SKS Splitting Beneath Mount St. Helens: Constraints on Subslab Mantle Entrainment

yy Observations of seismic anisotropy can provide direct constraints on the character of mantle flow in subduction zones, critical for our broader understanding of subduction dynamics. Here we present over 750 new SKS splitting measurements in the vicinity of Mount St. Helens in the Cascadia subduction zone using a combination of stations from the iMUSH broadband array and Cascades Volcano Observatory network. This provides the highest density of splitting measurements yet available in Cascadia, acting as a focused telescope for seismic anisotropy in the subduction zone. We retrieve spatially consistent splitting parameters (mean fast direction Phi: 74 degrees, mean delay time partial derivative t: 1.0 s) with the azimuthal occurrence of nulls in agreement with the fast direction of splitting. When averaged across the array, a 90 degrees periodicity in splitting parameters as a function of back azimuth is revealed, which has not been recovered previously with single-station observations. The periodicity is characterized by a sawtooth pattern in Phi with a clearly defined 45 degrees trend. We present new equations that reproduce this behavior based upon known systematic errors when calculating shear wave splitting from data with realistic seismic noise. The corrected results suggest a single layer of anisotropy with an ENE-WSW fast axis parallel to the motion of the subducting Juan de Fuca plate; in agreement with predictions for entrained subslab mantle flow. The splitting pattern is consistent with that seen throughout Cascadia, suggesting that entrainment of the underlying asthenosphere with the subducting slab is coherent and widespread.