We examine the scattered field generated at the Pacific trench of Mexico upon the incidence of teleseismic body waves and the waveguide properties associated with the structure of the subduction zone. In this study, we analyze seismic records from a portable array perpendicular to the trench that show a distinctive phase with a phase velocity of following body-wave phases. This signal is better observed following phases with high incident angles such as PS, SP, and SS arriving from the ocean side of the trench and for events at teleseismic distances with magnitudes larger than Mw 7.0, although weak scattered signals were detected for events down to Mw 6.2 and for direct P and S phases.
We found that the lateral transition between the dipping slab and continental crust, with an inclined boundary formed by the subducting slab at the trench, generates a strong discontinuity that scatters seismic waves, generating a secondary field with amplitude comparable to the incident wave. This phase appears for incident body-wave arrivals with characteristic wavelength comparable to the thickness of the continental crust. Analysis of the observed scattered field shows that the trapped waves are vertically polarized surface waves. Numerical modeling suggests that the impedance contrast at the trench and the geometry of the subducting slab form a waveguide-like structure that efficiently generates and transmits trapped seismic waves along the continental margin.