Mar. 24, 1998
BERKELEY, CA - Using seismic wave data gathered from tens of thousands of earthquakes, researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory have produced the first three-dimensional image of the Earth's entire structure, from the crust to the inner core. In creating their model, Don Vasco and Lane Johnson of the Lab's Center for Computational Seismology, found evidence that the outer core is not homogeneous, as has been long hypothesized. This information could help understand the Earth's magnetic field, according to the researchers. Their findings have been published in the February 1998 issue of the Journal of Geophysical Research.
The researchers used seismic data collected during the 1960s, '70s and '80s and measured the time the waves took to travel from the epicenter of each earthquake to seismographic stations located around the world. By using computers to analyze travel times from some 40,000 earthquakes, Vasco and Johnson were able to characterize the seismic velocity of materials which make up our planet.
"What we did is sort of like performing a CAT scan on the planet," Vasco said. "Just as a CAT scan uses thousands of rays to characterize a part of the human body, we used thousands of waves to characterize the makeup of Earth."
Until now, most researchers have focused on one region, such as the mantle, rather than the entire structure of Earth. Although they don't claim to have any definitive answers, Vasco said the work is another step in determining what the Earth's makeup is and how its structure affects our world. The three-dimensional structure of the Earth's mantle has only been determined over the past 20 years and now scientists are digging deeper and studying the inner and outer cores. It is thought that the outer core, which starts about 3,000 kilometers (1,850 miles) below the Earth's surface and is 2,300 km thick, is a liquid, with the viscosity not much different from water. This led some to conclude that the outer core has no real structure.
"We found indications of heterogeneity at the bottom of the outer core," said Vasco, describing the material as a iron-nickel-sulfur compound. High pressures and temperatures could be causing nickel-rich iron to solidify and depleting the nickel at the base of the outer core, Vasco said, which could help explain the Earth's magnetic field. The depleted iron is less dense than the surrounding core, causing it to rise, leading to convection and a magnetic field.
"We're interested in this area because there has been some recent modeling of the Earth's magnetic field. Their research found a rough symmetry in structure around the Earth's rotation axis, and this agrees with the symmetry of the magnetic field.
"None of this is unambiguous," Vasco says. "The big problem is trying to see inside the Earth, looking through the very heterogeneous crust and mantle at a volume that is relatively small."
While most of the research to date has been carried out on powerful desktop computers, Vasco is starting to use Cray T3E supercomputers at Berkeley Lab's National Energy Research Scientific Computing Center for his next stage of research. That work calls for analyzing other types of seismic data to check and refine their findings.
"Nobody knows for certain how the whole thing works," Vasco said of the Earth's interior.
Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, Calif. It conducts unclassified research and is managed by the University of California.
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