There were two main differences between the inner and outer waves.
First, and most prominently, the waves passing through the inner core moved faster in a systematic pattern over time. For example, waves passing through the inner core from a 2003 event arrived one-tenth of a second faster than those from a nearly identical 1993 event.
In addition, the waves themselves change in shape over timeanother sign of motion in the inner core, the researchers note.
"With those observations we can conclude that the changes must be from the inner core, and the reason we see these changes is because the inner core is moving," Song said.
Scientists attribute the change in travel time to how the waves interact with the inner core. Research suggests that the crystals in most of the iron core are structured in a grain, as in wood. Waves speed up or slow down depending on how they pass through the grain.
Song and colleagues believe that the "grain" has been steadily alligning itself in a way that provides a quicker path between the South Sandwich Islands and Alaska.
Their calculations suggest the inner core rotates at a rate of 0.3 to 0.5 degree per year faster than the mantle and crust. That's about 50,000 times faster than the tectonic plates move on the planet's surface.
"So, 0.3 to 0.5 degrees may not sound like so much, but within the solid Earth system, that is pretty fast," Song said.
The lumpiness of the inner core is thought to "scatter" the waves in different patterns, Song added.
Why the Core Spins
The heat of Earth's solid inner core churns the molten liquid outer core. The churning generates electric currents and, as a result, creates the planet's magnetic field.
Interaction between the electric current and magnetic field is thought to spin the inner core at the same rate that the liquid core just above it is turning, which tends to be slightly faster than the mantle.
Gary Glatzmaier is an earth scientist at the University of California, Santa Cruz. His 1995 computer simulations of the Earth's magnetic field suggested the inner core rotates more quickly than the crust.
The simulations inspired Song and Richards to look for observational data of the effect. Glatzmaier said their findings are "very important" for understanding Earth's magnetic field.
For example, Glatzmaier's most recent simulations, which include a gravitational torque between the inner core and mantle, suggest the inner core is rotating more slowly than these latest findings.
One possibility, he said, is that the value for the viscosity of the outer layer of the inner core is less than the figure he used in his computer model. Less viscosity at the surface of the inner core would allow the deep interior of the inner core to rotate faster, Glatzmaier said.
Glatzmaier noted that the new findings reflect the current rotation rate of the inner core.
"Many people think there's just one rate at which the inner core rotates. But that's unlikely. The rate likely keeps changing over time," he said. The simulations, he said, are only applicable for about the next 75 years.
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