At certain depths in Earth's mantle, the increasing pressure causes minerals to undergo phase changes, transforming to different crystal structures. Seismic waves change speed at these discontinuities, so analyzing seismic waves gives scientists information about the structure of the mantle.
To gain more detail about the fine structure of the transition zone beneath the western United States—the zone between the upper and lower mantles, bounded by discontinuities at 410 kilometers (255 miles) depth and 660 kilometers (410 miles) depth - Tauzin et al. analyzed seismic waves recorded at seismic stations of the U.S. transportable array. For instance, they imaged the area where the Gorda plate, which subducted under northern California, flattens and causes uplift of the 410 kilometer (255 mile) discontinuity under northern Nevada.
They also find that the transition zone is thicker below Washington, Oregon, and Idaho. In addition, they identify minor negative discontinuities (where seismic wave velocity decreases with depth rather than increases) at around 350, 590, and 630 kilometer (217, 367 and 391 mile) depths, and show that the 350 kilometer (217 mile) and 590 kilometer (367 mile) discontinuities extend over a wide region under those states.
The authors suggest that this might be related to increased water content in the transition zone there, or to a significant amount of oceanic material accumulated in the area.
Source: DOI: 10.1002/jgrb.50182/abstract