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Keywords:

  • anisotropy;
  • seismology;
  • mantle;
  • shear wave splitting

Abstract

Anisotropy in the Earth's upper mantle is a signature of past and present deformation. Here we present a new data set of ∼50,000 uniformly processed SKS shear wave splitting measurements that probe upper mantle anisotropy beneath seismic stations in the frequency band 0.02–0.1 Hz. The data set consists of measurements obtained at ∼2000 seismic stations from ∼2000 events. We identify several stations characterized by an apparent absence of shear wave splitting (so-called “null stations”). Station-averaged measurements are obtained by stacking shear wave splitting error surfaces. The stacked data set shows excellent agreement with a compilation of previous SKS measurements. The average amount of splitting beneath seismic stations (after error surface stacking) is 0.8 s, slightly lower than that found previously by vectorial averaging of non-null measurement splitting parameters. The data set disagrees, however, with an azimuthally anisotropic surface wave tomography model (DKP2005), suggesting that caution should be exercised when using such models for geodynamic interpretation, especially in continental regions. Studying our data set in detail, we find evidence that flow in the asthenosphere exerts partial control over SKS splitting in orogenic regions globally. In the active orogenic environment of the western USA, where we have the densest coverage, our data suggest that shallow asthenospheric flow is guided by a wall of thick lithosphere to the east.