Earth's Core Spins Faster Than Surface, Study Confirms

August 25, 2005

Analysis of nearly identical earthquakes that happened years apart proves that Earth's moon-size inner core rotates faster than the rest of the planet, a team of geophysicists report today.

The finding is "unambiguous" and should settle a nearly decadelong debate over the matter, said Xiaodong Song, a geophysicist at the University of Illinois at Urbana-Champaign.

Earth's iron core consists of a solid inner core about 1,500 miles (2,400 kilometers) in diameter—about the size of the moon—and a fluid outer core measures about 4,200 miles (7,000 kilometers) across.

The inner core plays an important role in the geodynamo—the process that generates Earth's magnetic field. Understanding how the inner core moves will allow scientists to better understand the geodynamo.

Song and Paul Richards of the Lamont-Doherty Earth Observatory in Palisades, New York, presented in 1996 observation data for the so-called super-rotation of the inner core.

The original finding was based on analysis of three decades of seismological records. Scientists have both confirmed and questioned the theory in the intervening years. Some scientists said the original finding could be a flaw in the data.

The newest research is reported in tomorrow's issue of the journal Science.

Kenneth Creager is a seismologist at the University of Washington in Seattle. He commented in an accompanying opinion piece that the research "removes any lingering doubt as to whether the inner core is rotating at a different rate than the mantle" (see diagram at right).

Identical Earthquakes

Song, Richards, and colleagues' evidence is based on side-by-side comparisons of seismic waves from 18 pairs of earthquakes that were nearly identical in location and magnitude. They differed only in date, and such similarity diminishes the margin for error.

The temblors occurred in the South Sandwich Islands near Antarctica and were detected by seismometers near Fairbanks, Alaska. On their pole-to-pole journey, the seismic waves traveled through the Earth, some through the inner core.

"Essentially, the waves that traveled through the area outside the inner core—the crust, the mantle, the outer core—are all the same," Song said. "Only when they travel through the inner core are they different."

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