New Framework of Combining Observations with Topographic Slope to Estimate VS30 and Its Application on Building a VS30 Map for Mainland China

ABSTRACT We propose a new framework of VS30 proxy based on Cokriging method and apply the framework to build a VS30 map for mainland China. This framework utilizes the VS30–topographic slope correlation in the cross-semivariogram to benefit VS30 estimation and has the following benefits: (1) the estimation results are consistent with the measurement data; (2) the estimation uncertainty can be represented by error variance at each unsampled location according to the spatial structure of VS30 and topographic slope; (3) the result map does not have artificial boundaries; and (4) the estimation results can reflect the spatial relation between VS30 and topographic slope and the spatial relation of local spatial environment of topographic slope. We quantify the performance of this framework and compare it with that of three other topographic slope-based VS30 proxy models, including original models developed from exogenous data and models developed from China local data. The result shows that the framework proposed in this article has the best performance. The framework is applied to 7797 borehole VS30 measurements to build a VS30 map for mainland China. The map can capture the high VS30 values in regions where the geological conditions are dominated by flat-lying rocks. Moreover, we consider the effect of sample bias that comes from oversampling of borehole profiles in flat terrain regions when applying borehole data in the proposed framework. We utilize the relation of VS30 and topographic slope to quantify this bias, and use a distance-related data spatial declustering method to eliminate it.