The Effect of Topography on the Seismic Wavefield

Abstract:

We perform a series of seismic wavefield simulations in 3D structural models in order to quantify the effect of topography on seismic waveforms. Our procedure involves three components: (1) constructing a finite-element, unstructured hexahedral mesh, (2) performing seismic wavefield simulations, and (3) quantifying differences between seismograms. We consider three different structural models: homogeneous, layered, and realistic 3D variations. For each structural model we produce a mesh with and without topography. For each of these meshes we produce a finer version in order to demonstrate the minimum resolvable period for the coarser meshes. The target regions is southern California which exhibit large topographic variations and also have abundant seismic stations that allow for comparisons with recorded data. We selected 137 events ranging from Mw 3.4 to 5 in addition to our initial Mw 7.6 point source to have a station and source specific analysis. Our simulations shows that the topography has the strongest effects on surface waves, whereby a phase shift of the main arrival occurs due to the different thickness of the uppermost layer (which includes the topography or not). Scattered waves are visible in the coda due to surface wave reflections that occur off the direct source-station path. Our analysis emphasizes seismograms with periods ≥ 1 s, with the motivation to understand the topographic signatures that may influence measurements used within adjoint-based tomographic inversions with earthquake data.