Download Volume II Accomplisments (28 Mb pdf). - IRIS

The Stratification of Seismic Azimuthal Anisotropy in the
Western US
Fan-Chi Lin (University of Colorado at Boulder), Michael H. Ritzwoller (University of Colorado at Boulder), Yingjie
Yang (University of Colorado at Boulder), Morgan P. Moschetti (University of Colorado at Boulder), Matthew J. Fouch
(Arizona State University)
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.
References
Azimuthal anisotropy in the crust, uppermost mantle, and asthenosphere and the
comparison between predicted and observed SKS splitting.
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).
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..
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.
2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics | II-187