not extend to 30 m depth. In order to use the data for these cases, a method to extrapolate the velocities must be devised. Two such methods are described herein and illustrated using data from 58 boreholes of KiK-net, for which the velocity model extends to at least 30 m. The simplest method assumes that, the lowermost velocity extends to 30 m results in significantly less bias than methods using correlations between shallow velocities and Vs(30). In addition, for all methods, the percent of misclassified sites was generally less than 10%. The site amplification factor was calculated in the Kanto region. Also, the maximum Peak Ground Acceleration (PGA) for two selected earthquakes was estimated and compared with the actual PGA. This study also emphasises the usefulness of the K-NET and KiK-net data currently available on the internet.