Gomer, B. M., R. M. Russo, H. Li, A. Montilla, C. R. Bina, and W. P. Richardson, Crustal shear-wave splitting from aftershocks of the 1997 Cariaco earthquake: The influences of crustal stress, local terranes, and the plate boundary zone, Report to Universidad de Oriente, Cumaná, Venezuela, 2000.
Abstract. Measurements of shear wave splitting delay times (delta_t) and fast polarization azimuths (phi) were made for 161 well-located crustal aftershock events following the July 9, 1997, Cariaco earthquake along the El Pilar fault. The phi and delta_t$ values are the manifestation of crustal anisotropy recorded at 9 sites in NE Venezuela. Delay times for most split shear waves were near 0.1 s regardless of the ray path length, indicating variations in delta_t were due to variability of intrinsic anisotropy. The observed differences in delay times are weakly dependent on back-azimuth, and are related to the anisotropy symmetry system of the Paria-Trinidad terrane and to the different crustal geology encountered along different wave paths. Fast polarization azimuths for 170 measurements span a wide range of values. Stacking the error surfaces of groups of measurements at individual stations generated one or sometimes two consistent phi values at each station. These values are variable throughout the study region, but indicate a strong influence from crustal extensive-dilatancy anisotropy (EDA). Stations located away from the main rupture zone along the El Pilar fault generally have phi values consistent with the maximum regional compressive tectonic stress inferred from earthquake focal mechanisms and relative plate velocity models. Stations located near the rupture zone produced phi values consistent with a background stress field that was perturbed by the presence of the El Pilar fault. These phi values are perpendicular to the El Pilar, indicating a lack of strong stress coupling across the fault.