Earthquake Swarm on Alaska’s North Slope

OK, the ground up there isn’t jumping so much it knocks down polar bears–this is just a laid-back local resident.

But a magnitude 6.4 quake is unusual on Alaska’s Arctic Ocean coast, and no one really knows what caused the Sunday event.

They call this region the North “Slope,” not the North “Trench,” because it’s not a subduction zone like the Aleutian Islands, where megathrust quakes are possible. There’s a lot of petroleum down there, and at the surface it is a national wildlife refuge. Apparently there is no drilling ongoing at the epicenter site, near an Inuit village called Kaktovic.

The Sunday quake was part of an ongoing swarm. There were a few events in the magnitude-6 range, with the 6.4 being the largest ever recorded up there. The rest are M3 or lower, and it’s still quite active up there along the Arctic Ocean today.


Nothing dire seems imminent, but it is unusual and worth noting. There hasn’t been much in the news, just this Arctic Today article (you’ll have to give them an email address to read it – sorry, but there are no alternative news sources) and this Alaska Earthquake Center post, which links to their continuously updated data pages. They’re really excited about it because they happened to have a dense array of seismometry equipment in place and now have a lot of data about the region’s geology, which isn’t well understood yet.

August 17, 2018: Here’s how last Sunday’s event looked as it travelled through that seismometer array that happens to be set up across Alaska at present.

Alaska Earthquake Center

August 22, 2018: Nothing much new in the news, though reportedly the petroleum people in the region did a flurry of inspections and found no equipment problems after the August 12th quake. The general epicenter area is still having lots of very low-level activity per today’s “latest earthquakes” USGS website (am guessing it’s the regional stress field adjusting to whatever rock formations “broke” down there).

September 18, 2018: An M5.1 in the general region of Kaktovik today. However, per the Alaska Earthquake Center event page, this may be related more to the Brooks Range than to the bigger August event on the North Slope.

Alaska’s geology is very complicated, even far from the Aleutian subduction zone.

January 28, 2019: Here is the Alaska Earthquake Center’s 2018 review of this event:

August 12 M6.4 Kaktovik

The year’s most unexpected earthquake, from a seismological perspective, was the Aug. 12 M6.4 quake in the Sadlerochit Mountains, 52 miles southwest of Kaktovik and 25 miles south of the Beaufort Sea coast. This was by far the largest earthquake ever recorded north of the Brooks Range in Alaska. It started a vigorous aftershock sequence, including a magnitude 6.0 earthquake at 1:15pm on the same day. That aftershock was the second largest earthquake ever recorded in the region.

The M6.4 mainshock and M6 aftershock were both felt widely, with reports coming in from Kaktovik west to Nuiqsut and as far south as Fairbanks. There were no reports of damage or injuries, and there was no impact on pipeline operations.

The earthquake ruptured an easterly trending strike-slip fault with motion similar to that of the Tintina and Denali faults to the south. So far we have recorded over 4,000 aftershocks, and the sequence is ongoing.

This region is poorly understood, so these earthquakes will be extremely important for studying North Slope seismicity. We are fortunate that this earthquake happened during the Transportable Array deployment in Alaska. Before 2013, the nearest stations would have been at Pump Station 1 and Pump Station 4. Instead, we have high quality recordings from a number of research-grade stations fairly close to the source.

Featured image: Alaska Region US Fish & Wildlife Service

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