Development of the Taiwan Generic Rock Seismic Velocity Profile

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 V_S30 (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 V_S profiles from PS-logging at strong-motion stations with V_S30 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 V_S profile to seismic bedrock (V_S of 3.5 km/s). A corresponding generic rock P-wave velocity (V_P) profile was developed using V_P from PS-logging measurements and an empirical relationship between V_P and V_S for the deeper part of the profile. The proposed Taiwan Generic Rock (TWGR) model has V_S30 of 754 m/s, Z_1:0 (thickness of sediments with V_S < 1:0 km= s) of 29 m, Z_2:5 (thickness of sediments with V_S < 2:5 km= s) of 2.1 km, and κ₀ (spectral decay slope) of 0.052 s. The V_S values of the TWGR profile at depths of 50 m–8 km are smaller than from the generic rock profile with V_S30 = 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.