HVSR- and VS30-Based Adjustment Terms for Geospatial Site Amplification Models for California

ABSTRACT This study presents a geotechnical site term for ground-motion models (GMMs) in California derived from horizontal-to-vertical spectral ratio (HVSR) and VS30 (time-averaged shear-wave velocity in the upper 30 m) data that can be used with a geospatial linear site model when measured geotechnical data are available. Spatially continuous site models, such as those based on geospatial data, have advantages for GMM implementation in that they provide a consistent site term for locations that lack measured geotechnical data. We propose enhancing these models with local geotechnical information where it is available. The proposed model was developed using microtremor HVSR (mHVSR; Wang et al., 2021), earthquake HVSR (eHVSR; Ji et al., 2022), and measured VS30 data (Ahdi et al., 2018; McPhillips et al., 2020; Kwak et al., 2021). The geotechnical parameters are analyzed for their ability to reduce site-to-site variability of peak ground acceleration, peak ground velocity, and pseudospectral accelerations from 0.01 to 10 s calculated from the residuals of the Boore et al. (2014; BSSA14) GMM, modified with a geospatial-based site term (Roberts et al., 2025). Geotechnical site terms are developed for sites with a VS30 measurement and for sites with mHVSR recordings that have a clear fundamental resonance peak or that meet the criteria of a flat recording. Compared to the base geospatial site term, the inclusion of the geotechnical site term results in an average reduction to the site-to-site variability of 6.5% for stations with VS30 measurements and 5.7% for stations with mHVSR measurements. This study illustrates the effectiveness of using geotechnical measurements to enhance geospatial linear site term models. We also highlight the potential for using the cost-effective HVSR technique to develop GMM site terms, using both eHVSR and mHVSR data, and sites with clear fundamental resonance frequencies and those without clear resonances.