Imaging the structure of the Sun Pyramid (Teotihuacán, Mexico) from passive seismic methods

Abstract

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.