Length-scale-dependent relationships between VS30 and topographic slopes in southern California

Abstract

In engineering seismology, the time‐averaged shear‐wave velocity ( ⁠ V S ⁠ ) of the upper 30 m of the crust ( ⁠V S30⁠ ) is the primary parameter used in ground‐motion models to predict seismic site effects. ⁠V S30⁠ is typically derived from in situ recordings of V S ⁠ , although proxy‐based approaches (using geologic and/or geomorphometric classifications) are provisionally adopted when measurement‐based ⁠V S30⁠ are sparse or not readily available. Despite the acceptance of proxy approaches, there are limited studies that examine the empirical relationships between ⁠V S30⁠ 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 ⁠V S30⁠ 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 ⁠V S30⁠ are weak but statistically significant. The correlations are improved when topographic slopes and relief are measured over length scales longer than typical hillslopes and ⁠V S30⁠ sites are separated by different geologic groups. This is likely because ⁠V S30⁠ ⁠ , 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.