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SUMMARY The Alaska–Aleutian subduction zone represents an ideal location to study dynamics within a mantle wedge. The subduction system spans several thousand kilometres, is characterized by a slab edge, and has ample seismicity. Additionally, the majority of islands along the arc house broad-band seismic instruments. We examine shear wave splitting of local-S phases originating along the length of the subduction zone. We have dense measurement spacing in two regions, the central Aleutians and beneath Alaska. Beneath Alaska, we observe a rotation in fast splitting directions near the edge of the subducting slab. Fast directions change from roughly trench perpendicular away from the slab edge to trench parallel near the boundary. This is indicative of toroidal flow around the edge of the subducting Alaska slab. In the central Aleutians, local-S splitting is primarily oriented parallel to, or oblique to, the strike of the trench. The local-S measurements, however, exhibit a depth dependence where deeper events show more consistently trench-parallel directions indicating prevalent trench-parallel mantle flow. Our local-S shear wave splitting results suggest trench-parallel orientation are likely present along much of the subduction zone excited by the slab edge, but that additional complexities exist along strike.
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Seismic Anisotropy and Mantle Flow in the Sumatra Subduction Zone Constrained by Shear Wave Splitting and Receiver Function Analyses
Abstract To systematically investigate seismic azimuthal anisotropy in the Sumatra subduction zone and probe mantle dynamics associated with the subduction of the Australian Plate beneath the Sunda Plate, a total of 169 pairs of teleseismic XKS (including PKS, SKKS, SKS) and 115 pairs of localSsplitting parameters are obtained using broadband seismic data recorded at ~70 stations. Additionally, crustal anisotropy in the overriding Sunda Plate is measured by analyzing the moveout ofP‐to‐Sconversions from the Moho using a sinusoidal function. Comparison between the three sets of anisotropy measurements obtained using shear waves with different depths of origin suggests that (1) the crust of the Sunda Plate is anisotropic with mostly trench‐parallel fast orientations and a mean splitting time of 0.28 ± 0.05 s; (2) the mantle wedge is azimuthally anisotropic with dominantly trench‐parallel fast orientations and splitting times ranging from 0.22 to 0.81 s, which generally increase with the focal depth; and (3) subslab anisotropy is mostly trench‐normal beneath the fore‐arc region with an averaged splitting time of 1.48 ± 0.06 s, and becomes trench‐parallel beneath the arc and back‐arc areas with a mean splitting time of 0.33 ± 0.04 s. The resulting lateral and vertical distributions of anisotropy obtained using splitting of three types of shear waves advocate the presence of an entrained subslab flow that is deflected by the mantle transition zone. The flow enters the mantle wedge through a slab window and flows horizontally parallel to the trench.
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- PAR ID:
- 10373776
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 21
- Issue:
- 2
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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