Tracking the Movement of Earth's Plates

Seward is migrating toward Fairbanks.

In the past year, the coastal community on the Kenai Peninsula has moved 35 millimeters-about one-and-one-half inches-closer to Fairbanks. That won't save you much gasoline on your next drive to Seward, but the creeping movement of areas all over Alaska is of keen interest to those who study the forces that cause earthquakes.

Jeff Freymueller, an assistant research professor of geophysics at the Geophysical Institute, tracks the fingernail-growth speed of Earth's plates with the help of satellites and Global Positioning System receivers.

The Global Position System, GPS, is a system of 24 satellites operated by the U.S. Air Force Space Command at Peterson Air Force Base, Colorado. As all 24 satellites zip around the globe at an altitude of about 12,500 feet, they broadcast radio signals that are picked up by GPS receivers, some of which can fit in the palm of your hand. A computer within the GPS unit instantly compares the distances from the satellites to the receiver, then calculates the geometry and pinpoints the location of the GPS unit.

Some airplane pilots and ocean-liner captains have called the GPS the greatest invention since the wheel. Scientists find it useful, too.

"It's opened up an entire new way of studying active processes within the earth," Freymueller said.

For more than a decade, Freymueller has used GPS to track the movement of Earth's plates--gigantic fragments of rock and soil that are more than 1,000 miles across and are up to 40 miles thick. About a dozen plates make up the Earth's crust. Alaska is a seismic hot spot because much of the state sits on or near where these plates meet. The Pacific Plate collides with and dives below the North American Plate along the Pacific coastline of Alaska. Movement of the plates in 1964 caused the Good Friday Earthquake.

Each year, Freymueller and his colleagues travel to the same spots off the Seward and Sterling highways. After hiking in to remote areas, they set up GPS receivers to find out how much each area has moved in relation to another area. These GPS units alerted Freymueller to Seward's snail crawl in the direction of Fairbanks. Other measurements showed the complexity of Earth's movement in Alaska: Sterling, a town about 50 miles from Seward, hasn't moved a millimeter toward Fairbanks; Nikiski is moving about a centimeter a year in the opposite direction Seward is moving. All the measurements are accurate to within 2 to 4 millimeters, Freymueller said.

Some of the movement Freymueller records is due to the motion of Earth's plates, and some is due to ground motion along faults, weak areas in Earth's crust that give way during an earthquake. Along fault lines, the ground surface usually "sticks" with the friction between rocks, Freymueller said. Though the surface along a fault often doesn't move except during an earthquake, warmer rock about 10 to 12 miles beneath the fault continues moving, as does the ground surface farther away from the fault. By measuring the movement of Seward, for example, Freymueller said he's measuring motion that will also be seen along local faults during an earthquake.

By clocking the rates Alaska towns and other spots are moving in relation to one another, Freymueller hopes to better understand the interaction of the Pacific and North American plates where they meet under Alaska.

The GPS measurements also have confirmed the apparent encroachment of Anchorage. Though Anchorage probably slipped away from Fairbanks during the 1964 earthquake, in years without a major earthquake Alaska's largest city creeps toward Fairbanks about a centimeter every year. Don't worry. At that rate, the two cities won't collide for another 40 million years or so.