Geometric Parametrization of Sedimentary Basins in Southern California for Site Response Analysis and Modelling

Rashid Shams, University of Southern California

Site response in sedimentary basins is partially governed by mechanisms associated with three-dimensional features. This includes the generation of propagating surface waves due to trapped and refracted seismic waves, focusing of seismic energy due to basin shape and size, and resonance of the entire basin sediment structure. These mechanisms are referred to as basin effects and they lead to a significant increase in the amplitude and duration of the observed ground motions from earthquake events. Currently, ground motion models (GMMs) incorporate basin effects using the time-averaged shear-wave velocity in the upper 30 m (V_S30), and the isosurface depths (depth to a particular shear wave velocity horizon, z_x). This approach captures site response features associated with the basin but uses parameters that are one-dimensional in nature and therefore are limited in their description of the lateral and other three-dimensional (3D) contributing effects. This work explores geometric features as predictive parameters in the development of region-specific models to improve the characterization of site response in sedimentary basins. In this work we constrained basin shape using depth to sedimentary basement (depth to a particular shear wave velocity horizon i.e., z_1.5 and z_2.3) and depth to crystalline basement (z_c,b) which are derived and validated using systematic exploration of geological cross sections and Community Velocity Model (CVM) profiles over Los Angeles Basin (LAB). Finally geometric parameters such as includes Standard deviation of zcb, Standard deviation of Absolute difference between z_1.5 and z_cb, distance from basin margin, and Spatial Area of Influence based on V_S30 are computed based on finalized shape. Residual analysis is employed to access derived geometric parameters for their ability to reduce bias and uncertainty in basin site response analysis.