Length‐Scale‐Dependent Relationships between VS30 and Topographic Slopes in Southern California

Abstract

AbstractIn engineering seismology, the time‐averaged shear‐wave velocity (VS) of the upper 30 m of the crust (VS30) is the primary parameter used in ground‐motion models to predict seismic site effects. VS30 is typically derived from in situ recordings of VS, although proxy‐based approaches (using geologic and/or geomorphometric classifications) are provisionally adopted when measurement‐based VS30 are sparse or not readily available. Despite the acceptance of proxy approaches, there are limited studies that examine the empirical relationships between VS30 and topographic attributes measured from various length scales and different resolutions of the digital elevation model. In this study, we examine the relationships between compiled VS30 measurements from 218 sites in southern California and topographic metrics of slope and relief measured over various length scales. We find that the correlations between topographic metrics and VS30 are weak but statistically significant. The correlations are improved when topographic slopes and relief are measured over length scales longer than typical hillslopes and VS30 sites are separated by different geologic groups. This is likely because VS30, especially on the rock sites, is better reflected in topographic metrics that capture large‐scale topographic relief, as well as landscape positions such as hilltops and valley bottoms.