The Stratification of Seismic Azimuthal Anisotropy in the Western US

The Stratification of Seismic Azimuthal Anisotropy in the Western US Azimuthal anisotropy in the crust, uppermost mantle, and asthenosphere and the comparison between predicted and observed SKS splitting.
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Knowledge of the stratification of seismic anisotropy in the crust and upper mantle would aid understanding of strain partitioning and dynamic coupling in the crust, lithospheric mantle, and athenospheric mantle. It has been difficult, however, to obtain an integrated, self-consistent 3D azimuthally anisotropic model for the crust and upper mantle based on both SKS splitting and surface wave measurements due to the rather different sensitivities of the two wave types. We applied surface wave tomography, including ambient noise tomography (ANT) and eikonal tomography, to data from the Transportable Array (TA) component of EarthScope/ USArray to estimate the 3D anisotropic structure of the crust and uppermost mantle. These results were combined with SKS splitting measurements to investigate the deeper anisotropic structures. Figure 1 shows the anisotropic properties of the (a) middle-to-lower crust, (b) uppermost mantle, and (c) asthenosphere in our final model, where the fast propagation direction and anisotropic amplitude are represented by the orientation and length of the yellow/red bars on a 0.6° spatial grid. Isotropic shear wave speeds at depths of 15 and 50 km are color coded in the background of (a)-(b), and the fast direction is shown in the background in (c). The comparison of observations of SKS splitting (blue, red, or black) and predictions (yellow) from the 3D model of anisotropy model shown in (a)-(c) are also shown in (d), where the fast direction and splitting times are summarized by the orientation and length of the bars. The blue, red, and black colors of the observed measurements identify differences with the model predictions of the fast axis directions: Blue: 0o-30o, Red: 30o- 60o, Black: 60o-90o. The inferred stratification of anisotropy demonstrates complex and highly variable crust-mantle mechanical coupling. Regional-scale azimuthal anisotropy is dominated by relatively shallow tectonic processes confined to the crust and uppermost mantle, although the patterns of anisotropy in the crust and mantle are uncorrelated. The more homogeneous asthenospheric anisotropy broadly 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 systems. These results would not have been possible without the TA, and future work will involve applying the method to new TA stations to the east.
</p><p>References
</p><p>
Lin, F., Ritzwoller, M. H. & Snieder, R. Eikonal tomography: surface wave tomography by phase front tracking across a regional broad-band seismic array. Geophys. J. Int. 177, 1091-1110 (2009).
</p><p>Lin, F., Ritzwoller, M. H., Yang, Y., Moschetti, M. P., & Fouch, M. J. The stratification of seismic azimuthal anisotropy in the western US. Submitted to Nature Geosci..
</p><p>Acknowledgements: Data used in this study were made available through EarthScope and the facilities of the IRIS Data Management Center. This work has been supported by NSF grants EAR-0711526 and EAR-0844097.</p>

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