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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 ~3,5 km/s following body-wave phases. This signal is better observed following phases with high incident angles such as PS, SP, and 55 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. [ABSTRACT FROM AUTHOR]

The Sun Pyramid in Teotihuacán, Mexico, belongs to the cultural heritage of Mexico and the world. Built nearly a millennium ago, the Pyramid is poorly known in terms of its mechanical properties. In order to assess these properties, we measured and studied the ambient seismic noise which has become a popular tool to explore non-invasively geological and human-made structures. This research is aimed to contribute to the structural diagnosis and seismic vulnerability assessment of this emblematic Mexican pre-Hispanic monument. We analyzed the horizontal-to-vertical spectral ratio and the cross-correlations of seismic ambient noise recorded during four hours by 15 broadband stations, distributed on different levels of the structure, and propose a three-dimensional velocity model for the Sun Pyramid in Teotihuacán. The processed data includes the stacking of cross-correlations. We identified arrivals corresponding to a rich mixture of body and surface waves. Group velocity dispersion curves were extracted from the symmetrized cross-correlation of vertical displacements between station pairs. The retrieved travel times between station pairs allowed a tomographic analysis based on the Fast Marching Method (FMM). Besides, simultaneous inversions of the local dispersion curves (obtained from the group velocity tomography) and the HVSR mitigate non-uniqueness of the retrieved P- and S-wave velocities. Our model reveals two main features of the Sun Pyramid: i) The low-velocity anomalies in the cover layer correlate well with the presence of support walls or counterforts that, apparently, were part of the construction technique seeking to provide support to weak parts of the Sun Pyramid. ii) The core of the structure, with soft material subjected to the action of infiltrated water, is susceptible to causing wall failures. These results could be useful to design conservation strategies for this emblematic Mexican pre-Hispanic monument. • Cross correlation of ambient seismic noise is applied to a structure with topography. • A projection scheme is used to correct the wave propagation velocities. • Joint inversion of dispersion curves and H/V spectral ratio improves resolution. • The model for the Sun Pyramid shows a dome structure with some low-velocity anomalies. [ABSTRACT FROM AUTHOR]