Crustal and Upper‐Mantle Structure Below Central and Southern Mexico.

The Mexican subduction zone is a complex convergent margin characterized by a slab with an unusual morphology, an abnormal location of the volcanic arc, and the existence of slow earthquakes (slow slip events and tectonic tremors). The number of seismic imaging studies of the Mexican subduction zone has increased over the past decade. These studies have revealed the seismic wave velocity structure beneath Central and Southern Mexico. However, the existing tomographic models are either confined to specific areas or have coarse resolution. As a consequence, they do not capture the lateral and finer‐scale variations in the mantle and the crust structure. Here, we fit frequency‐dependent traveltime differences between observed and synthetic seismograms in a three‐dimensional model of Central and Southern Mexico to constrain crustal and upper mantle seismic velocity structures jointly. We use ∼3,300 seismic records, filtered between 5 and 50 s, from 74 regional earthquakes. We perform point spread function tests to assess the resolution and trade‐offs between model parameters. Our tomographic model reveals several seismic wave velocity features. These features include the geometry of the Cocos slab, the partial melting zone beneath the Trans‐Mexican Volcanic Belt, and the Yucatan slab diving below Los Tuxtlas Volcanic Field that are consistent with previous studies. We also identify horizontal structures in the lower crust and the shallow upper mantle that were not previously identified. These structures include slow seismic wave velocity anomalies that may be correlated with ultraslow seismic velocity zones and high conductivity regions, where slow earthquakes have been identified. Plain Language Summary: The Trans‐Mexican Volcanic Belt is one of the most enigmatic geological regions in Mexican territory because it is the superficial expression of volcanism associated with the subduction system in Mexico. The Mexican subduction has received significant attention from the scientific community due to the diversity of earthquakes produced in the region (large earthquakes, tectonic tremors, and silent earthquakes). Previous studies have investigated the velocity structure of the Mexican subduction zone. However, the existing velocity models are confined to specific areas or have a low resolution. In this work, we present a new model of the velocity structure of the crust and shallow upper mantle of central and southern Mexico. Our velocity model reveals several seismic wave velocity features consistent with previous studies and new low‐velocity anomalies correlated with tectonic tremors and silent earthquake regions, which increase the understanding of the Mexican subduction zone's complexity. Key Points: A three‐dimensional full seismic waveform model of Central and Southern Mexico for 5–50 s periods was constructed via adjoint methodsThe shear wave velocity model indicated two continuous anomalies in the shallow upper mantle and the lower crustThese anomalies were associated with the ultraslow seismic velocity zones and the high conductivity regions where slow earthquakes have been identified [ABSTRACT FROM AUTHOR]