Seismic anisotropy in the Guerrero Gap, Mexico: Preliminary results

Abstract

Three component seismograms of low magnitude earthquakes (Md < 3) recorded at the central station Tetitlan of the seismic network located in the Guerrero gap, Mexico show evidence of shear wave splitting. The data set contains several months of activity from the installation of the network in 1987 until late 1989. Even though the sampling rate is only 75 samples/sec and the signal to noise ratio is not very large, it was possible to estimate in most cases the polarization direction of the first shear wave as well as shear‐wave splitting time delays. Most polarizations show a direction which correlates with the main compressive stress in the area due to subduction of the Cocos Plate beneath the North American Plate. Two groups of hypocenters can be distinguished: a shallow group up to 25 Km in depth and a deeper group from 32 to 45 Km. As there is no difference in the polarization direction pattern of both groups and the time delays do not increase with depth, the shear wave splitting is interpreted in terms of an anisotropic layer above 25 km in depth. Filtering the seismograms to enhance the peak amplitude frequency band does not change the observed polarization directions but helps identify shear wave splitting.