According to the U.S. Geological Survey earthquake site, the quake occurred just after 6:35 p.m.
Some of the most intense shaking was felt on the Kenai Peninsula…about a half hour into the Kenai Peninsula Borough Assembly meeting.
It was still rocking almost a minute later.
“Just guessing here- it was probably felt by a majority of Alaskans, so that makes it notable,” says Michael West, a state seismologist and director of the Alaska Earthquake Center. “It occurred fairly deep in the earth, about 75 miles below the surface and because of that, it was felt over a very wide area. On the flip side, because it was deep, that means that nobody was right next to the earthquake. That is, even if you were right on top of it, you were still 75 miles away.”
As a result, the shaking wasn’t nearly as strong as it would have been had it been shallower.
The epicenter was 44 miles south-southwest of Redoubt Volcano on the west side of Cook Inlet. It was almost due west of Anchor Point, near Pedro and Chinitna Bays.
West says it’s not directly related to the recent magnitude 6.9 quake in the Aleutians. That one was shallow and has had vigorous aftershocks. This quake has had few aftershocks and none were very significant.
He says this is a fairly normal type of quake for Alaska, though it was larger than usual, which makes it notable.
“From a plate tectonics perspective, the Pacific plate, that is the area of earth that is the Pacific Ocean is converging on Alaska,” says West. “The two are moving together a couple of inches per year. In this competition between the mainland of Alaska and the Pacific plate, Alaska wins and the Pacific plate is being thrust down into the earth in a process that we refer to as subduction.”
The stress caused by that thrusting manifests itself as an earthquake.
One aspect of this particular quake that had social media abuzz was its length. It was felt for more than 45 seconds in some places, which puts it at the upper end for many common seismic events.
West says the actual rupture in the earth was over and done with in just a few seconds. But the resulting seismic waves bounce around and reverberate.
“An analogy we’ve used this evening is like a crowd in a stadium, echoes that bounce around and around and around. And the seismic waves do the same thing. Typically, the further away you are, the longer you may feel that shaking,” says West.
West says his office received reports of dishes falling off of shelves and lots of shaking, but hadn’t heard any reports of injuries or serious damage.
So, while it was an evening surprise for much of the state, its effects luckily weren’t severe.