Earthquakes in the Nenana Basin in Interior Alaska last longer and feel much stronger than quakes of comparable magnitude in other places.
University of Alaska Fairbanks scientists are studying why.
It’s all about the reverberation.
Below the beautiful lakes and swamps in the Minto Flats region is a giant bowl, and it’s filled with gravel. UAF Geophysical Institute seismology professor Carl Tape said to imagine the flat surface of the land like the surface of the ocean, and if you removed the water, you’d see interesting things revealed.
“If you remove all this dirt that’s been deposited, there’s a lot of interesting features,” he said. “These deep bowls, depression, exists west of the town of Nenana, where the Tanana River kind of takes a turn and flows north, toward the Minto Flats area before it goes west and eventually into the Yukon. So in that area of Minto Flats, what looks very flat and normal at the surface has a lot of really interesting features beneath.”
Tape worked with graduate student Kyle Smith, placing 13 seismic monitors across the area over four years, from 2015 to 2019. In that time, they collected data from 48 local and regional earthquakes. The monitors, the first seismic stations installed in Minto Flats, are part of the Fault Locations and Alaska Tectonics from Seismicity project funded by the National Science Foundation.
“That area has not been really studied that much,” said Smith. “And we wanted to know how the basin moved, because people want to do a lot of projects in that region. So it’s good to know what happens when there’s a big earthquake.”
They found that the seismic waves get amplified as they bounce back and forth off the sides and bottom of the sedimentary basin. So people in the flats perceive the earthquakes as bigger than they actually are. They also incorporated discussions with people who live and work in Minto into their research.
The basin west of Nenana and south of Minto is 56 miles long and 7.5 miles wide. It was filled in over millions of years with sediment brought by the rivers from the Alaska Range. Smith said it could be the deepest such basin in Alaska.
“ It’s up to seven kilometers deep, so that maybe about four miles,” Smith said.
That is deeper than Denali is high.
The seismic monitors recorded earthquakes lasting longer on the gravel-filled basin than on harder ground. Both Tape and Smith noted the measured difference between the Nenana Ridge under the Parks Highway, and the flats below.
“The ridge between Nenana and Fairbanks, when you drive along that, you’re high up on pretty close to rock. The ground moves very differently in that kind of material than it does down in Minto Flats.”
“There’s certain places that, because of the exceptional topography below ground in this case, is big basin Bowl amplifies the ground motion and makes it last longer. And we know that because we put the stations out there to record earthquakes,” Tape said.
“The shaking is like 10 times less compared to if you were just downhill from that. So it’s pretty amazing how stark the difference is caused by whatever the underlying geology is,” Smith said.
Smith was raised in the Navajo Nation, got his Ph. D. at UAF, and is on a new research assignment in Taiwan.
Tape said many UAF scientists are targeting the basin for research.
“But the same features are what led, you know, like Doyon, for example, to drill exploratory wells and acquire geophysical data because of the prospects for oil and gas,” Tape said.
Although the research shows how the ground would likely move during certain kinds of earthquakes, Smith cautions that actually predicting earthquakes is a long way off.
“ We will have a better idea of how much the ground will be shaking from some kind of earthquakes, but that doesn’t tell us when and where the earthquake is occurring,” Smith said.
Smith will give a talk on the research at Sandia National Laboratories in May.
