Earthquake ground‐motion prediction models usually define site conditions based on the time‐averaged shear‐wave velocity in the upper 30 m (VS30). Proxy‐based estimations of VS30 are commonly used, if velocity measurements are not available. We compile a soil‐profile database for the Beijing plain area (China), using data from research documents and technical reports. The database contains 479 soil profiles, 463 of which have depths greater than 30 m. We develop regional relationships for the Beijing plain area for extrapolating the time‐averaged shear‐wave velocity to a given depth less than 30 m to VS30, and then compare the performance of available models. We find that the second‐order polynomial model (Boore et al., 2011), based on data from Japan, provides an overprediction, whereas the linear model (Boore, 2004) calibrated on data from California underestimates VS30.
We develop relationships for estimating VS30 based on proxies such as ground slope gradients from radar‐derived digital elevation models (DEMs) and surface geology at different scales. We find that local VS30 data in the Beijing plain are generally lower than existing 30 arcsec gradient‐based global models. Regression results show a modest correlation between VS30 and topographic ground slope for several DEM resolutions (3, 15, 30, and 60 arcsec). Geology‐based proxies are more effective than ground slope for VS30 estimation in the analyzed area. We propose a bilinear model based on geologic ages and depositional environments for estimating VS30, which shows a statistically significant trend for application in the Beijing plain area.
Online Material: Figures showing topographic ground slopes and correlations of VS30 with topographic slope from digital elevation model (DEM) data and a table summarizing data from the 463 boreholes.