Constraints on Lowermost Mantle Anisotropy beneath the Eastern Pacific from SKS-SKKS Splitting Discrepancies

Constraints on Lowermost Mantle Anisotropy beneath the Eastern Pacific from SKS-SKKS Splitting Discrepancies An example of an anomalous SKS-SKKS splitting measurement at station UNM. Columns show the radial (dashed) and transverse (solid) components of the phase (left panel), the initial (dashed) and corrected (solid) particle motion (center panels), and the energy map of the transverse component (right panels). The SKS phase (top row) exhibits null or near-null splitting, while the SKKS phase (bottom row) exhibits well-constrained splitting parameters of φ = 63°, δt = 2.6 sec.
Measurements of the birefringence of SKS and SKKS phases are commonly used to infer seismic anisotropy and flow in the upper mantle beneath a seismic station. Comparisons of SKS and SKKS splitting for stations around the globe have demonstrated that in 95% of cases, SKS and SKKS phases for the same event/station pair yield the same splitting parameters [Niu and Perez, 2004] In an important minority of cases, however, these measurements diverge, and in this case the difference in splitting must be attributed to anisotropic structure in the lower mantle far away from the receiver side. Strongly discrepant SKS-SKKS splitting has been observed at broadband stations in western Mexico and California [Long, 2009]. In particular, strong SKKS splitting with fast polarization directions near ~60° and delay times of up to ~ 3 sec is observed for a group of raypaths that sample the lowermost mantle beneath the eastern Pacific Ocean. The region most likely to cause the observed discrepancies is the deepest mantle, as this is the least similar portion of the SKS/SKKS path. Because there is considerable seismological evidence for anisotropy in the D” layer, and because there is laboratory and seismological evidence that the bulk of the lower mantle is seismically isotropic, my preferred interpretation of the anomalous SKKS splitting observed in western North America is that it is due to complex anisotropy in the D” region at the base of the mantle. The anomalous SKKS splitting appears to delineate a region of fairly uniform azimuthal anisotropy in the D” layer beneath the eastern Pacific; this anisotropy coincides geographically with a sharp lateral gradient in isotropic S wavespeed structure at the base of the mantle. One possible model to explain the observations invokes a broad region of high-strain deformation associated with the impingement of the Farallon slab upon the CMB beneath North America, inducing the LPO of lowermost mantle minerals to produce a region of anomalous D” anisotropy at its edge.
</p><p>Houser, C., G. Masters, P. Shearer, and G. Laske (2008), Shear and compressional velocity models of the mantle from cluster analysis of long-period waveforms, Geophys. J. Int., 174, 195-212.
</p><p>Niu, F., and A. M. Perez (2004), Seismic anisotropy in the lower mantle: A comparison of waveform splitting of SKS and SKKS, Geophys. Res. Lett., 31, L24612.
</p><p>Long, M. D. (2009), Complex anisotropy in D” beneath the eastern Pacific from SKS-SKKS splitting discrepancies, Earth Planet. Sci. Lett., 283, 181-189.</p>


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