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Published May 20, 2011 | public
Journal Article

Probing Asthenospheric Density, Temperature, and Elastic Moduli Below the Western United States


Periodic ocean tides continually provide a cyclic load on Earth's surface, the response to which can be exploited to provide new insights into Earth's interior structure. We used geodetic observations of surface displacements induced by ocean tidal loads to constrain a depth-dependent model for the crust and uppermost mantle that provides independent estimates of density and elastic moduli below the western United States and nearby offshore regions. Our observations require strong gradients in both density and elastic shear moduli at the top and bottom of the asthenosphere but no discrete structural discontinuity at a depth of 220 kilometers. The model indicates that the asthenosphere has a low-density anomaly of ~50 kilograms per cubic meter; a temperature anomaly of ~300°C can simultaneously explain this density anomaly and inferred colocated minima in elastic moduli.

Additional Information

© 2011 American Association for the Advancement of Science. Received 7 January 2011; accepted 23 March 2011. Published online 14 April 2011. We thank H. Kanamori, D. L. Anderson, M. Gurnis, and two reviewers for informative discussions and helpful reviews of early versions of this work. This material is based on raw data provided by the Plate Boundary Observatory operated by UNAVCO for EarthScope (www.earthscope.org). Supported by NSF grants EAR-0350028 and EAR-0732947, Ministry of Education, Culture, Sports, Science & Technology of Japan grant-in-aid for scientific research 20740254, JSPS Postdoctoral Fellowships for Research Abroad, and the Gordon and Betty Moore Foundation. Data are available from UNAVCO Archive collections (http://facility.unavco.org/data/). This paper is Caltech Tectonics Observatory contribution 150 and Seismolab contribution 10056. We thank T. Takasu for providing us his GPS analysis codes (GPS Tools, version 0.6.4).

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