Shallow travel-time tomography below southern Mexico

Abstract

P and S wave travel-time tomographies as well as a Vp/Vs ratio image of the crust below southern Mexico were developed using data from the Meso-American Subduction Experiment (MASE) broad-band temporary network. The profile used in the tomography starts at the Pacific coast and runs 205 km inland perpendicular to the trench with a depth of 55 km. Results show fast P and S-wave velocities from the coast to 40 km inland in the descending section of the Cocos slab, a low P-wave anomaly between 50 km and 90 km above the corner where the slab becomes subhorizontal and low P and S-wave velocities above the slab between 90 and 205 km. The Vp/ Vs image shows two areas with high values: (1) the zone where the slab descends from the coast to 60 km inland; and (2) between 90 km - 160 km from the coast where Non-Volcanic Tremors (NVT) are also found to occur. Low Vp/Vs values are found where the slab bends (60 km - 90 km) probably due to it being a highly stressed, dry region of the crust. Normal Vp/Vs values are found within the crust farther than 160 km from the coast despite strong evidence of high pore fluid pressure in that region. This is probably due to the proportional reduction of the P-wave velocity with the S-wave velocity due to high temperature and low effective pressure.