Frequency-Dependent Comparison of Vs30 and Site Period (Ts) as Predictors of Earthquake Ground Motion Intensity Measures

Joy I. Ndamukunda, Chunyang Ji, & Yongfei Wang

Submitted September 7, 2025, SCEC Contribution #14791, 2025 SCEC Annual Meeting Poster #TBD

Most existing studies use Vs30, the time-averaged S-wave velocity for the top 30 m of soils, as the site proxy to develop the site terms of the Ground Motion Model (GMMs). However, Vs30 is only related to the influence of shallow site conditions on ground motion. Meanwhile, limited attention has been given to their frequency-dependent predictive performance, leaving a gap in understanding whether Vs30 is an effective predictor for high-frequency (e.g., Peak Ground Acceleration (PGA) and Short-Period Spectral Acceleration) versus low-frequency (e.g., Peak Ground Velocity (PGV) and Long-Period Spectral Acceleration) intensity measures (IMs). Since high-frequency ground motions are generally more affected by these shallow layers, it is reasonable to expect that Vs30 may perform better than site period (Ts) in predicting high-frequency IMs. Conversely, Ts, which corresponds to the peak amplification at the site of interest, can capture both shallow and deep site effects. Thus, Ts could outperform Vs30 for low-frequency IMs. This study aims to explore whether Vs30 or Ts is a better proxy for characterizing site conditions.

A subset of the NGA-West2 GM database across California was selected to test these hypotheses by comparing with the site term of GMM residuals. Data points were plotted and analyzed in ArcGIS Pro, with regions grouped into three geographic clusters: Group 1 (Los Angeles, Ventura, San Bernardino, and Orange counties), Group 2 (Sonoma, Solano, Napa, Marin, Contra Costa, San Francisco, Alameda, San Mateo, Santa Clara, Santa Cruz, Monterey, and San Benito), and Group 3 (Riverside and Imperial counties). Within each group, the correlations between the site proxy (Vs30 and Ts) and site residuals were examined through linear regression analyses. The linear regression results indicate that Vs30 slightly outperformed Ts in predicting both high- and low-frequency IMs across most regions. However, the small R2 (e.g., 0.03 for the regression between Vs30 and PGA) for the regressions suggests that linear regression may not fully capture the trend between Vs30 or Ts and site residuals. Although the linear correlations with Ts were not obvious, this work still observed that the performance of Ts is better in Northern California, which potentially supports the inclusion of Ts into the site term for the region-specific GMM in this area. For future work, nonlinear regression will be adopted to further study the performance of Vs30 and Ts in ground motion predictions.

Key Words
Ground Motions, Ground Motion Models, Spectral Acceleration, Intensity Measures, Low-Intensity Measures, High-Intensitve Measures, Soil Proxy, GIS, Peak Ground Acceleration, Peak Ground Velocity, Vs30, Ts, Site Period, Regression Analysis, Linear-Regression Analysis

Citation
Ndamukunda, J. I., Ji, C., & Wang, Y. (2025, 09). Frequency-Dependent Comparison of Vs30 and Site Period (Ts) as Predictors of Earthquake Ground Motion Intensity Measures. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions (GM)