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ASU-generated research highlighted in 'Science'

September 25, 2009

"How do you peer into the heart of a continent to see what makes it tick?" That's the question that Richard Kerr asks in the 25 Sept. issue of Science. The answer is the National Science Foundation's EarthScope project, a project that is revolutionizing how scientists see earth's interior.

One component of the EarthScope Project is USArray, a continental-scale seismic observatory which comprises a dense network of permanent and portable seismometers designed to record seismic waves from earthquakes all over the world to help image the deep earth. The seismic stations of the USArray Transportable Array, one of the four major components of the USArray facility, record earthquakes from around the globe. As the seismic waves propagate through the earth, they are affected by the structure and composition along their travel paths, which allow scientists to deduce details of the structure between source and station.

Imaging of crustal and upper mantle structure is an important starting point for understanding the dynamic processes and history of our planet. Many of the images of earth's interior that geophysicists have constructed up to this point have been quite blurry, but as the transportable array slowly marches across the lower 48 states, features of the earth's interior are being revealed in unprecedented resolution.

The array consists of an 800-kilometer wide (497-mile wide) grid of 400 highly capable seismometers 70 kilometers (44 miles) apart. Over the course of 12 years, the seismic stations are moved in a leapfrog fashion to cover the whole United States. The first group of 400 seismometers was deployed in the Western part of the United States. Today, it stretches 2,000 kilometers (1,243 miles) along the Rocky Mountains from the Canadian border to the Mexican border and into the Midwestern U.S. The stations continuously roll across the U.S. at a rate of just over one station per day, every day of the year.

Kerr quotes Edward Garnero, a seismologist and professor in the School of Earth and Space Exploration in the College of Liberal Arts and Sciences, as saying that researchers "are jumping up and down" with all the new data USArray is providing. Since USArray's inception, Garnero has been using the data for many projects; he is one of the many researchers in the school involved with and reliant upon the data from the transportable array.

"Once you use the data for a project, it is really hard to go back to other networks of seismic data," Garnero explains. "USArray ‘listens' to the earth with such incredible fidelity, owing to the really dense station coverage over such a large area, with state of the art equipment."

Many research projects have enabled substantially more detailed images thanks to the USArray, many of which have been ASU-generated results. In the 24 May issue of Nature Geoscience, seismologist John West, a graduate student in the School of Earth and Space Exploration, in collaboration with ASU geophysics professor Matthew Fouch and colleagues, reported the discovery of a 500-kilometer tall (311-mile-tall) drip beneath South-Central Nevada.

In the article, Kerr highlights ASU seismic models. Last year, ASU graduate student Jeff Roth published a paper showing some of the first tomographic images. Kerr also brings ASU into the discussion pertaining to the analysis of the images of the region beneath Yellowstone. Rather than a contorted columnar plume, Kerr says that "Fouch and his colleagues see a bent, thin ‘hot sheet' extending between shallow and deep blobs of hot rock in their processed seismic data."

Kerr notes that although the images generated through USArray data are now sharper, it doesn't mean that all the scientists are coming to the same conclusion about the interpretation of the images. He points out that much more work is left to be done to understand the details of the images, and quotes Fouch as saying that with each group's different processing of the same data, "you can let tomography become a Rorschach test."