Shear Wave Velocity Profile Informed Basin Depth Extrapolation Model For z1.0 Estimation

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

Submitted August 30, 2026, SCEC Contribution #15636, 2026 SCEC Annual Meeting Poster #TBD

Ground motion models (GMMs) incorporate sedimentary basin effects using basin depth parameters (zx), which represents the depth to specific shear wave velocity (VS) isosurfaces (typically where x = 1.0 km/s or 2.5 km/s). Basin depths are often estimated using community velocity models (CVMs), which are three-dimensional (3D) representations of seismic velocities that extend from the ground surface to considerable depths (>15 km). CVMs are often based on tomographic imaging and geophysical measurements, but shallow velocity structures are often poorly resolved by the data available. This is particularly true for shallow velocity structures, which are represented using a geotechnical layer. The geotechnical layer may not be accurate at a particular site, and site-specific velocity measurements can therefore provide superior accuracy to CVMs. However, many velocity profiles do not extend to VS = 1.0 km/s and therefore do not provide a direct measurement of z1.0. This paper develops a basin depth extrapolation model based on invasive VS profiles in California that reached or exceeded VS = 1.0 km/s. The profiles were truncated at velocity horizons ranging from 500 m/s to 950 m/s in 50 m/s intervals, and a model was developed to estimate the remaining depth to z1.0. Model performance was subsequently tested and evaluated using an independent dataset of non-invasive VS profiles (VS ≥1000 m/s) in CA. The model is conditioned on the velocity at the bottom of the profile, as well as the velocity gradient over the bottom portion of the profile. Mixed-effects regression was used to define model coefficients for different surface geology classifications. The model was subsequently used to estimate z1.0, with quantified uncertainties, for an additional 622 profiles in California that extend to firm horizons (VS ≥ 500 m/s) but not to VS = 1000 m/s. The proposed approach is useful for z1.0 estimation at sites with velocity profiles that do not extend to z1.0, particularly at sites where CVMs are unavailable or known to be limited in accuracy.

Citation
Shams, R., Nweke, C. C., Buckreis, T. E., Brandenberg, S. J., & Stewart, J. P. (2026, 08). Shear Wave Velocity Profile Informed Basin Depth Extrapolation Model For z1.0 Estimation. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions (GM)