SEISMIC ANISOTROPY BENEATH THE CONTIGUOUS UNITED STATES FROM SHEAR WAVE SPLITTING ANALYSIS UTILIZING ALL THE USARRAY AND OTHER STATIONS

We present a uniform database of SKS, SKKS, and PKS splitting parameters measured at over 3000 broadband seismic stations in the contiguous United States with data available at the IRIS Data Center. About 30,000 pairs of splitting measurements demonstrate systematic spatial variations with the majority of the fast orientations subparallel to the absolute plate motion (APM) direction of the North American plate, suggesting a dominant plate-motion induced mantle anisotropy originated from the relative movement of the lithosphere and the asthenosphere. Systematic deviations of the fast orientations from the APM are found around the western, southern, and eastern margins of the North American craton, probably as a result of deflection of mantle flow around the cratonic root. The average splitting time over the entire study area is about 1.0 s which is comparable to the global average, and large splitting times of about 1.5 s are found above the subducting Juan de Fuca plate and the along the southern edge of the North American craton. The cratonic interior shows a significantly smaller-than-normal splitting time of about 0.7 s, and the fast orientations are inconsistent with the orientation of ancient orogenic zones, suggesting a negligible contribution of the lithosphere to the observed azimuthal anisotropy. The new database provides first-order constraints to geodynamic models and improved understanding of the origin of seismic anisotropy beneath continents.