Report GIRS 2020-12: Site Response of Southern California Sedimentary Basins and Other Geomorphic Provinces

By: Chukwuebuka C. Nweke, Jonathan P. Stewart, Scott J. Brandenberg

DOI: 10.34948/N3159F

Seismic site response can be influenced by a variety of physical mechanisms that include amplification due to resonance, nonlinearity, topographic effects, impedance contrasts, and contributions from two- or three-dimensional wave propagation in sedimentary basins. Current ground motion models use ergodic procedures that average these effects over many sites globally by conditioning on the time-averaged shear wave velocity in the upper 30 m (VS30), and in some cases, on the depth to a shear wave velocity isosurface (zx) that is also known as the basin depth parameter.

Current site amplification models conditioned on VS30 reflect, in an average sense, most of the aforementioned physical mechanisms, including basin effects. The site response contributions from basin effects are associated with a differential depth parameter taken as the difference between depth for a particular site and average basin depth conditioned on VS30. The basin amplification models are “centered”, in the sense that they predict changes in ground motion amplification for non-zero differential depths. The changes in ground motion amplification are positive and negative for sites with positive and negative differential depths, respectively. The models predicting this behavior are derived using data from both northern and southern California, and for sites situated within sedimentary basin structures but also other geomorphic provinces (e.g., sedimentary structures of different scales and sites with shallow soil overlying rock).

Full abstract available in the report.