Alaska Earthquakes Reveal Earth's Wandering Ways

Alaskans who live in the Interior were recently reminded that the earth isn't a passive, lifeless hunk of rock and soil. Many people who live near Fairbanks now remember precisely what they were doing at 9:23 p.m. on October 5th when a magnitude 6.2 earthquake rattled buildings and provided conversation fodder for the next day.

Earthquake stories are common in Alaska because we live in the most seismically active state in the U.S., by far. Three of the largest earthquakes in recorded history have happened here, including the second-strongest quake ever recorded. The March 28, 1964, earthquake--a whopping magnitude 9.2--destroyed much of Anchorage, Valdez and Cordova.

There are good reasons why Alaska shakes more than any other state. The earth's crust is made up of about a dozen or so plates--gigantic fragments of rock that are more than 1,000 miles across and are up to 40 miles thick, according to Bruce Bolt in his book, Earthquakes. Although we consider the ground beneath us to be relatively stable, the surface we walk our dogs on, drive to work on, and build our houses on is moving.

To visualize plates, imagine the earth as an orange. A woman peels the orange, ending up with 12 pieces of orange rind. She then glues those sections of peel back on the orange from where she pulled them off. The peel that made up most of North America represents the North American plate, and the peel that covers most of the Pacific Ocean is the Pacific plate.

Alaska is a seismic hot spot because much of the state sits on or near where these plates meet. The Pacific plate collides with the North American plate along the Pacific coastline of Alaska as both plates creep along, fueled by heat generated deep within the earth's core.

The Pacific plate moves northwest past the panhandle of Southeast Alaska and then slides beneath the North American plate at southern Alaska and the Aleutian Islands. The plate travels about 2.5 inches a year, or about as fast as human fingernails grow, according to Alaska Deputy State Seismologist Charlotte Rowe, who works for the Earthquake Information Center at the Geophysical Institute.

The energy of moving plates is transferred to rocks, which would groan at the strain if they could. Rocks are surprisingly elastic though; they compress and bend with the incredible force a plate applies. After what may be hundreds or thousands of years, the built-up pressure causes a weak section of rock, known as a fault, to violently fail. We feel this huge release of energy as an earthquake.

The spot at which rock first slipped against rock, called the hypocenter, is usually miles below the ground surface. Shock waves created by the rupture travel outward from the hypocenter in all directions like ripples from a rock dropped into a pond. Seismologists mark the ground surface above the hypocenter with a dot known as the epicenter.

Aftershocks, smaller earthquakes that follow a larger one, occur as the stress from a fault is transferred to smaller rocks. The smaller rocks then crumble for a few days until friction causes movement along a fault to cease.

Particularly strong earthquakes happen at plate boundaries, such as the Pacific coast of North America. As the Pacific and North American plates slowly collide, major earthquakes occur along their meeting place from Alaska to the California coast and southward. Since so much of Alaska straddles the border of two plates, we have a lot more earthquake potential than other areas of the U.S. We live in a place where we're often reminded the earth is a dynamic planet--even the seemingly still surface below our feet.