A new strategy for developing Vs₃₀ maps

Despite obvious limitations as a proxy for site amplification, the use of time-averaged shear-wave velocity over the top 30m (Vs₃₀) is useful and widely practiced, most notably through its use as an explanatory variable in ground motion prediction equations (and thus hazard maps and ShakeMaps, among other applications). Local, regional, and global Vs₃₀ maps thus have diverse and fundamental uses in earthquake and engineering seismology. As such, we are developing an improved strategy for producing Vs₃₀ maps given the common observational constraints available in any region for various spatial scales. We investigate a hierarchical approach to mapping Vs₃₀, where the baseline model is derived from topographic slope because it is available globally, but geological maps and Vs₃₀ observations contribute, where available. Using the abundant measured Vs₃₀ values in Taiwan as an example, we analyze Vs₃₀ versus slope per geologic unit and observe minor trends that indicate potential interaction of geologic and slope terms. We then regress Vs₃₀ for the geologic Vs₃₀ medians, topographic-slope, and cross-term coefficients for a hybrid model. The residuals of this hybrid model still exhibit a strong spatial correlation structure, so we use the kriging-with-a-trend method (the trend is the hybrid model) to further refine the Vs₃₀ map so as to honor the Vs₃₀ observations. Unlike the geology or slope models alone, this strategytakes advantage of the predictive capabilities of the two models, yet effectively defaults to ordinary kriging in the vicinity of the observed data, thereby achieving consistency with the observed data.