Monday, June 16, 2014

Re: [Geology2] Earthquake 'swarm' near Noatak continues, puzzling scientists



Isostasy?


On Mon, Jun 16, 2014 at 6:41 AM, Lin Kerns linkerns@gmail.com [geology2] <geology2@yahoogroups.com> wrote:
 

Earthquake 'swarm' near Noatak continues, puzzling scientists

Jillian RogersThe Arctic Sounder
June 15, 2014

Another powerful earthquake shook the Northwest Arctic earlier this month. It is the fourth magnitude 5.5 quake to rock the region in six weeks. Like the previous three, last Friday's episode was initiated about 10 miles from Noatak and was measured at a depth of 10 miles.

"The whole house shook," said Herbert Walton, the tribal administrator in Noatak. "We're concerned."

Walton said there was no major damage or injuries that he was aware of, though the first set of quakes in mid-April did cause a few cracks in the IRA building.

"There are plenty of people wondering if there is going to be a bigger one, because every time it happens, they seem to be getting bigger," Walton said.

The first two quakes happened on April 18, while the third shook the area on May 3. All four were about the same magnitude and are now being referred to as an "earthquake swarm," said Mike West, a seismologist with the Alaska Earthquake Center.

The four major quakes have been accompanied by more than 250 "unusually vigorous" aftershocks, West said.

"They all have the same cause; the same fault motion," he said. "And they occur in more or less the same place."

West said vigorous aftershocks are not uncommon, but normally they simmer down over time. The fact that this series of shakes is not losing strength is part of the reason scientists are referring to the occurrences as a swarm, rather than a sequence.

Earthquakes relieve pressure in the earth, and because these strong quakes and aftershocks are still happening, that tells experts that the stress was not fully relieved with the first set of quakes.

"In an area like Noatak, this is very unusual behavior," West said.

Swarms are more common around volcanoes and geothermal sites, but since there are neither in the area in question, West said seismologists are scratching their heads trying to find a comparable episode in mainland Alaska. 

Last month, technicians traveled to Kotzebue and Noatak where they held public information meetings and installed temporary seismic stations in both communities. The instruments will allow scientists to better understand what exactly is happening and record all the aftershocks, even the less jarring ones.

"Those two stations are behaving perfectly," West said. "The difference is that we know far more about the earthquake Friday night than we do about the ones in April."

For example, they can now trust the depth reading, and are closer to understanding the orientation of the fault by detecting the smaller aftershocks, all of which is valuable in figuring out why these earthquakes are happening. Experts also know that the fault line spans about 19 miles.

What seismologists still don't know is whether or not a bigger earthquake is on the horizon.

"There is nothing to suggest a larger earthquake; earthquake swarms are characterized by earthquakes of the same size," West said. "But I would be lying if I said there's no possibility of a larger earthquake."

There is no evidence to suggest that the quakes will grow in size, he reiterated.

"It's a very tricky subject. This is a very unusual situation," West said.

Because this is new territory as far as recorded seismic data, those studying the quakes have nothing to compare information to, leaving them limited as far as what they can tell the general public.

As for those in Noatak, Walton said, they are still wondering what all these quakes mean. And each time the ground rumbles, locals are getting calls from surrounding villages asking the same question. Last month, the town meeting in Noatak with the technician from the Alaska Earthquake Center was full with curious locals, but experts are limited on what they can explain because they simply don't know why it's happening or if it will continue.

"This is a significant thing and it's a challenge to raise awareness without becoming alarmist," West said.

http://www.alaskadispatch.com/article/20140615/earthquake-swarm-near-noatak-continues-puzzling-scientists
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