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

Journal name:
Nature Geoscience
Volume:
4,
Pages:
55–61
Year published:
DOI:
doi:10.1038/ngeo1036
Received
Accepted
Published online

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.

At a glance

Figures

  1. Major tectonic setting and examples of 2-psi azimuthal anisotropy for Rayleigh waves.
    Figure 1: Major tectonic setting and examples of 2-psi azimuthal anisotropy for Rayleigh waves.

    a, Triangles identify the seismic stations. Yellow and red lines are the plate and tectonic boundaries. CAS: Cascade Range; CV: Central Valley; GB: Great Basin; HLP: High Lava Plains; CP: Colorado Plateau; SRP: Snake River Plain. Black arrows give relative motions between the Pacific (PA) and North American plates (NA), the Juan de Fuca plate (JdF) and NA, and NA and the hotspot reference frame (HS; ref. 24). Red stars give locations shown in bg and Fig. 2d,e. bg, Examples of 12-, 26- and 38-s-period Rayleigh wave phase velocity measurements. Green dashed lines give the best fitting 2-psi curves.

  2. Example azimuthal anisotropy variation and dispersion in the study region.
    Figure 2: Example azimuthal anisotropy variation and dispersion in the study region.

    ac, Maps of 12, 26 and 38s period Rayleigh wave phase velocity azimuthal anisotropy on a 0.6° spatial grid. The fast propagation direction and anisotropic amplitude are presented by the orientation and length of the red bars. d,e, An example of anisotropy dispersion curves for a location in northern Nevada between periods of 12 and 54s with associated uncertainties. The red lines are the best fitting dispersion curves based on the crustal and uppermost mantle model shown in Fig. 3a,b.

  3. Azimuthal anisotropy in the crust, uppermost mantle, and asthenosphere and predicted SKS splitting.
    Figure 3: Azimuthal anisotropy in the crust, uppermost mantle, and asthenosphere and predicted SKS splitting.

    a,b, Crust and uppermost mantle in models A, B and C. c, Asthenospheric layer in model C. Fast propagation directions and anisotropic amplitudes are given by the orientations and lengths of the yellow/red bars on a 0.6° spatial grid. Background colours represent isotropic shear wave speeds at depths of 15 and 50km in a,b, and the fast direction is shown in the background in c. d, Predicted SKS measurements based on model C, where the background colour also gives the split time.

  4. Comparison of predicted and observed SKS splitting and comparison of anisotropy between different layers.
    Figure 4: Comparison of predicted and observed SKS splitting and comparison of anisotropy between different layers.

    a, Observed SKS splitting (blue, red or black) compared with predictions (yellow) from model C (Fig. 3a–c) where bars summarize the fast direction and splitting time. Blue, red or black colours give differences in fast directions: blue: 0°–30°, red: 30°–60°, black: 60°–90°. b,c, Differences between the observed and predicted (model C) directions and times in a. d, Differences in fast directions between the crust and uppermost mantle. e, Differences in fast directions between the uppermost mantle and the asthenosphere in model C.

References

  1. Savage, M. K. Seismic anisotropy and mantle deformation: What have we learned from shear wave splitting? Rev. Geophys. 37, 65106 (1999).
  2. Ozalaybey, S. & Savage, M. K. Shear-wave splitting beneath western United States in relation to plate tectonics. J. Geophys. Res. 100, 1813518149 (1995).
  3. Savage, M. K. & Sheehan, A. F. Seismic anisotropy and mantle flow from the Great Basin to the Great Plains, western United States. J. Geophys. Res. 105, 1371513734 (2000).
  4. Silver, P. & Holt, W. The mantle flow field beneath western North America. Science 295, 10541057 (2002).
  5. Savage, M. K. Seismic anisotropy and mantle deformation in the western United States and southwestern Canada. Int. Geol. Rev. 44, 913937 (2002).
  6. Becker, T. W., Schulte-Pelkum, V., Blackman, D. K., Kellogg, J. B. & O’Connell, R. J. Mantle flow under the western United States from shear wave splitting. Earth Planet. Sci. Lett. 247, 235251 (2006).
  7. Marone, F. & Romanowicz, B. The depth distribution of azimuthal anisotropy in the continental upper mantle. Nature 447, 198201 (2007).
  8. Zandt, G. & Humphreys, E. Toroidal mantle flow through the western US slab window. Geology 36, 295298 (2008).
  9. West, J. D., Fouch, M. J., Roth, J. B. & Elkins-Tanton, L. T. Vertical mantle flow associated with a lithospheric drip beneath the Great Basin. Nature Geosci. 2, 438443 (2009).
  10. Moschetti, M. P., Ritzwoller, M. H., Lin, F. & Yang, Y. Seismic evidence for widespread western-US deep-crustal deformation caused by extension. Nature 464, 885889 (2010).
  11. Buehler, J. S. & Shearer, P. M. Pn tomography of the western United States using USArray. J. Geophys. Res. 115, B09315 (2010).
  12. Lin, F., Moschetti, M. P. & Ritzwoller, M. H. Surface wave tomography of the western United States from ambient seismic noise: Rayleigh and Love wave phase velocity maps. Geophys. J. Int. 173, 281298 (2008).
  13. Yang, Y., Ritzwoller, M. H., Lin, F., Moschetti, M. P. & Shapiro, N. M. Structure of the crust and uppermost mantle beneath the western United States revealed by ambient noise and earthquake tomography. J. Geophys. Res. 113, B12310 (2008).
  14. Sabra, K. G., Gerstoft, P., Roux, P., Kuperman, W. A. & Fehler, M. C. Surface wave tomography from microseisms in Southern California. Geophys. Res. Lett. 32, L14311 (2005).
  15. Shapiro, N. M., Campillo, M., Stehly, L. & Ritzwoller, M. H. High-resolution surface-wave tomography from ambient seismic noise. Science 307, 16151618 (2005).
  16. Pollitz, F. F. Observations and interpretation of fundamental mode Rayleigh wavefields recorded by the Transportable Array (USArray). J. Geophys. Res. 113, B10311 (2008).
  17. 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, 10911110 (2009).
  18. Bensen, G. D. et al. Processing seismic ambient noise data to obtain reliable broad-band surface wave dispersion measurements. Geophys. J. Int. 169, 12391260 (2007).
  19. Smith, M. L. & Dahlen, F. A. Azimuthal dependence of Love and Rayleigh-wave propagation in a slightly anisotropic medium. J. Geophys. Res. 78, 33213333 (1973).
  20. Barruol, G. & Kern, H. Seismic anisotropy and shear-wave splitting in lower-crustal and upper-mantle rocks from the Ivrea zone—experimental and calculated data. Phys. Earth Planet. Inter. 95, 175194 (1996).
  21. Montagner, J-P. & Nataf, H-C. A simple method for inverting the azimuthal anisotropy of surface waves. J. Geophys. Res. 91, 511520 (1986).
  22. Rumpker, G. & Silver, P. G. Apparent shear-wave splitting parameters in the presence of vertically varying anisotropy. Geophys. J. Int. 135, 790800 (1998).
  23. Montagner, J-P., Griot-Pommera, D-A. & Lavé, J. How to relate body wave and surface wave anisotropy? J. Geophys. Res. 105, 1901519027 (2000).
  24. Gripp, A. E. & Gordon, R. G. Young tracks of hotspots and current plate velocities. Geophys. J. Int. 150, 321361 (2002).
  25. Zhang, S. Q. & Karato, S. Lattice preferred orientation of olivine aggregates deformed in simple shear. Nature 375, 774777 (1995).
  26. Holt, W. E. Correlated crust and mantle strain fields in Tibet. Geology 28, 6770 (2000).

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Affiliations

  1. Center for Imaging the Earth’s Interior, Department of Physics, University of Colorado at Boulder, Boulder, Colorado 80309, USA

    • Fan-Chi Lin,
    • Michael H. Ritzwoller,
    • Yingjie Yang &
    • Morgan P. Moschetti
  2. School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA

    • Matthew J. Fouch

Contributions

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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The authors declare no competing financial interests.

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