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Complex and variable crustal and uppermost mantle seismic anisotropy in the western United States


The orientation and depth of deformation in the Earth is characterized by seismic anisotropy1—variations in the speed of passing waves caused by the alignment of minerals under strain into a preferred orientation. Seismic anisotropy in the western US has been well studied2,3,4,5,6,7,8,9,10,11 and anisotropy in the asthenosphere is thought to be controlled by plate motions and subduction6,7,8,9. However, anisotropy within the crust and upper mantle and the variation of anisotropy with depth are poorly constrained. Here, we present a three-dimensional model of crustal and upper mantle anisotropy based on new observations of ambient noise12 and earthquake13 data that reconciles surface wave and body wave9 data sets. We confirm that anisotropy in the asthenosphere reflects a mantle flow field controlled by a combination of North American plate motion and the subduction of the Juan de Fuca and Farallon slab systems6,7,8,9. We also find that seismic anisotropy in the upper mantle and crust are largely uncorrelated: patterns of anisotropy in the crust correlate with geological provinces, whereas anisotropy in the upper mantle is controlled by temperature variations. We conclude that any coupling between anisotropy in the crust and mantle must be extremely complex and variable.

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Figure 1: Major tectonic setting and examples of 2-psi azimuthal anisotropy for Rayleigh waves.
Figure 2: Example azimuthal anisotropy variation and dispersion in the study region.
Figure 3: Azimuthal anisotropy in the crust, uppermost mantle, and asthenosphere and predicted SKS splitting.
Figure 4: Comparison of predicted and observed SKS splitting and comparison of anisotropy between different layers.

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Instruments [data] used in this study were made available through EarthScope (; EAR-0323309), supported by the National Science Foundation. The facilities of the IRIS Data Management System, and specifically the IRIS Data Management Center, were used for access to waveform and metadata required in this study. The IRIS DMS is funded through the National Science Foundation and specifically the GEO Directorate through the Instrumentation and Facilities Program of the National Science Foundation under Cooperative Agreement EAR-0552316. This work has been supported by NSF grants EAR-0711526 and EAR-0844097.

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F-C.L. carried out ambient noise and earthquake tomography for the Rayleigh-wave measurements, computed the three-dimensional inversion and co-wrote the paper. M.H.R. guided the study and co-wrote the paper. Y.Y. and M.P.M contributed surface-wave analysis tools. M.J.F. assembled and carried out SKS splitting measurements. All authors discussed the results and provided comments on the manuscript.

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Correspondence to Fan-Chi Lin.

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The authors declare no competing financial interests.

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Lin, FC., Ritzwoller, M., Yang, Y. et al. Complex and variable crustal and uppermost mantle seismic anisotropy in the western United States. Nature Geosci 4, 55–61 (2011).

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