Evaluating empirical regression equations for Vs and estimating Vs30 in northeastern Taiwan

The shear wave velocity of shallow sediments is very important in seismic wave amplification and thus Vs30 is a well-known parameter for site classification. Based on the relationship between Vs and soil indexes, the empirical regression equations were evaluated using more than 600 data sets from the Ilan area and the Taipei Basin. Multivariable analysis, which can increase the accuracy of regression equations, was used in this study. Three extrapolations were compared, and then the most accurate bottom-constant extrapolation was adopted to estimate Vs30 at 16 boreholes, none of which reached a depth of 30 m. Ultimately Vs30 was derived for 110 free-field strong motion stations, and the stations in northeastern Taiwan were reclassified according to the Vs30-based NEHRP provisions. Regression equations of Vs and the extrapolation of Vs30 were also applied to boreholes with only N-values and corresponding depths of less than 30 m for assessing the Vs30 and NEHRP class.