Margin-wide continental crustal anisotropy in the Mexican subduction zone

Abstract

We present new shear wave anisotropy measurements in the continental crust along the Mexican subduction zone obtained from tectonic tremor. The new measurements were made in the states of Jalisco, Colima, Michoaćan and Oaxaca. To make a complete analysis of the anisotropic crustal structure, we also include previous measurements reported in Guerrero using tremor signals. Since tectonic tremor is abundant along the Mexican subduction zone, it offers an opportunity to determine anisotropy parameters in this region. Polarization and splitting analyses were performed using broad-band, three-component seismograms. Results show that splitting times range between 0.07 and 0.34 s. These values are similar to the splitting magnitudes typically observed in the continental crust. Fast polarization azimuths are variable in Jalisco, Colima and Michoaćan, but some of them tend to align with the regional stress field (margin-normal and maximum horizontal compressive stresses). On the other hand, the fast axes at the remaining stations are margin parallel, suggesting that in this case anisotropy could be controlled by active crustal faulting or geological structures. In Oaxaca, fast polarization directions tend to align with Tertiary inactive faults and are oblique with respect to the local stress field, which suggest that anisotropic geological structures are the source of anisotropy.