Freak anomaly: Low-seismic Alaska area shaken by mysterious series of quakes

A swarm of identical earthquakes has hit Alaska – taking place in the same low-seismicity area in the northwest, all at the same 5.7 magnitude. The fifth and latest event took place on Monday, leaving scientists puzzled.

The eerily similar tremors, which have been taking place regularly since April 18, woke people up at around 4 am. No one in the Inuit Eskimo community in Noatak is used to earthquakes - there are no major active fault lines and the latest such incident took place back in 1981.

The area is about 100km north of the Arctic Circle. The April swarm struck about 30km from Noatak at a depth of about 16km. Just as with previous temblors, there were no injuries, apart from minor structural cracks in Noatak.

The latest tremor epicenter was located northeast of the village, the Alaska Earthquake Center reported.

Herbert Walton told the Arctic Sounder that “the whole house shook… we’re concerned.”

While many slept through the quake, locals still fear “there’s going to be a bigger one, because every time it happens, they seem to be getting bigger,” Walton said.

The regularity is a little bit offset. While the first two events happened in rapid succession on April 18, the third event did not happen until May 3. But all four were about the same magnitude, which is part of the reason the events are being treated as a group and called a “swarm” by Mike West, with the Earthquake Center.

Each event has since been accompanied by 300 smaller aftershocks, their magnitude reaching 3 at times. A 4.2 foreshock was also present this time, just before the main event hit. It was then followed by no less than 10 aftershocks.

What is worrying to West is that the aftershocks are “unusually vigorous” and that the temblors “all have the same cause; the same fault motion” and “occur in more or less the same place.” What is strange here is that unlike other strong aftershocks, these ones do not have a tendency to reduce in magnitude – they pulse in the same way.

Scientists so far have no answers as to what could have caused this. Ruppert thinks that tremors don’t happen unless there is an active fault in the area which does not appear to be the case as all the faults have long been mapped. The only remaining possibility is that there’s a hidden fault scar that seismologists missed due to it being hidden by vegetation or glacial deposits.

There is nothing more that can be done at this point, apart from taking down readings from seismic sensors the researchers set up a month ago, about 150km south in the town of Kotzebue. This should provide the Earthquake Center with much more accurate readings regarding the depth and origin of every event and aftershock.

“At this point, we don't really understand the nature of these earthquakes,” Ruppert told AP. When asked about a nearby mine less than 80km to the north, she said the relationship between it and the swarm of quakes is not very likely, because “all mining activities are very near the surface… and all the earthquakes are miles below the surface.”

The events are a huge mystery and researchers are on the ready for a larger earthquake; despite there being nothing to suggest it, events like this are usually followed by a much larger temblor.

“It’s a very tricky subject. This is a very unusual situation,” Walton told the newspaper.