New technique uses “seismic garbage” to view Earth's interior

24 April 2005 | 14:03 Code : 4984 Geoscience events
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Seismologists have long relied on earthquakes or expensive tools like explosives to help create images of Earth's interior

Seismologists have long relied on earthquakes or expensive tools like explosives to help create images of Earth's interior, but a new method created by University of Colorado at Boulder (CU-Boulder) researchers will produce quicker, cheaper and clearer images, researchers believe.

Rather than waiting for earthquakes, scientists have recovered surface-wave information from normal seismic noise that is constantly produced by fluctuations in the Earth's atmosphere and oceans. Measuring surface waves is important because the information helps define a clearer picture of the Earth's interior, according to Michael Ritzwoller, director of CU-Boulder's Center for Imaging the Earth's Interior. The method is described in the March 11 issue of the journal Science

"This new technique will give us a better fundamental understanding of the planet by providing much better resolution of Earth's interior" Ritzwoller says. 

The new method promises significant improvements in the resolution and accuracy of crust and upper mantle images down to 60 miles or more within the Earth, particularly when used with seismic projects like USArray, according to Nikolai Shapiro, a research associate in the Center for Imaging the Earth's Interior and the study's chief author. 

Coupled with existing and emerging technology, such as USArray, the new measuring technique will lead to a better fundamental understanding of the structure of the planet and may help save lives in the process, Ritzwoller said. A component of the National Science Foundation's (NSF) EarthScope program, USArray includes hundreds of portable seismometers that in coming years will be moved over the entire country, producing images of the Earth's interior to aid in earthquake risk assessment. 

"The authors' application of what used to be 'seismic noise' to the detailed mapping of the crust and upper mantle will have significant impact on earth science and on seismic hazard mitigatio," says James Whitcomb, head of NSF's deep Earth processes section, which funded the research. "This innovative research foretells what's to come from EarthScope." 

Researchers have for years been constructing tomographic images of Earth's crust and upper mantle from waves generated by earthquakes. That method, known as seismic tomography, reconstructs Earth's inner structure on a computer screen, slice by slice. The new technique is similar, but is based on organizing ambient seismic noise, which is typically discarded as seismic "garbage." 

Seismic tomography is like doing a medical CT scan of the Earth, Ritzwoller said. But when people have a CT scan, doctors are in control and can make images at will. Seismologists can't control when an earthquake happens, so they can either wait for another one or set off explosives to create their own image-generating waves. 

"To move beyond these limitations requires observational methods based on seismic sources other than earthquakes, which is what our method offers" says Shapiro

REFERENCE: www.geolsoc.org.uk

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