For the past decade, USArray’s large and dense grid of seismometers has gradually collected data on seismic waves across the contiguous United States. Using these data, seismologists calculate the speed of a seismic wave, which is influenced by the material it passes through. Variations in these speeds allow researchers to generate a three-dimensional map of deep Earth structures that is somewhat analogous to a medical computed tomography (CT) scan. Such maps chart areas with different compositions or that are especially cold or hot in the Earth’s underlying crust and upper mantle.
As part of a series of studies striving to improve the methods used to produce these deep Earth maps, Shen and Ritzwoller have created a new type of three-dimensional model using a novel technique that jointly inverts data from earthquakes, ambient noise, and other sources collected beneath more than 1800 stations, then projects the results onto a map of the contiguous United States. The high resolution of the new model, which extends to a depth of 150 kilometers, highlights prominent structural differences beneath the eastern, central, and western United States, including the Cascadia Subduction Zone and the Snake River Plain in the Pacific Northwest and the Reelfoot Rift in the Southeast.
The new model also reveals some previously unknown features that warrant further study, including three relatively low velocity areas in the upper mantle beneath the Appalachians—one centered beneath northern Georgia, a second below the Blue Ridge Mountains in western Virginia—and an especially prominent anomaly beneath New England’s White and Green Mountains. Intriguingly, both the Virginia and New England anomalies are confined to the shallow mantle above 80 kilometers depth and are areas that previous research has tentatively linked to a Cretaceous hot spot track.
The results of this study, including the new methodology, discussion of potential error sources associated with the model, and the improved resolution of these deep Earth maps, will be an important reference for other researchers interested in seismic tomography and the structure of the crust and upper mantle beneath the United States. (Journal of Geophysical Research: Solid Earth, doi:10.1002/2016JB012887, 2016)
—Terri Cook, Freelance Writer