WASHINGTON - The giant iron ball at the center of the Earth appears to be spinning a bit faster than the rest of the planet.

The solid core that measures about 1,500 miles in diameter is spinning about one-quarter to one-half degree faster, per year, than the rest of the world, scientists from Columbia University's Lamont-Doherty Earth Observatory and the University of Illinois at Urbana-Champaign report in Friday's issue of the journal Science.

The spin of the Earth's core is an important part of the dynamo that created the planet's magnetic field, and researcher Xiaodong Song said he believes magnetic interaction is responsible for the different rates of spin.

The faster spin of the core was proposed in 1996 by two of the current study's authors, Paul Richards of Lamont-Doherty and Song, now an associate professor at Illinois.

The researchers studied the travel times of earthquake waves through the Earth, analyzing what are called couplets. Those are earthquakes that originate within a half-mile or so of one another but at different times.

They analyzed 30 quakes occurring in the South Atlantic and measured at 58 seismic stations in Alaska and found differences in the travel times and shape of the waves, indicating differences in the core as the waves passed through the center of the Earth.

Analyzing those differences, they calculated that the core is spinning slightly faster than the rest of the planet and is a bit lumpy.

That solid inner core is surrounded by a fluid outer core about 4,200 miles across.

Since the planet is divided into 360 degrees of longitude, a core spinning one-quarter to one-half degree faster than the outer surface could take between 700 and 1,400 years to get one full revolution ahead.

But Song said in a telephone interview that he expected that rate to vary over time and sometimes the core might be spinning slower than the rest of the planet.

"What we see right now is a snapshot of a long time process between the magnetic field and the inner core," he said. "I do expect to see this rate change with time."

"What is surprising for us is that we could actually see it in such a short time scale," he said, noting the measurements had been made over less than a decade.

Geologists are used to thinking in terms of thousands or millions of years for geological processes, he said.

The work was funded by the U.S. National Science Foundation and the Natural Science Foundation of China.

The initiation and propagation of earthquake ruptures depend upon the mechanical behavior of fault rocks and fluids at depths of several kilometers or more. Using borehole geophysical measurements in conjunction with laboratory studies, USGS scientists determine the temperature, stress, and fluid-pressure conditions at the depths where earthquakes occur and characterize the mechanical behavior of fault-zone materials at realistic in-situ conditions. This knowledge is combined with surface-based geophysical observations, measurements of tectonic strain accumulation, and other information to yield improved models of the earthquake cycle.

A major goal of the rock physics laboratories at the U.S. Geological Survey is to improve our understanding of the physics of seismogenic faulting. Laboratory studies of rock properties are conducted under pressures and temperatures that simulate conditions deep in the earth where earthquakes are generated. These studies include strength and frictional behavior of rocks and fault zone materials, the velocity of seismic waves through rock, as well as the role of fluids and fluid flow in fault zones.

How is this data used?

Information on rock properties is combined with other geophysical observations to improve our models of the earthquake process, such as earthquake triggering, recurrence, rupture propagation, and ground motion. This in turn is necessary to understand earthquake hazards and risk in earthquake-prone areas.

• Effect of water on the strength of fault gouge minerals
• How mineral reactions at high temperatures can change rock permeability
• Strength and permeability studies of core samples from the Nojima Fault in Japan (1995 Kobe earthquake)
• Shear wave splitting in foliated rocks
• Understanding tidal triggering of earthquakes
• Laboratory models of aftershock patterns
• Changes in physical properities of rocks prior to simulated earthquakes:
   -Permeability and dilatancy
   -Electrical resistivity
   -P and S wave velocities and wave attenuation
   -Microseismicity patterns (acoustic emission)