Seismic Anisotropy beneath the Shumagin Islands Segment of the Aleutian-Alaska Subduction Zone

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Journal of Geophysical Research-Solid Earth
Journal Date: 
Sep 10
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Lithospheric strain and mantle flow in subduction zone back arcs reflect the fundamental dynamics of lithospheric subduction. In this study we inferred possible strain geometries in the mantle beneath the eastern Aleutians by mapping seismic anisotropy with shear wave splitting. We analyzed S phases of local events recorded by the Shumagin Seismic Network and obtained splitting parameters (fast direction phi and splitting time delta t) and their 95% confidence regions. To avoid complexity from converted phases, data were restricted to paths at incidence angles less than 32 degrees and to events deeper than 50 km. The S phases sample the forearc and back are of the Shumagin segment from 159.7 degrees W to 163.8 degrees W at depths up to 216 km. In our data set of 84 local events, 44 of 1,27 S phases yielded constrained splitting parameters with consistent fast directions thar are approximately parallel to the volcanic are (roughly 60 degrees). The remaining 83 S phases produced null measurements, and the fast directions inferred from these phase polarizations agree with the fast-axis alignment in the constrained measurements. Splitting times vary from 0.1 to 0.35 s and show an overall increase with hypocentral depth. Observed splitting parameters are well matched by predicted values for olivine-rich mantle wedge models with similar to 1% SV anisotropy where the olivine b axis is are-orthogonal and the a axis is vertical (no are-parallel strain) or arc-parallel (no vertical strain). These results arc consistent with mantle wedge strain models in which are-normal compression is accompanied by arc-parallel or vertical shearing or extension.


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