Enhancements in earthquake ground motion assessment through regional Vₛ₃₀ characterization and Vₛ₃₀ scaling
Local site conditions can have significant effects on the intensity of ground shaking caused by earthquakes. Vₛ₃₀, the time-averaged shear wave velocity in the upper 30m, is a prevalently utilized parameter in numerous predictive ground motion models (GMMs) and building codes to account for site effects. Although the database of in-situ measurements of Vₛ₃₀ is constantly expanding, Vₛ₃₀ measurements are not yet available for the vast majority of ground motion recording stations. This research aims to provide reliable estimates of Vₛ₃₀ of these stations using site-specific application of the P-wave seismogram method and to evaluate the regional Vₛ₃₀ scaling effects of ground motions using the new site characterization information and ground motion recordings. The P-wave seismogram method relates Vₛ₃₀ to the vertical and horizontal amplitudes of the P-wave arrival from earthquake recordings. The method is applied to estimate the Vₛ₃₀ at 194 ground motion recording stations in California to verify the accuracy of this method. Comparison with the measured Vₛ₃₀ validates the performance of the P-wave seismogram technique. Additionally, this study suggests the use of Horizontal-to-Vertical Spectral Ratio (HVSR) measurements to identify and correct the potential bias in the P-wave seismogram estimated Vₛ₃₀. A Texas-specific Vₛ₃₀ map is created using a region-specific geologic proxy, geostatistical kriging, and observations of Vₛ₃₀ measurements and Vₛ₃₀ estimates from the P-wave seismogram method. The Texas-specific Vₛ₃₀ map is compared with the Vₛ₃₀ map currently available from the USGS, which uses topographic slope to predict Vₛ₃₀. Across much of Texas, the Texas-specific Vₛ₃₀ map predicts higher Vₛ₃₀ values than the USGS Vₛ₃₀ map derived from a topographic slope proxy, except for the Gulf Coast region, where the values are comparable. Due to the generally larger values of Vₛ₃₀ in the Texas-specific Vₛ₃₀ map, generally smaller ground motions are predicted for Texas. We empirically investigate the bias in the median NGA-East GMMs when used in combination with the site amplification model for central and eastern North America (CENA) using newly available ground motion recordings from Texas, Oklahoma, and Kansas and improved Vₛ₃₀ estimates from the Texas-specific Vₛ₃₀ map. A regional GMM is built with the reference empirical approach. Comparison of the site amplification model described by the new GMM with other site amplification models for CENA suggests similar Vₛ₃₀ scaling of ground shaking for Vₛ₃₀ less than about 760 m/s, but differences for Vₛ₃₀ of 760 m/s relative to the GMM reference condition of 3,000 m/s.
Ground motion model, Vₛ₃₀, Site amplification
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