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.