Seismic Anisotropy of the Mexican Subduction Zone Based on the Surface Waves, Shear Wave Splitting, and Higher Modes.

Abstract:

The Mexico subduction zone is characterized by both steep and flat subduction, and a volcanic arc that appears to be oblique to the trench. It has excellent seismic data coverage due to the 2005-2007 Middle America Subduction Experiment (MASE) and the permanent Mexican stations. Here, we study the anisotropy of the region using Surface waves, shear-wave splitting measurements, and higher modes. Our goal is to verify and complement the three-dimensional model of shear-wave velocity and anisotropy in the region constructed using Rayleigh wave phase velocity dispersion measurements (Stubailo et al., JGR, 2012) and constrain the depth of the shear-wave splitting anisotropy with the help of the n1-3 overtones.

The 3D model contains lateral variations in shear wave velocity consistent with the presence of flat and steep subduction, as well as variations in azimuthal anisotropy, that suggest a tear between the flat and steep portions of the slab. Shear-wave splitting is effective for studying mantle anisotropy beneath the receivers and has a better lateral resolution than the Rayleigh wave phase velocity dispersion measurements, although it suffers from a poor depth resolution. To better resolve the anisotropy at depth, we also calculate the anisotropy based on the higher mode surface waves of different overtones for Mexican stations using least-squares fitting of the synthetic higher mode seismograms to the data collected from the deep earthquakes. The three methods allow us to separate the anisotropy and its strength at different depths.

We will report on our shear-wave splitting and higher mode results, and their comparison, and present evidence that anisotropy under Mexico is of deep origin.