They report their observation in the July 18 Nature . The surface of the core, at its equator, is moving about 20 km per year relative to the liquid outer core. In contrast, the continents creep across Earth's surface at only a few centimeters per year. Gary A. Glatzmaier of Los Alamos says he was surprised that Song and Richards could actually observe the core's rotation. The Lamont-Doherty scientists could not have detected the core's movement without the help of recent discoveries about inner Earth.
Waves go most slowly in the plane of the equator and most quickly when heading poleward.
To explain the differences in speed, researchers proposed that the iron crystals in the inner core have a distinct orientation, like the grain in wood. Waves moving along this crystalline grain travel faster than those going against it. In the last 4 years, seismologists have uncovered evidence that the grain is skewed slightly from an exact north-south alignment. Song and Richards realized that this axis of anisotropy, which currently points toward Siberia, should shift over time if the core rotates separately. To test the idea, they pulled out old recordings made in central Alaska of earthquakes that occurred in the South Atlantic.