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Depth Dependent Azimuthal Anisotropy in the Western US Upper
Mantle
Huaiyu Yuan (Berkeley Seismological Laboratory, University of California-Berkeley), Barbara Romanowicz (Berkeley
Seismological Laboratory, University of California-Berkeley)
We present the results of a joint inversion [Yuan and Romanowicz, 2010] of long period seismic waveforms and SKS splitting
measurements for 3D lateral variations of anisotropy in the upper mantle beneath the western US, incorporating recent datasets
generated by the USArray deployment as well as other temporary stations in the region. We find that shallow azimuthal
anisotropy (Figure 1) closely reflects plate motion generated shear in the asthenosphere in the shallow upper mantle (70-150
km depth), whereas at depths greater than 150 km, it is dominated by northward and upward flow associated with the extension
of the East-Pacific Rise under the continent, constrained to the east by the western edge of the north-American craton, and to
the north, by the presence of the East-West trending subduction zone. (Figure 1 here) The strong lateral and vertical variations
throughout the western US revealed by our azimuthal anisotropy model reflect complex past and present tectonic processes. In
particular, the depth integrated effects of this anisotropy (Figure 2) explain the apparent circular pattern of SKS splitting measurements
observed in Nevada without the need to invoke any local anomalous structures (e.g. ascending plumes or sinking
lithospheric instabilities [Savage and Sheehan, 2000; West et al., 2009]): the circular pattern results from the depth-integrated
effects of the lithosphere-asthenosphere coupling to the NA, Pacific and JdF plates at shallow depths, and in the depth range 200-
400 km, northward flow from the EPR channeled along the craton edge and deflected by the JdF slab, and more generally slab
related anisotropy. With the accumulating high quality TA data, surface wave azimuthal anisotropy makes it possible to resolve
complex depth dependent anisotropic domains in the North American upper mantle. (Figure 2 here)
References
Savage, M. K., and A. F. Sheehan (2000), Seismic anisotropy and mantle flow from the Great Basin to the Great Plains, western United States,
J. Geophys. Res., 105(6), 13,715-713,734.
West, J. D., M. J. Fouch, J. B. Roth, and L. T. Elkins-Tanton (2009), Vertical mantle flow associated with a lithospheric drip beneath the Great
Basin, Nature Geosci., 2(6), 439-444.
Yuan, H., and B. Romanowicz (2010), Depth Dependent Azimuthal anisotropy in the western US upper mantle, Earth Planet. Sci. Lett., in revision.
Acknowledgements: We thank the IRIS DMC and the Geological Survey of Canada for providing the waveforms used in this study and K. Liu,
M. Fouch, R. Allen, A. Frederiksen and A. Courtier for sharing their SKS splitting measurements with us. This study was supported by NSF
grant EAR-0643060. This is BSL contribution #10-05.
− 130 ° − 120 ° − 110 ° − 100 ° − 130 ° − 120 ° − 110 ° − 100 ° − 130 ° − 120 ° − 110 ° − 100
− 130 ° − 120 ° − 110 ° − 100 ° − 130 ° − 120 ° − 110 ° − 100 °
45 °
45 °
40 °
40 °
35 °
70 km 100 km 150 km 30 °
° − 130 ° − 120 ° − 110 ° − 100 °
− 130 ° − 120 ° − 110 ° − 100 ° − 130 ° − 120 ° − 110 ° − 100
45 °
40 °
(a)
(b)
30 °
45 ° Apparent SKS
40 °
Measurements
SKS Prediction
35 °
°
Figure 1. Azimuthal anisotropy variations with depth. Black bars indicate the fast axis direction
and the bar length is proportional to the anisotropy strength. Blue, green and red arrows show
the absolute plate motion (APM) directions of the North American, JdF, and the Pacific plates
respectively, computed at each location using the HS3-NUVEL 1A model.
Figure 1
200 km 300 km 400 km 30 °
NA APM Pacific APM Juan de Fuca APM Anisotropy 1%
Rocky Mountain Front
Strength 2%
35 °
− 130 ° − 120 ° − 110 ° − 100 ° 35 °
30 °
(c)
Delay time 1s
2s
Figure 2. Comparison of observed and predicted station averaged
SKS splitting direction and time. Red bars indicate observations
and are shown in the left panels only, for clarity. Black
bars indicate the model predictions. Predicted splitting is
shown for integration of the models over, (a) the full depth
range Figure of the 2 azimuthal anisotropy models, (b) the top 150 km
of the models, and (c) the portion of the model between 150
and 500 km, respectively.
II-186 | 2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics