A bizarre "swarm" of earthquakes in recent weeks around the northwest Alaska village of Noatak is confounding scientists and unsettling residents who last felt a good shake more than 20 years ago.
A magnitude 5.7 earthquake that struck just after 4 a.m. Monday marks the fifth quake of exactly that size measured near the village since April, according to the Alaska Earthquake Information Center. Hundreds of aftershocks, some measuring magnitude 4 or greater, have also rippled through the area.
The seismic activity is about 12 miles northeast of Noatak, a village of about 500 people near the Chukchi Sea coast north of Kotzebue, and 25 miles south of the Red Dog Mine.
The shaking started on April 18 with two magnitude 5.7 quakes 12 minutes apart, according to the earthquake center at the University of Alaska Fairbanks. Three more of the same magnitude came on May 3, June 9 and Monday, along with over 300 smaller quakes.
"In April, when we had our first one, I woke up one morning, I was drinking my coffee, I saw it in my mind: I need to take my family to the hills, to the mountains," Alvin Ashby, a 48-year-old subsistence mapper for the Northwest Arctic Borough, said Monday.
"About an hour later, we had an earthquake."
The latest one, a 10-second rumbler, woke Ashby from a sound sleep the day after a successful bearded seal hunt on the Chukchi Sea coast.
"I took a shower, went to bed and woke up to a shake," he said.
This kind of sustained earthquake activity is highly unusual in this remote part of arctic Alaska. The last time a good-sized earthquake hit the area was the early 1990s, locals say.
There have been no reports of injuries or significant damage, but the shaking left cracks in the village council office, which is more than 20 years old and on stilts, said Dawn Schaeffer, the council's administrative assistant.
Local concerns prompted the borough to invite the UAF earthquake researchers to the village, Schaeffer said.
Natasha Ruppert, a seismologist at the earthquake information center, traveled to Noatak with a seismic technician a month ago to meet with residents, council officials and students. The tech installed a seismic sensor there and in Kotzebue, where the quakes are also being felt. There are about 400 seismic sensors around Alaska but few in northwest Alaska, Ruppert said. The closest to Noatak was at Red Dog Mine; the next was 300 miles away.
The new sensors will help researchers pinpoint the exact locations for earthquakes, she said.
Earthquake researchers are working with geologists to learn more about the tectonics of the region, Ruppert said. A database compiled by the Alaska Division of Geological and Geophysical Surveys shows no active fault structures for hundreds of miles but some very old faults mapped in the area.
"Maybe some of those faults are getting reactivated," she said, adding that the area is also near much more active tectonic areas on the Seward Peninsula.
Ruppert said she sees no connection between the earthquakes and mining activity at Red Dog, an open-pit surface mine that's one of the largest zinc concentrate producers on the planet and home to about 600 employees and contractors this time of year.
There are no volcanoes anywhere near Noatak, according to Cheryl Searcy, a seismologist and geophysicist with the Alaska Volcano Observatory. The closest volcanoes are in the Aleutian Islands and in the Gulf of Alaska. These are "tectonic" quakes caused by pressure building on underground plates, Searcy said.
It also doesn't appear that thawing permafrost is a factor, Ruppert said. Permafrost extends maybe hundreds of feet and these earthquakes are several miles deep.
Seismologists call these clusters of quakes "swarms" to differentiate them from the more conventional kind of event where there's one big main quake and then some aftershocks and then nothing — at least for while. A swarm is what seismologists call a sequence of similar-sized earthquakes in similar locations over a period of time.
Earthquake swarms are more common in parts of the Lower 48 such as California, according to Julia Dutton, a geophysicist with the U.S. Geological Survey's National Earthquake Information Center in Golden, Colo. USGS researchers are also seeing "a huge amount" of earthquakes in Oklahoma thought to be associated with fracking techniques used in oil and gas production, Dutton said.
Meanwhile, Noatak residents are keeping a close watch on the earthquake center's web site -- www.aeic.alaska.edu-- and Facebook page, where on Monday they posted comments like "That was a big one" and "Each time this happens, I always pray to God that this ends sometime soon."
Researchers are telling people the swarm shouldn't last too much longer, though since this event is unique it's impossible to say for sure.
"They are concerned, of course," Ruppert said. "It's been years since anybody felt any earthquakes in that area and now for two months they've been having lots of activity."
Ashby said he's spent much of his life in Noatak and never experienced such a concentration of good-sized quakes.
"We'll get a little bit of water supply ready just in case," he said. "I got my Coleman fuel ready and a little bit of firewood."
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