Finding the Foundation of the Alaska Range
“Stomp your foot,” said Doug Christensen, a seismologist with the Geophysical Institute, as we stood in a grassy clearing north of Healy. I obliged, striking the earth three times with the sole of my boot.
“Yeah, that looks good,” he said while looking at a tiny computer screen. Three bursts of squiggly lines showed the seismometer he and his coworkers had installed was picking up subtle vibrations felt within the ground. Christensen, graduate student Liz Meyers, and others are installing 36 seismometers to take the pulse of central Alaska. One of their goals is to find the foundation of the Alaska Range.
Christensen, State Seismologist Roger Hansen, and Geoff Abers of Boston University received funding from the National Science Foundation to install temporary earthquake-detection sites along the Parks and Denali highways, the Petersville Road, and the Denali National Park road all the way to Wonder Lake. When they finish in early June, the scientists will have planted seismometers about every 10 kilometers in a line from Nenana to Talkeetna.
The seismologists dig holes and cement seismometers in muskeg, tundra, and the forest floor on a line that cuts north-south through the Alaska Range. After installing seismometers about the size of a loaf of bread inside a plastic barrel and snaking a few wires outside to a set of batteries and solar panels, the seismologists test the stations by stomping the ground and looking for a response on a small computer screen. The scientists will visit the stations every few weeks this summer to retrieve a disc of information and replace it with a new one.
With the new, tightly spaced network, the scientists will get a precise view of what’s going on beneath the Alaska Range. What underlies the highest mountain in North America is still a mystery, but scientists now have a few guesses. Mountains are often like icebergs, Christensen said, composed of light rock held up by a much larger mass of rock beneath the surface. Mt. McKinley and other Alaska Range mountains may be floating on an incredible chunk of rock below. Another theory is that pressure within Earth’s crust, a 20-mile thick layer of rock, may be creating enough tension to suspend the Alaska Range at its current height. Mountains held up by this means sometimes shrink when earthquakes relieve surface pressure.
At the end of this summer, the scientists should have all stations running in a string across the Alaska Range, in locations ranging from a teacher’s backyard in Talkeetna Junction to a patch of woods near Anderson. Seismometers at the sites will pick up earthly vibrations from around the world, such as the signal from a 7.2 magnitude earthquake in Turkey. Meyers recently gave the teacher in Talkeetna Junction a printout of seismic waves from the Turkey earthquake recorded in his backyard. Another station recorded the footfalls of a bear that left its teeth marks on equipment Meyers checked after the bear’s departure. Though the bear’s vibrations won’t help the seismologists discover Mt. McKinley’s foundation, there’s a pretty good chance that earthquakes around the world will.
