UCLA

Ground motion models (GMMs) use depth to shear wave velocity (VS) isosurfaces to quantify site response effects arising from sediment thickness, with z1.0 (i.e., the depth to Vs = 1,000 m/s) often used. However, direct measurements of z1.0 are rare, and z1.0 is poorly resolved in tomographic measurements often used to develop community velocity models (CVMs), particularly in shallow layers and near basin edges. To address this challenge, we present an algorithm for estimating z1.0 for shear wave velocity profiles that do not reach VS = 1,000 m/s. The model is conditioned on the shear wave velocity at the bottom of the profile, VS (zp), and the average gradient near the bottom of the profile (dVS/dz). Using 253 direct z1.0 measurements from the shear wave velocity profile database (VSPDB), we regressed coefficients for VS (zp) = 500 to 1,000 m/s. We then used the algorithm to extend 622 VS profiles that reach ≥500 m/s, but not 1,000 m/s, and quantify distributions of z1.0 based on surficial geologic unit. These relationships are useful at locations where a velocity profile and a reliable CVM-based estimate are not available. The resulting data set enhances the development of site response terms in GMMs, especially in regions where basin depth data are sparse or unreliable, supporting more accurate seismic hazard assessment.