Alaska Seismometers Clock the Spin of Earth's Inner Core

At the bottom of the world, an earthquake shakes the South Sandwich Islands. Waves of energy travel through the center of the earth, reaching Alaska in about 20 minutes. Seismometers near Fairbanks feel the shudder.

By studying this pathway of earthquake vibrations through the earth, scientists at Columbia University in New York recently discovered that the earth's inner core, a 500-mile ball of iron, is moving faster than the earth's surface. This spinning ball-within-a-ball may be a major force generating the earth's magnetic field.

Using earthquake information gathered over the last 30 years at the College International Geophysical Observatory in Fairbanks, seismologists Xiaodong Song and Paul Richards calculated that earthquake vibrations from the South Sandwich Islands are reaching Alaska faster as the years progress. Speedier seismic waves tell the researchers something is going on deep within the planet.

Although no one has dug a 3,000-mile tunnel to the center of the earth (seven miles is the limit for modern drills), years of observations lead scientists to believe the earth is composed of a series of shells. The innermost layer is an orb of solid iron slightly smaller than the moon, Song said. The inner core was formed, and grows, by pressure Song said is 3 million times greater than the force exerted on us by the earth's atmosphere.

The solid inner core of the earth is surrounded by a sea of molten metals known as the outer core. The outer core is capped by the mantle, a thick layer of dense, solid rock. The crust, a 12-to 40-mile layer where earthquakes occur, is the shell of the earth upon which we walk our dogs.

Song and Richards determined the inner core is rotating under our feet by more than 12 miles each year at its equator. They did so by counting the seconds it took earthquake waves from the South Sandwich Islands, located between South America and Antarctica, to reach the College Observatory. Along that pole-to-pole route, seismic signals pass through the inner core. Signals pass fastest through the inner core at a certain angle Song calls the "axis of anisotropy."

Song and Richards' study indicates that South Sandwich earthquake waves reach Alaska 0.3 seconds quicker now than they did in the 1960s. By attributing that change to the shift in the inner core's axis of anisotropy, the researchers tallied the accelerated spin of the inner core.

Alaska got involved in the discovery because it's located at the top of the globe and the College International Geophysical Observatory provides years of accessible data, Song said. Jack Townshend, long-time chief of the College Observatory (which became the College International Geophysical Observatory in 1996), said seismic signals have been recorded there since the late 1940s and at other University of Alaska Fairbanks locations since 1955. Song plans to gather the College records of earthquakes around the South Sandwich Islands in the 1940s and 1950s.

"We should see larger (earthquake signal travel time) differences between now and then," Song said.

So what does it all mean, this iron moon spinning within the earth? If we learn more about the inner core of the earth, we learn more about earth's magnetic field, Song said. The energy given off as the molten outer core freezes to the solid inner core drives the motions of the outer core. This motion creates the earth's magnetic fields, Song said. With the revelation that the inner core is spinning, Song and Richards have defined a large moving part within the earth's magnetic-field generator.

The earth's magnetic field polarizes the planet. This make compass needles point north. It also makes winter life in the north a bit more bearable by helping to create the auroral oval, the ring-shaped region around the North Pole where the northern lights dance.