News — 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 researchers will produce quicker, cheaper and clearer images.
Rather than waiting for earthquakes, the researchers have now recovered surface wave information from ambient seismic noise that is constantly produced by fluctuations in the Earth's atmosphere and oceans. Measuring surface waves is important because it helps scientists get a clearer picture of the Earth's interior, according to Michael Ritzwoller, director of CU-Boulder's Center for Imaging the Earth's Interior and a physics professor.
"This new technique will give us a better fundamental understanding of the planet by providing much better resolution of Earth's interior," Ritzwoller said. "It also will diminish what is known in seismology as the 'tyranny of earthquakes,' which means having to wait for an earthquake to happen to do our jobs."
The new method promises significant improvements in the resolution and accuracy of crustal and upper mantle images down to 60 miles or more within the Earth, particularly when used in tandem 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.
A paper on the technique appears in the March 11 issue of the journal Science. Co-authors include Ritzwoller of CU-Boulder and Michel Campillo and Laurent Stehly of the Laboratory of Geophysics and Tectonophysics at the Joseph Fourier University in Grenoble, France.
In a process similar to a medical CT scan, researchers have for some years been constructing tomographic images of Earth's crust and upper mantle from waves generated by earthquakes. This method, known as seismic tomography, reconstructs Earth's inner structure on a computer screen, slice by slice. The new CU-Boulder method is similar, but is based on organizing ambient seismic noise, which is typically discarded as seismic garbage.
While the new method offers no help in predicting when earthquakes will happen, it can provide information for risk assessment. And it will be a very useful tool for USArray researchers, Ritzwoller said.
"The risk that the public faces is not only from the earthquake, but how waves emanated from the earthquake interact with the inner-structure of the Earth," Ritzwoller said. This new measuring technique, coupled with existing and emerging technology such as USArray, will lead to a better fundamental understanding of the structure of the planet and may help save lives in the process, he said.
A component of the National Science Foundation's program EarthScope, USArray is a massive seismic project using hundreds of portable seismometers that in coming years will be moved across the entire country, producing images of the Earth's interior. The project's goal is producing new data about the Earth's interior, including earthquake risk assessment. The CU-Boulder findings should be extremely useful to USArray researchers, according to Ritzwoller.
Seismic tomography is like doing a medical CT scan of the Earth, Ritzwoller said. During a CT scan, the body goes into the machine, which takes multiple X-rays and then uses a computer to construct cross-sectional views of the body or body parts. Seismologists have previously relied on waves generated from earthquakes to reconstruct images of the inner Earth.
However, 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, or they can set off explosives to create their own waves to generate images of the Earth's interior.
But each of these methods has drawbacks. "To move beyond these limitations requires observational methods based on seismic sources other than earthquakes, which is what our method offers," said Shapiro.
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