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Seismic Anisotropy under Central Alaska from SKS Splitting
Observations
Douglas Christensen (Geophysical Institute, University of Alaska Fairbanks), Geoffrey Abers (Lamont Doherty Earth
Observatory of Columbia University)
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 percent.
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].
References
Christensen, D.H., G.A. Abers (2010). Seismic anisotropy under Alaska from SKS splitting observations, J. Geophys. Res., 115, B04315,
doi:10.1029/2009JB006712.
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.
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.
207˚
65˚
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SKS Splitting Observations
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km
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1 sec
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2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics | II-193