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

Abstract

Observations of seismic anisotropy can provide direct constraints on the character of mantleflow in subduction zones, critical for our broader understanding of subduction dynamics. Here wepresent over 750 new SKS splitting measurements in the vicinity of Mount St. Helens in the Cascadiasubduction zone using a combination of stations from the iMUSH broadband array and Cascades VolcanoObservatory network. This provides the highest density of splitting measurements yet available inCascadia, acting as a focused“telescope”for seismic anisotropy in the subduction zone. We retrieve spatiallyconsistent splitting parameters (mean fast directionΦ: 74°, mean delay time∂t: 1.0 s) with the azimuthaloccurrence of nulls in agreement with the fast direction of splitting. When averaged across the array, a90° periodicity in splitting parameters as a function of back azimuth is revealed, which has not beenrecovered previously with single‐station observations. The periodicity is characterized by a sawtooth patterninΦwith a clearly defined 45° trend. We present new equations that reproduce this behavior based uponknown 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 motionof the subducting Juan de Fuca plate; in agreement with predictions for entrained subslab mantleflow. Thesplitting pattern is consistent with that seen throughout Cascadia, suggesting that entrainment of theunderlying asthenosphere with the subducting slab is coherent and widespread. Show more