POLARITIES OF P-WAVES AND S-WAVES, AND SHEAR-WAVE SPLITTING OBSERVED FROM THE BUCARAMANGA NEST, COLOMBIA

The Bucaramanga nest, Colombia, is an intense seismic source centered at 6-degrees-48'N, 73-degrees-10'W and 161-km depth beneath the eastern Andes. During a 3-week period, 161 microearthquakes (m(b) less-than-or-equal-to 4.3) were recorded by a 260-by-140-km array of 12 three-component University of Wisconsin (UW) seismographs centered above the nest. Eight-eight percent of the events recorded were from the nest, providing a unique opportunity to study natural events that have different source parameters but nearly the same ray paths. Shear wave splitting analysis shows that the average polarization direction of the first-arriving shear waves from the nest events at nine of the seismographs is N12-degrees-E +/- 18-degrees. After corrections based on the amount of discrepancy between the observed average P wave azimuth and the expected event-to-station azimuth, average S wave polarization is N6-degrees-E +/- 15-degrees. Variation in time delays with azimuth and epicentral distance appears to be due to the varying angles between the ray path and the principal axes of the anisotropic system. Shear wave splitting observed from 17 events along the Wadati-Benioff zone outside the nest shows that the time delays increase with increasing depth, and defines a wedge-shaped anisotropic volume above the subducting plate. The anisotropic layer has a maximum thickness of approximately 60 km at the depth of the nest and is thinning updip. The anisotropy most likely results from aligned anisotropic minerals such as olivine and orthopyroxene in a localized mantle flow in the lithosphere above the Wadati-Benioff zone, a region weakened by active fluid migration and mobilized by the heating and shearing along the suducting slab.