Developing Spatially Continuous Site Terms for Ground Motion Models Across the U.S.

Shiying Nie, Maggie Roberts, Elise Meyer, Laurie Baise, James Kaklamanos, & Weiwei Zhan

Submitted September 7, 2025, SCEC Contribution #14625, 2025 SCEC Annual Meeting Poster #TBD

In ground motion models (GMMs), site terms are typically derived from site-specific geotechnical parameters such as VS30 (time-averaged shear-wave velocity to 30 m) and Z1.0 (depth to the 1.0 km/s shear-wave velocity horizon). However, these parameters are often only available at specific sites, making them spatially inconsistent and subject to uncertainties due to proxy-based approaches. Our group aim to developing spatially continuous site models for GMM site terms using broadly available geospatial data. For California, we developed a region-specific model using the DesignSafe Ground Motion Database (Ji et al., 2022; Roberts et al., 2025a), using surficial geology, geomorphic provinces, and the topographic position index (TPI), all of which are continuous across the state. In the Central and Eastern U.S. (CEUS), we used NGA-East GMMs with physiographic provinces and a continuous sediment thickness model as inputs (Elise et al., 2025). For the Western U.S., we contributed to the NGA-West 3 project by developing a model based on physiographic provinces, detailed California geology, and a topography-derived basin term (Nie et al., 2025; Nie et al., 2025 report). In addition to map-based adjustments, we recommend incorporating site-specific measurements, such as HVSR and VS30, where available, to refine predictions at individual sites (Roberts et al., 2025b). These geospatial models not only largely reduce site residuals in GMMs but also provide continuous spatial coverage over the specific area.

Key Words
Site effect

Citation
Nie, S., Roberts, M., Meyer, E., Baise, L., Kaklamanos, J., & Zhan, W. (2025, 09). Developing Spatially Continuous Site Terms for Ground Motion Models Across the U.S.. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions (GM)