Seismic velocity structure of the Guerrero gap, Mexico

Abstract

A two-dimensional velocity structure of the Guerrero gap was obtained by applying a damped least square method to hypocenters of local seismicity recorded by a telemetric network situated on the Guerrero coast, above Cocos plate subduction zone. The region was parameterized by a mesh of 64 cubes in six layers, a total of 384 blocks. The results of 3-D inversion showed that differences of P-wave velocity values among blocks along the strike of the subduction zone were ∼0.25 km/s, effectively showing a two-dimensional symmetry. A 2-D inversion taking into account velocity similarities among the 2-D bands generated megablocks. A final inversion procedure yields P-wave velocity values ranging from 5.4 to 8.2 km/s, and S-wave values from 3.2 and 4.7 km/s, suggesting a continental crust with a thickness of ∼32 km composed of four flat megablocks with a P-wave velocity interval of 5.4 to 7.1 km/s. The Moho interface lies at ∼32 km depth and above a mantle wedge between continental and oceanic crust. The downgoing oceanic crust has three layers (7.2-7.7 km/s), dipping at an angle of ∼26°. A sharp velocity change at a depth of ∼30 km suggests a phase change from basalt to eclogite (7.2 to 7.6 km/s). The mantle has an average velocity of 8.2 km/s. The new velocity model reduced the error in locations and fits better the characteristics of the Guerrero gap.