Mesh Generation for Short-Period Seismic Wave Propagation Based Upon the Spectral- Element Method: Southern California.

The Spectral Element Method (SEM) has been successfully applied to simulate ground motion in Southern California and the Los Angeles Basin for period up to 2 sec. Nowadays, simulations at shorter period are computationally feasible, but they require both a realistic geological model and a detailed unstructured hexahedral mesh. Aiming to include the effect on the seismic propagation due to subsurface geology, topography and low- velocity sedimentary basins, we have generated a 3D unstructured hexahedral mesh of Southern California suitable for shorter period simulations. The grid honors an updated description of the Salton Sea and of the sedimentary basins of Los Angeles, San Fernando and Ventura. We discuss some criteria to determine the geological details that need to be honored, analyzing 2D cross sections of the region for simulations at 10 Hz. We have focused in particular on a profile crossing the Santa Monica Mountains, investigating a possible lensing effect due to a large overthrust. The grid is nearly automatically generated on a massively parallel machine by GEOCUBIT, a Python script collection based upon CUBIT (Sandia Laboratory, www.cubit.sandia.gov). CUBIT is an advanced 3D unstructured hexahedral mesh generator that offers tremendous opportunities in assessing the quality of a mesh both in terms of geometrical complexity and numerical accuracy. For 3D simulations, we have applied SPECFEM3D, which accommodates anisotropy attenuation, free surface topography, fluid-solid boundaries and absorbing boundary conditions. For the 2D simulations we use SPECFEM2D, developed by Roland Martin, Dimitri Komatitsch, Céline Blitz and Nicolas Le Goff (2008).