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Geophysics
Super-rotation for Earth’s core

A new earthquake study supports the 1996 finding that Earth’s inner core is spinning faster than the planet’s mantle and crust — at a rate detectable on human timescales, but about one-third of the rate first suggested.

“It’s exciting to see something deep in the interior actually changing on a timescale comparable to one’s lifetime.”
Gary Glatzmaier,
University of California, Santa Cruz

Jian Zhang, a seismology doctoral student at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, N.Y., and Xiadong Song, a seismologist at the University of Illinois in Urbana-Champaign, reported in the Aug. 26 Science that the 2,440-kilometer-wide inner core is rotating 0.3 to 0.5 degrees faster per year than the mantle and crust. Researchers suspect that this rate changes over time; however, at the current rate, it would take the inner core about 900 years to “lap” the crust and get one revolution ahead.

Refinement of the rate is of particular interest to scientists who study Earth’s magnetic field, which is generated by convection and rotation in the fluid outer core. The magnetic field shields Earth from solar radiation and periodically reverses polarity (see Geotimes, September 2005). What exactly causes the “super-rotation” is not yet fully understood, but researchers believe that, gripped by the magnetic field, the inner core is being dragged along by the fast-moving outer core, while gravity from the mantle is holding it back.

“The better the measurements that seismologists can give us,” the more accurate the models will be, says Gary Glatzmaier, a geophysicist at the University of California, Santa Cruz, who first noted in 1995 that the inner core was rotating slightly faster in geodynamo models.

Zhang and Song compared “doublets” — matching seismic waves generated by pairs of similarly sized earthquakes that occurred in the same location but years apart. They examined the time it took for 18 doublets from 30 earthquakes that occurred between 1961 and 2004 in the South Atlantic, near the South Sandwich Islands, to travel to Alaska.

On average, they found that the waves arrived 0.0090 seconds faster per year, which they attribute to the changing orientation of the core due to rotation. Because the solid iron crystals of the inner core align, much like the grain of wood, the wave speeds are affected by whether the waves travel with or against the “grain.” Additionally, they note that variability, or “lumpiness,” in the core’s structure may also explain the travel time differences.

The 1996 study, by Song and co-author Paul Richards, a Lamont seismologist, used a similar method involving paired earthquakes, but was criticized by some researchers who noted that the quakes were not geographically close enough to produce suitably matched seismic waves. That study had calculated an inner core rotation rate of 1 degree per year faster.

The latest study, however, more precisely located similar quakes with matching waveforms, which indicate that the travel times have indeed changed, says Guy Masters, a seismologist at the Scripps Institution of Oceanography in La Jolla, Calif. However, Masters, who found an inner core rotation rate of 0.1 degree per year faster using a method that relies on seismic waves with much longer wavelengths, says that the final calculation of a rotation rate hinges on the variability in the inner core’s structure.

Still, the finding of changes in the core occurring over such short periods of time is quite significant, Glatzmaier says. “Most things change on a timescale much longer than 100 years, so it’s exciting to see something deep in the interior actually changing on a timescale comparable to one’s lifetime.”

Sara Pratt
Geotimes contributing writer

Links:

"Supercomputer models Earth’s magnetic field," Geotimes, September 2005

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