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Evidence for shear-wave anisotropy in the mantle wedge beneath south central Alaska

Seismology and Volcanology Department Meteorological Research Institute, 1-1 Nagamine, Tsukuba 305, Japan stefan{at}mri-jma.go.jp
Geophysical Institute University of Alaska Fairbanks, Fairbanks, Alaska 99775-7320

Abstract

We investigate shear-wave polarization and splitting for more than 200 earthquakes with hypocentral depths ranging from 20 to 180 km beneath southcentral Alaska. Data were recorded at three sites (stations SKN, AUI, and RED). A preferred polarization parallel to the strike of the Wadati-Benioff Zone (WBZ) (N25°E) is found for a majority of events. However, some regions show systematic differences in polarization. For a subset of nine events, we compute focal mechanisms and find no correlation between the theoretical shear-wave polarization caused by the fault-plane orientation and the observed polarization. Delays of the slow versus the fast S-wave range from less than 0.05 to 0.25 sec for station SKN, and up to 0.4 sec for stations AUI and RED. A statistically significant correlation of delay with depth is found (r = 0.78), with the largest delays corresponding to the deepest events. This indicates that the source of the anisotropy cannot be located in the crust. Based on the ray path geometry, the source cannot be solely located in the subducting slab either and must therefore originate in the mantle wedge. Our results for the subduction zone beneath Alaska are consistent with similar studies in other subduction zones that describe slab-parallel anisotropy in the mantle wedge.

Footnotes

^* Present address: Institute of Geophysics; ETH Hoenggerberg; CH-8093, Zurich, Switzerland; +41-1-6336625; E-mail: stefan@seismo.ifg.ethz.ch

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