Improving Iso-surface Depth (z1.0) Estimates for California Sites from Measured Profiles and Geology-Based Proxy Models for Ground Motion Studies

Rashid Shams, Chukwuebuka C. Nweke, Tristan E. Buckreis, Scott J. Brandenberg, & Jonathan P. Stewart

Submitted September 7, 2025, SCEC Contribution #14252, 2025 SCEC Annual Meeting Poster #TBD

Accurate estimates of subsurface shear wave velocity structure are essential for predicting ground motions and ensuring the resilience of lifeline infrastructure during earthquakes. The depth to a shear wave velocity of 1000 m/s (z1.0) is a key parameter in ground motion models (GMMs), yet it is often unavailable or inaccurately represented, especially outside sedimentary basins. Currently, z1.0 is seldom measured but rather is estimated using community velocity models (CVMs), which are three-dimensional representations of the subsurface geology developed using tomographic inversions or rule-based methods and constrained with deep site investigation data. The resulting resolution of these CVMs can be too coarse to capture shallow subsurface structures needed to estimate z1.0, especially for sites located outside of basins or close to basin margins. In this study, we compiled 253 measured z1.0 values from the Shear Wave Velocity Profile database and developed an extrapolation model to estimate z1.0 at an additional 875 sites using partial VS profiles. These values were aggregated within surface geologic units to create a regional z1.0 database with associated uncertainty. This geology-based approach provides more reliable estimates where community velocity models are sparse or unreliable. The resulting database enhances seismic hazard models and supports more accurate site response analyses, directly benefiting the seismic design and risk assessment of critical infrastructure systems and advancing community resilience goals across California.

Citation
Shams, R., Nweke, C. C., Buckreis, T. E., Brandenberg, S. J., & Stewart, J. P. (2025, 09). Improving Iso-surface Depth (z1.0) Estimates for California Sites from Measured Profiles and Geology-Based Proxy Models for Ground Motion Studies. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology