Seismic Anisotropy under Central Alaska from SKS Splitting Observations

Seismic Anisotropy under Central Alaska from SKS Splitting Observations Figure. Compilation of SKS splitting results. Each measurement is plotted as a red line parallel to the fast direction, with length proportional to the delay time. The observations are plotted at the 100 km depth projection of the ray, in order to demonstrate the back-azimuthal pattern. Locations of seismic stations from the BEAAR experiment are shown by the black dots. Thick black lines show contours to slab seismicity (50 and 100 km). Thin dark lines show active faults, and white lines show roads.
Seismic anisotropy under central Alaska is studied using shear wave splitting observations of SKS waves recorded on the Broadband Experiment Across the Alaska Range (BEAAR), 1999-2001. Splitting results can be divided into two distinct regions separated by the 70 km contour of the subducting Pacific plate. Waves that travel through the thicker mantle wedge show fast directions that are parallel to the strike of the slab. These slab-parallel directions appear to indicate along-strike flow in the mantle wedge, and splitting delay times increase with path length in the mantle wedge suggesting anisotropy of 7.9 ± 0.9 per- cent. The region of along-strike flow corresponds to high seismic attenuation and hence high temperatures. Along-strike flow here may be driven by secular shallowing of the slab driven by subduction of buoyant Yakutat-terrane crust, or by torroidal flow around the east end of the Aleutian slab. Waves traveling southeast of the 70 km contour sample the Pacific plate and the nose of the mantle wedge; they show fast directions that parallel the direction of plate motion. These fast directions are most likely due to flow under the subducting Pacific plate and/or anisotropy within the subducting Pacific lithosphere. The high splitting delay times (0.8-1.7 sec) associated with these convergence-parallel directions cannot be produced in the mantle wedge, which is 10-30 km thick here. Thus, anisotropy shows a sharp 90° change in fabric associated with the onset of high-temperature wedge flow. Results have been published in Christensen and Abers [2010].
</p><p>Christensen, D.H., G.A. Abers (2010). Seismic anisotropy under Alaska from SKS splitting observations, J. Geophys. Res., 115, B04315, doi:10.1029/2009JB006712.
</p><p>Acknowledgements: The BEAAR deployment benefited from contributions by many people, including contributions from the Alaska Earthquake Information Center, the IRIS-PASSCAL Instrument Center, and a large number of students who helped with the deployment and its analysis. This work supported by National Science Foundation grant EAR-9725168.</p>


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