Denali Fault quake still source of study a decade later
FAIRBANKS — The Denali Fault earthquake only rumbled for about 3 minutes when it shook Interior Alaska a decade ago, but some of the reverberations of that powerful seismic event are still being felt today.
The 7.9 magnitude quake, which rattled the area Nov. 3, 2002, has been the subject of research and fascination since. Scientific studies of the quake, whose epicenter was about 90 miles south of Fairbanks, spawned new national standards for building bridges and are providing a deeper understanding of how earthquakes affect frozen ground.
“Really, this last decade has been huge for understanding earthquake hazards in Interior Alaska as a whole, and the Denali Fault in particular,” said Peter Haeussler, an Anchorage-based research geologist with the U.S. Geological Survey.
The Denali Fault quake was a monster — the largest inland earthquake in North America in nearly 150 years — and its west-to-east shockwave was powerful enough that it was felt as far away as Louisiana. Roads were sheared apart along the fault line in the Interior, and some glaciers literally were ripped in two.
Natasha Ruppert, the acting state seismologist with the Alaska Earthquake Information Center, said geologists always had known that the Denali Fault had the potential to produce a huge quake. But the 2002 temblor gave scientists an opportunity for study that hadn’t previously existed.
A decade later, research connected to that quake has affected the way seismologists look at earthquakes throughout the world.
“If it wasn’t for the 2002 earthquake, if the fault had never ruptured, that research would definitely be a low priority over something else,” Ruppert said.
Haeussler said the side-to-side slipping action of the Denali Fault earthquake made it of particular interest to researchers. There were only about a half-dozen quakes of comparable size last century of that variety, known as strike-slip earthquakes.
During a strike-slip earthquake, an object on one side of the fault could end up 15 feet to the right or left of an object that it previously was right next to. Other types of earthquakes involve land either being pushed together or pulled apart along a fault line.
Another strike-slip fault line is the San Andreas Fault in California. Understanding the behavior of those types of earthquakes is a high priority, since the San Andreas passes through some of the country’s most heavily populated areas.
Researchers from California, which is in the process of revising its seismic maps, flocked to Alaska in 2002 to research a fault that exhibits the same characteristics.
“I think that grabbed people right away,” Haeussler said.
Another notable aspect of the Denali Fault quake is that it traveled along three separate fault lines — the previously undiscovered Susitna Glacier Fault, the Denali Fault and the Totschunda Fault.
Some researchers had thought an earthquake would come to an immediate halt at the intersection of fault lines. After seeing the 2002 earthquake clearly break that rule — moving more than 200 miles while shifting along three faults — that thinking has changed, Haeussler said.
“It just became blatantly obvious that you could have one rupture that goes from the next fault to the next to the next,” he said.
The Denali Fault earthquake is perhaps most notable for its effect on cold-weather seismic research. The way Interior bridges responded during the quake offered a unique look at how frozen soil behaves during a seismic event. Some of the revelations led to new standards for how bridges are built in the U.S.
For a bridge on unfrozen ground, the area at risk for structural failure during an earthquake is far underground. Following the Denali Fault quake, researchers found that the weak point on frozen soil — which acts like concrete during an earthquake — is just below the surface.
“I think the engineers knew that frozen layer was different,” said Zhaohui “Joey” Yang, associate director at the Alaska University Transportation Center. “They didn’t know how different.”
The earthquake also offered new insights into soil liquefaction, which was first observed during the 1964 Good Friday Earthquake in Southcentral. While the surface layer of soil stays frozen, the sand beneath it turns to liquid during a seismic event, causing the frozen crust to suddenly shift.
“If you happen to have your bridge in this layer, your bridge is gone,” Yang said.
The earthquake also was notable for pointing out some untested design successes.
When the trans-Alaska pipeline was built in the mid-1970s, engineers knew they’d need to contend with earthquake zones along the way, most notably the Denali Fault. The pipeline was built with a zig-zag design along much of its route, allowing it to stretch and move if the ground beneath it shifted.
When the Denali Fault earthquake hit, the pipe slid sideways 18 feet — two feet less than its limit — and moved vertically five feet. Haeussler said it’s the first time a structure designed to span a fault had survived a major quake.
“They pretty much nailed it dead on,” Haeussler said of the pipeline engineers.
The pipe wasn’t realigned after the 2002 earthquake, said Alyeska Pipeline Services Co. spokeswoman Lynda Sather, but additional horizontal supports were installed so it could withstand more movement.
“The pipeline performed as designed, so we really didn’t have to change anything,” Sather said. “It did what we wanted it to do.”
Mike Dean, the chief structural engineer for the Fairbanks architecture firm Design Alaska, said the Denali Fault quake didn’t offer a major shakeup in local building designs.
Builders in the U.S. had their standards overhauled a decade earlier, when damage from the Northridge Earthquake in California caused the industry to take a new look at its building codes. Because of those tightened standards, Dean said, the effect of the Denali Fault earthquake was roughly a third of what the firm’s newest buildings were designed to withstand.
When is the next one?
A decade after the Denali Fault quake suddenly rattled the Interior, one thing still is tough to predict: Will Fairbanks ever see anything like that quake again?
Haeussler said that by examining the accumulated sediments along a fault — a process known as paleoseismology — scientists can gauge how often quakes have occurred historically. The Denali Fault has had a rupture about the size of the 2002 quake every 400 to 600 years, he said.
It means that the part of the fault that shifted a decade ago probably is done with major moves for the rest of our lifetimes — but the Fairbanks area isn’t necessarily in the clear. Numerous other secondary faults are scattered throughout the Interior, and most of them are largely unstudied. Haeussler said one of them caused a 7.3 magnitude quake near Fairbanks in 1937, and events like that remain a constant possibility.
“People have tried to understand (those faults) better,” Haeussler said. “They’ve been more elusive about revealing their mysteries.”