Low-velocity zone atop the 410-km seismic discontinuity in the northwestern United States
The seismic discontinuity at 410 km depth in the Earth's mantle is generally attributed to the phase transition of (Mg,Fe)_2SiO_4 from the olivine to wadsleyite structure. Variation in the depth of this discontinuity is often taken as a proxy for mantle temperature owing to its response to thermal perturbations. For example, a cold anomaly would elevate the 410-km discontinuity, because of its positive Clapeyron slope, whereas a warm anomaly would depress the discontinuity. But trade-offs between seismic wave-speed heterogeneity and discontinuity topography often inhibit detailed analysis of these discontinuities, and structure often appears very complicated. Here we simultaneously model seismic refracted waves and scattered waves from the 410-km discontinuity in the western United States to constrain structure in the region. We find a low-velocity zone, with a shear-wave velocity drop of 5%, on top of the 410-km discontinuity beneath the northwestern United States, extending from southwestern Oregon to the northern Basin and Range province. This low-velocity zone has a thickness that varies from 20 to 90 km with rapid lateral variations. Its spatial extent coincides with both an anomalous composition of overlying volcanism and seismic 'receiver-function' observations observed above the region. We interpret the low-velocity zone as a compositional anomaly, possibly due to a dense partial-melt layer, which may be linked to prior subduction of the Farallon plate and back-arc extension. The existence of such a layer could be indicative of high water content in the Earth's transition zone.
Additional Information© 2004 Nature Publishing Group. Received 12 August; accepted 14 November 2003. We thank J. Ni, R. Aster, and the rest of the RISTRA team for the use of their data set, and we acknowledge all who have contributed to the PASSCAL arrays and IRIS DMC. We also thank H. Gilbert for providing receiver-function profiles. Reviews from D. Anderson, P. Asimow, T. Ahrens, B. Savage and M. Simons also helped to clarify early versions of this paper. We benefited from discussions with H. Gilbert, D. Anderson, H. Kanamori, Y. Tan, S. Ni and M. Gurnis. We thank L. Zhu for sharing his receiver function modelling code. This study was supported by the NSF and is a contribution to Division of Geological and Planetary Science, Caltech.
Supplemental Material - nature02231-s1.doc