Carpeting the Denali Fault with earthquake sensors

Author: 
Ned Rozell
Article Number: 
2,305
April 21, 2016

NEAR MILLER CREEK — Crouching amid scratchy spruce branches and surrounded by feet of snow, Amir Allam jabs half-frozen soil with the spikey base of a white cylinder. The seismologist twists the 6-pound seismometer to orient it northward. Then he clicks a cable to a magnetic connection on top.

"Starting operation," says a tinny voice that sounds like a woman from London. The words come from a thick tablet attached to the cable. In less than 10 minutes, Allam has deployed another shake-detection instrument on one of Alaska's greatest earthquake producers, the Denali Fault.

Allam, assisted during this season of bear emergence by shotgun-toting UAF student Nick Lock, will install dozens more seismometers in a dense grid straddling the fault. A weak point in Earth's crust ruptured here in 2002, slicing the highway and shoving the nearby Trans-Alaska Pipeline on its extra-long rails.

A team of five including seismologist Carl Tape of the Geophysical Institute installed almost 200 of the instruments during three sunny days in mid-April. They favored shoving the seismometers into ground at the base of spruce trees, where there was less snow to shovel and ample soil to receive the spikes.

The fading snowpack was still about four feet deep. Cold nighttime temperatures formed a crust that could support the scientists' weight until early afternoon.

"Conditions are pretty good right now," Lock said at noon, "But once everything starts melting they're the worst."

"We've sunk in chest deep," says Allam, who, like Lock, is big as a bear.

In their backpacks, Allam and Lock each carry as many as a half dozen rugged little seismometers owned by researchers at the University of Utah, where Allam works (he was a postdoctoral scholar at UAF). Allam asked his advisor Fan-Chi Lin if he could bring the instruments to Alaska and learn more about the Denali Fault.

Lin said yes. Carl Tape had the local connections to shelter the science team in a log cabin belonging to a friend. The plan was on.

Allam and Tape will leave the seismometers in place for one month. They'll return in May to pick them up in a season with less snow and more mosquitoes.

In those 30 days, the instruments will record hundreds of earthquakes. The data from the dense network should give seismologists a better idea of the Denali Fault's character.

They already know huge earthquakes happen on the fault. A rupture there caused a magnitude 7.9 earthquake on November 3, 2002, tearing a 200-mile line across the face of Alaska, through soil and glacial ice.

The fault is an ancient trench through central Alaska maintained by Earth's crustal forces shoving in opposite directions. Framed by the mountains of the Alaska Range, the Denali Fault is easy to spot on a map.

"It's a big frowny face in the middle of Alaska," Allam said.

The scientists want to see what the fault looks like beneath the surface. They hope to define the damage zone, a band of rocks broken by past earthquakes. There, energy waves can be trapped during an earthquake, intensifying the shaking.

While Allam and Lock blanketed part of where the fault ripped the surface during the 2002 earthquake, Tape and graduate students Kyle Smith and Yadong Wang worked closer to Canwell Glacier, which lies in a valley maintained by the Denali Fault. In brilliant sunshine, they snowshoed over piles of gravel shoved by the glacier. They stopped at spruce trees, placing their instruments beneath them.

Dozens of seismometers now rest quietly in forest and near glacier in the middle of Alaska, waiting for the ground to speak.

Since the late 1970s, the University of Alaska Fairbanks' Geophysical Institute has provided this column free in cooperation with the UAF research community. Ned Rozell is a science writer for the Geophysical Institute.