Spatiotemporal characterization of crustal anisotropy in the source area of the 2017 puebla-morelos earthquake

Abstract

Spatiotemporal variations in seismic anisotropy are investigated using local crustal and upper mantle earthquakes with magnitudes greater than M_w 2.5, occurred in the Puebla-Morelos region of Central Mexico, an area historically impacted by significant seismic activity. Data were collected from five permanent stations located more than 70 km from the epicentral area of the 2017 Puebla-Morelos earthquake (M_w 7.1). Shear-wave splitting parameters were obtained from 276 seismic waveforms recorded between 2011 and 2020, providing insights into crustal anisotropy behaviors. The overall dominant fast polarization (ϕ) is oriented WNW86.7°ESE, which deviate from the regional maximum stress field (S_{HR max}), typically oriented NE-SW to ENE-WSW. Instead, it aligns with the WNW-ESE structural trend parallels to the Middle American Trench, suggesting that subduction-derived tectonic fabric exert strong control on seismic anisotropy in the region. Anisotropic observations reveal notable spatial differences in both tectonic and anisotropic frameworks across the study area. The northern-northwestern sector appears more susceptible to stress-induced anisotropy. This sector also exhibits elevated crack densities, with normalized delay times (δt km⁻¹) reaching up to 30 ms km⁻¹, and exhibit monoclinic and transverse isotropic features, suggesting a greater capacity for structural deformation. In contrast, the western and eastern sectors display anisotropy primarily controlled by pre-existing structural features with orthorhombic features. Although ϕ measurements in these sectors align with S_{HR max}, there is not conclusive evidence of stress-induced anisotropy. These patterns reveal that static stress changes affecting crustal anisotropy during the preseismic and coseismic stages of the Puebla-Morelos mainshock were not uniformly expressed across the region. Such changes appear to be more clearly captured in the northern-northwestern sector, highlighting a greater sensitivity to the accumulation of structural deformation prior to a major earthquake. Overall, these findings underscore the heterogeneous response of different crustal blocks to evolving stress conditions.