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

Abstract

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 Φ: 74°, mean delay time ∂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° 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 Φ with a clearly defined 45° 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.

Short-scale laterally varying SK(K)S shear wave splitting at BFO, Germany — implications for the determination of anisotropic structures

“…8a) which coincides with the theoretical 'null' direction for the two-layer case (Silver and Savage 1994). Our observed jump in ϕ (SC method) of around π/2 appears to be larger than the synthetic error or jump for a one-layer anisotropy model proposed by Eakin et al (2019) (Fig. 7).…”

“…A crucial point is that ϕ values have 45° slopes over backazimuth and a jump when the backazimuth coincides with the null direction. Eakin et al (2019) proposed that for real data with a more realistic SNR of 5-10 also the SC method tends to show such a behavior. Silver and Savage (1994) showed that also for two-layer anisotropy, a jump in ϕ can appear for the SC method.…”

Anisotropic structure of the Australian continent

“…The pair with the maximum correlation represents the fast direction and delay time of the split. One limitation of this method is a systematic misorientation of 45 °at near-null directions; this can be accounted for with modeling of the splitting parameters, detailed in Section 3.3 (Wüstefeld and Bokelmann, 2007;Eakin et al, 2019). To check for the quality of splits, we also calculate splitting parameters using the minimum energy and eigenvalue methods (Silver and Chan, 1991).…”

Constraints on Olivine Deformation From SKS Shear‐Wave Splitting Beneath the Southern Cascadia Subduction Zone Back‐Arc

“…Even if the slab supplies some modest amount of water beneath the CSZ, it might not directly influence the upper mantle below our study area, given that the slab at this position is located within the transition zone (Figure 2). We propose that material influx from the subslab mantle (Figure 8), which is likely dominated by subslab entrainment (Eakin et al, 2019) and anticipated to be relatively hydrated, toward the mantle above the slab in the back-arc might explain the moderately high water contents. A combination of poloidal and toroidal flow in the CSZ system, suggested by several authors (e.g., Long, 2016;Zandt & Humphreys, 2008;Zhou et al, 2018;Zhu et al, 2020), may play a role in transporting and distributing subslab mantle into the back-arc.…”