The effect of topography on the seismic wavefield

Abstract

Active tectonic settings exhibit deformation manifested by earthquakes and by strong topographic variations due to erosion and uplift. Seismic waves from these earthquakes will clearly be influenced by the topographic variations, but it is challenging to isolate the effects of topography from the effects of variations in 3D seismic wave-speed structure. Here we design a realistic numerical experiment to investigate the effects of topography on the regional seismic wavefield. We choose southern California as a target region. We perform several sets of 3D seismic wavefield simulations for 137 earthquake sources ranging from Mw3.4 to 5.4. We test the influence of topography within a homogeneous model and a layered model, and for each model we establish the shortest resolvable period for each path between a source and station. By examining the path-specific shortest resolvable periods, we are able to make some generalizations. Topography has the strongest influence on surface waves, particularly for waveforms with travel paths that are nodal to the source radiation; in these directions, the wave amplitudes are relatively low, so any multi-pathing or scattering effects due to topography are more easily identified. The topographic effects are stronger for shorter periods and for longer paths. The influence of topography on the seismic waveforms arises from both the change in the topographic surface, but also the change in the wave-speed structure that arises from perturbing the topography for a 1D (or 3D) wavespeed model. These generalizations of the influence of topography provide a basis for further numerical investigations or for where to search within a regional set of observations for the topographic effects. Topography should be included within simulation-based seismic imaging applications, especially those at high frequencies, in order to eliminate the possibility of attributing topographically-caused waveforms to subsurface variations in structure.