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 sta tions of the RESNOM seismic network in northern Baja California, Mexic o, were used to search for probable shear-wave anisotropic characteris tics in the region of the Cerro Prieto fault. Shear-wave splitting was identified in many of the three-component records analyzed. We measur ed 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 thos e records displaying shear-wave splitting. For station CPX, which is n earest the Imperial Valley region to the north, the preferred polariza tion direction found in this study (azimuth 180 degrees +/- 10 degrees ) coincides with the direction of the regional maximum compressive str ess determined for the region. This polarization direction can be inte rpreted in terms of the ''Extensive Dilatancy Anisotropy'' model as th e effect of vertical parallel aligned cracks. The preferred polarizati on direction measured at LMX, however, gives an azimuth of 45 degrees +/- 5 degrees. Thus, it appears that faults and fractures aligned obli que to the main tectonic trend have a greater influence on the anisotr opic characteristics of the crust south of Cerro Prieto volcano than t hat of the regional stress field. Time delays between slow and fast S waves observed at CPX appear constant from 1988 to 1992 while delays m easured at LMX for the same interval indicate a small increase with ti me which cannot be attributed to azimuthal variations of paths.