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Whole lotta shakin’
Major Alaskan quake
rocks alum, seismologists

By Tim Stoddard

For most people, it would have been the wrong place at the wrong time, but not for Josh Stachnik. On November 3 at 1:12 p.m., Stachnik’s pickup truck began to dance a jig across a remote stretch of the Denali highway. Stachnick (GRS’02), a staff seismologist with the Alaska Earthquake Information Center, and two colleagues had been deploying seismometers that morning along the dirt highway in the hope of recording aftershocks from a magnitude 6.7 earthquake that had rattled the region on October 23. What they got was much more than an aftershock.

Geoff Abers, a CAS associate professor of earth sciences, with Josh Stachnik (GRS’02) doing fieldwork near Mount McKinley during the summer of 2000. Photo courtesy of Geoff Abers

“I had just driven through a big field of potholes,” Stachnik says, “when it felt like the truck was bucking me from the back end. It started jumping all over the road. I thought I had a flat tire or had lost a wheel or something so I stopped. I got out and realized that the trees were flopping back and forth and the truck was still swinging in the breeze.”

For the next three minutes, Stachnik felt the earth move under his feet as the most powerful earthquake recorded on the planet so far this year unleashed its seismic wrath across that sparsely populated region of Alaska. Not only was Stachnik one of the closest human beings to the epicenter of the magnitude 7.9 temblor, he certainly was the closest geologist. According to Geoff Abers, a CAS associate professor of earth sciences and Stachnik’s former thesis advisor, it was the largest earthquake to occur on land in the United States since 1857.

Centered approximately 75 miles south of Fairbanks on the Denali fault, the quake triggered countless landslides, but resulted in only minimal structural damage and amazingly few injuries. The 800-mile-long Trans-Alaska pipeline, which carries one million barrels of crude oil a day from Alaska’s North Slope to the lower 48, was shut down for several days after the quake broke a half-dozen energy-absorbing supports.

Most geologists would give their eyeteeth to have been in Stachnik’s shoes that quiet Sunday afternoon, experiencing the power of an M7.9 in a safe location. And the seismology community is now clamoring to see the data from the instruments that were in the right place at the right time. Until now, there have been only a few instances where seismometers were positioned close to the epicenter of a major earthquake, says Rachel Abercrombie, a CAS assistant professor of earth sciences, who studies the mechanics of earthquakes.

“The Denali quake was a big deal,” says Abercrombie. “The problem with our field is that we always want to have observations of earthquakes within a few kilometers of their starting point, and we can never predict where that will be. I think there are many seismologists who are downloading data from this quake right now, and if there are some nice recordings — which is perhaps what we’ll get from Josh’s work — then they could be really useful.”

Stachnik says that it’s too soon to assess the quality of this data because seismologists in Alaska are still scrambling to analyze their recordings and to deploy additional instruments to catch aftershocks. But the seismologists at the University of Alaska, Fairbanks, believe that the data from Stachnik’s instruments will shed light on some of the long-standing questions about earthquakes.

	The motion of the ground along the Denali fault was evident along roads such as this one. Photo by Akihito Ito

One of the main questions of interest to Abercrombie and other seismologists is how big earthquakes get started. Since the mid-1990s, geologists have debated two possible models. In the cascade theory, a major quake is triggered by a series of smaller ones that set off a chain reaction along the fault. The baby quakes can happen anywhere along the fault at any time, making it virtually impossible to predict when and where the big one will occur. An alternate theory called the nucleation model has been proposed by some seismologists. In this scenario, pressure builds along a region of the fault until the two sides begin creeping past each other at a weak point. The slow creeping then accelerates into a rapid slip that releases the earthquake’s energy.

Abercrombie has not yet perused the data from the November 3 quake, but she says that after glancing at data collected from seismometers in California, it looks like a small event immediately preceded the big quake. If this prelude has the seismic signature of a normal small earthquake, it would support the cascade model, she says. If it’s a peculiar creeping signal, however, then it will support the nucleation model.

The seismic waves from the quake reportedly sloshed lakes in Louisiana, Abercrombie says, and in Harvard, Mass., the waves shook a seismometer back and forth by about four centimeters. “The seismometers will be able to pick up the surface waves of this earth-quake on their first trip around the world,” she says, “then their second and third trips.” It takes about three hours for the surface waves to circumnavigate the globe. “The earth is essentially ringing like a bell after an earthquake of this magnitude, which enables people to use the seismograms to do all sorts of interesting things, like find out what’s in the inner core.”

In the meantime, seismologists will begin poring over the data collected from the Denali quake. Abercrombie says that there are already plans to devote a special session to this earthquake at the fall meeting of the American Geophysical Union. And for once, seismologists can express their enthusiasm over a big earthquake with a clear conscience. “The nice thing about this quake is that we can legitimately get excited because there were no serious casualties,” she explains. “So often in an earthquake like this, it’s a mixture of enthusiasm and tragedy. Now we can focus on the amazing event and not have to think about the victims of collapsed buildings.”

15 November 2002
Boston University
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