Continuous Maps of z1.0 and z2.5 for California from Integrated Data Sources to Support Site Response Modeling

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

Submitted September 7, 2025, SCEC Contribution #14859, 2025 SCEC Annual Meeting Poster #TBD

Community Velocity Models (CVMs) are 3D representations of subsurface seismic velocity structure. Although originally developed to support simulation models, CVMs are commonly used in ground motion model development as well. Specifically, they are queried to extract isosurface depths (e.g., z1.0 and z2.5) by identifying where specific velocity horizons are encountered within a 1D vertical profile. CVMs have limitations when applied to the assignment of site-specific parameters at shallow depths. Specifically, their derivation is often focused on representing deeper, subsurface structures that are important for seismological modeling of low-frequency wave propagation. However, in many engineering applications, shallower features (typically in the upper 1 km of the crust) are also important for modeling site response over a wide range of frequencies. Near-surface features are relatively poorly constrained in CVMs, which often omit shallow stratigraphy and localized geologic variability that is observed in nature. To overcome this, some CVMs incorporate geotechnical layers to enhance the accuracy of near-surface velocities in high-resolution sub-regions, however the regional-scale models that are default for sites outside of these high-resolution sub-regions do not provide realistic predictions of near-surface properties. We present a framework to develop spatially continuous maps of z1.0 and z2.5 across California that combines high-resolution depth estimates from sub-regional CVMs (e.g., CVM-Hv15.1.1, CVM-S4.26.M01, and SFCVMv21.1) with lower-resolution depth estimates from the USGS National Crustal Model. For z1.0, the framework incorporates empirically derived geology-based depth estimates for pre-Quaternary sites outside of high-resolution sub-regions that are often predicted as having z1.0 = 0 by CVMs, despite having time-averaged shear wave velocity in the upper 30 m less than 1 km/s. The main benefits of this approach are that it (1) provides spatially continuous depths without non-physical discontinuities, such as might occur at the boundaries of CVMs, (2) utilizes high-resolution data where available, and (3) incorporates geology-based adjustments for sites with very shallow depths.

Citation
Buckreis, T. E., Shams, R., Nweke, C. C., Brandenberg, S. J., & Stewart, J. P. (2025, 09). Continuous Maps of z1.0 and z2.5 for California from Integrated Data Sources to Support Site Response Modeling. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Community Earth Models (CEM)