STRESS ORIENTATION AND ANISOTROPY BASED ON SHEAR-WAVE SPLITTING OBSERVATIONS IN THE CERRO-PRIETO FAULT AREA, BAJA-CALIFORNIA, MEXICO

Digital seismograms continuously recorded from 1988 to 1992 by two stations of the RESNOM seismic network in northern Baja California, Mexico, were used to search for probable shear-wave anisotropic characteristics in the region of the Cerro Prieto fault. Shear-wave splitting was identified in many of the three-component records analyzed. We measured the polarization direction of the leading S wave inside the S-wave window as well as the delay times between fast and slow phases on those records displaying shear-wave splitting. For station CPX, which is nearest the Imperial Valley region to the north, the preferred polarization direction found in this study (azimuth 180 degrees +/- 10 degrees) coincides with the direction of the regional maximum compressive stress determined for the region. This polarization direction can be interpreted in terms of the ''Extensive Dilatancy Anisotropy'' model as the effect of vertical parallel aligned cracks. The preferred polarization direction measured at LMX, however, gives an azimuth of 45 degrees +/- 5 degrees. Thus, it appears that faults and fractures aligned oblique to the main tectonic trend have a greater influence on the anisotropic characteristics of the crust south of Cerro Prieto volcano than that of the regional stress field. Time delays between slow and fast S waves observed at CPX appear constant from 1988 to 1992 while delays measured at LMX for the same interval indicate a small increase with time which cannot be attributed to azimuthal variations of paths.