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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