Development of the Taiwan Generic Rock Seismic Velocity Profile

Abstract

Reference velocity profiles for ground-motion models (GMMs) provide more information about the site condition represented by the GMM than the simple site parameters used in the GMMs, such as VS30 (time-averaged S-wave velocity of top 30 m strata from the surface). A reference velocity profile for generic soft-rock site conditions is developed for Taiwan using multiple data sets to span the depth range from 0 to 16 km. The measured VS profiles from PS-logging at strong-motion stations with VS30 between 610 and 930 m/s (i.e., 760 m/s ± 20%) were selected to define the top 30 m of the profile. The velocity profiles obtained from multiple existing geophysical studies were then used to extend the VS profile to seismic bedrock (VS of 3.5 km/s). A corresponding generic rock P-wave velocity (VP) profile was developed using VP from PS-logging measurements and an empirical relationship between VP and VS for the deeper part of the profile. The proposed Taiwan Generic Rock (TWGR) model has VS30 of 754 m/s, Z1.0 (thickness of sediments with VS<1.0  km/s) of 29 m, Z2.5 (thickness of sediments with VS<2.5  km/s) of 2.1 km, and κ0 (spectral decay slope) of 0.052 s. The VS values of the TWGR profile at depths of 50 m–8 km are smaller than from the generic rock profile with VS30=760  m/s for California, which lead to different site amplifications between soft-rock sites in Taiwan and California. The TWGR provides information on the applicability of the large empirical data set of strong-motion recordings from Taiwan to other regions.