News Release 05-151
Earth's Core Rotates Faster Than Its Crust, Scientists Say
Proof ends longstanding debate
August 26, 2005
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Scientists have ended a long debate by proving that Earth's core rotates faster than its surface.
Their research measured differences in the time it took seismic waves generated by nearly identical earthquakes to travel through Earth's inner core.
According to geologists Jian Zhang of the Lamont-Doherty Earth Observatory (LDEO), Xiaodong Song of the University of Illinois at Urbana-Champaign and other co-authors of a paper in the Aug. 26 issue of the journal Science, Earth's iron core is rotating approximately 1 degree per year faster than the rest of the planet.
"Whether the Earth's core spins faster than its surface has been a hotly debated topic," says Robin Reichlin, program director in the National Science Foundation (NSF)'s Division of Earth Sciences, which funded the research. "These new observations provide compelling support that it does."
The scientists studied waveform doublets--earthquakes that are detected at the same seismic recording station in two different places, at two different times. A Sept. 2003, earthquake in the Atlantic Ocean near the South Sandwich Islands that was also detected in Ala., provided a near-exact match with one that had occurred in Dec.1993.
The seismograms were almost identical for shocks that had traveled only in the mantle and outer core. But seismic waves that had traveled through the inner core looked slightly different: they had made the trip through the Earth faster in 2003 than in 1993.
"The similar seismic waves that passed through the inner core show changes in travel times," says Song. "The only plausible explanation is the faster rotation of the inner core."
In all, the geologists analyzed 18 "doublets" from the South Sandwich Islands that were detected at Ala. seismic stations between 1961 and 2004.
"For decades, people thought of the Earth's interior as changing very slowly over millions of years," said scientist Paul Richards of LDEO, a co-author of the paper. "These results show that we live on a remarkably dynamic planet. They also underscore the fact that we know more about the moon than we know about what's beneath our feet. Now we need to understand what is driving this difference."
In addition to Zhang, Song and Richards, co-authors of the paper are Illinois graduate students Yingchun Li and Xinlei Sun and research scientist Felix Waldhauser. The work was also funded by the Natural Science Foundation of China.
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