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

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Science  14 Apr 2011:
DOI: 10.1126/science.1202584


Periodic ocean tides continually provide a cyclic load on the Earth’s surface, the response to which can be exploited to provide new insights into Earth’s interior structure. We use 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 220 km depth. The asthenosphere has a low density anomaly of ∼50 kg/m3. A temperature anomaly of ∼300°C can simultaneously explain this density anomaly and an inferred co-located minima in elastic moduli.