UCLA

Seismic scattering is the direct consequence of the changes in the elastic properties of the medium. These so-call heterogeneities play a fundamental role for the understanding of the propagation of seismic waves and their possible effect on the analysis of seismic risk. This dissertation examines the scattering properties of Middle America region at continental scale, and provides a discussion of the current methods aimed to model the effect of small-scale heterogeneities in the crust.\newline

Using data from a portable array deployed in Mexico perpendicular to the trench, we identified and modeled trapped crustal waves that were previously undocumented. When low frequency f<1Hz body phases from teleseismic events reach the trench from the ocean side, the lateral discontinuity between the continental and oceanic crust behaves as a line source that generates surface waves. This trapped mode travels with a significantly small attenuation through the crust as if it were propagating through a waveguide structure. Interestingly, conversion rates for this phase can reach up to 60% the amplitude of the incident phase in the vertical component. The scattered wave is better observed following the SP, PS, and SS phases for events in the southern hemisphere. Numerical models using a boundary element formulation support this hypothesis. Comparison between the simulations and the seismic records suggested that the structure formed by the flat-subducting slab and the crust behaves very similar to a sedimentary basin but at a larger scale. These observations could explain differences in attenuation along different paths of propagation in this area.