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