Effects of Topography on Ground Motion in Southern California and the Wasatch Front Regions

Abstract:

We examine the effect of realistic surficial topography by conducting a set of deterministic 3D ground motion numerical simulations in the Southern California, and the Wasatch Front regions. We incorporate the highly heterogeneous surficial topography of both regions by implementing a Virtual Topography scheme into Hercules, the octree-based finite-element earthquake simulator developed by the Quake Group at Carnegie Mellon University. We used ∼80×80×40 km3 volumes to perform fmax = 5 Hz simulations subjected to rupture sources representative of large earthquakes. Each region was simulated using three different models: (i) realistic 3D velocity structure with realistic topography (CMP model); (ii) realistic 3D velocity structure without topography (SQD model); and (iii) homogeneous half space with realistic topography (HMG) model. Our results illustrate how realistic topography greatly modifies the ground response. In particular, they highlight the importance of the combined interaction between source-effects, source-directivity, focusing, soft-soil conditions, and the 3D topography. We provide quantitative evidence of this interaction by the inclusion of maps of topographic amplification factors between the CMP and the SQD models.