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Wednesday’s ‘megathrust’ magnitude 8.2 quake was Alaska’s largest in more than 50 years

No serious damage reported after coastal residents temporarily sought higher ground
This graphic shows the location and preliminary magnitude of an earthquake that struck at 10:15...
This graphic shows the location and preliminary magnitude of an earthquake that struck at 10:15 p.m. Wednesday, July 28, 2021.(Alaska's News Source Weather Team)
Published: Jul. 29, 2021 at 9:47 PM AKDT|Updated: Jul. 30, 2021 at 2:22 PM AKDT
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ANCHORAGE, Alaska (KTUU) - A magnitude 8.2 earthquake struck off the coast of the Alaska Peninsula Wednesday night, sending coastal communities for higher ground and generating a small tsunami wave, but no major damage.

It was Alaska’s largest earthquake since 1965, when a magnitude 8.7 earthquake struck at the Rat Islands in the Aleutian Chain. The state’s largest earthquake to date remains the magnitude 9.2 Good Friday earthquake of 1964.

Wednesday’s quake struck about 64 miles southeast of Perryville at 10:15 p.m. and triggered a tsunami warning for much of the Gulf of Alaska coast. The U.S. Geological Survey reported it was about 20 miles deep. The earthquake did produce a small tsunami — with waves between 0.4-0.7 feet in some coastal communities — but the warning was canceled about two hours later.

No serious damage has been reported, according to Jeremy Zidek, spokesperson for the Alaska Division of Homeland Security and Emergency Management.

Before the all-clear, coastal towns from Sand Point on the Aleutian Chain to Seward and Homer on the Kenai Peninsula evacuated residents to higher ground, with local schools opening as shelters. In Homer, Police Chief Mark Robl said about 2,000 people were evacuated off the roughly 4.5-mile long Homer Spit.

In the summer, the Spit’s many RV campgrounds and sections for tent camping fill up with tourists from the Lower 48 and elsewhere in Alaska alike.

“We had some good campground hosts out there who started going around telling those folks who didn’t know what the tsunami system was and what they should do, and it was actually a pretty orderly evacuation,” Robl said.

Cars evacuate off the Homer Spit after the community got a tsunami warning the evening of...
Cars evacuate off the Homer Spit after the community got a tsunami warning the evening of Wednesday, July 28, 2021 following a magnitude 8.2 earthquake off the Alaska Peninsula.(Photo courtesy Crisi Osier Matthews)

In Sand Point, where potential tsunami activity was projected to start first, police department administrator Denise Mobeck sounded the alarm for her community herself.

The tsunami warning alarm is not automatic in Sand Point, she explained, so she went to the building where it’s located to activate it.

Mobeck was in bed when she first felt the quake.

“It started getting stronger and stronger,” she said. “And I ended up going out into the living room because I started hearing glass break.”

She said it felt like the earthquake lasted about two minutes.

In Perryville, the community closest to the quake’s epicenter, Sarah Kosbruk caught her vehicles being violentely jostled in her driveway on film.

Perryville, Alaska was closest to the epicenter of last night's magnitude 8.2 earthquake. Not only were objects in Sarah Kosbruk's house shaking, so were the vehicles outside. Read more: https://bit.ly/3lbFmhO

Posted by Alaska's News Source on Thursday, July 29, 2021

Though communities on the Alaska Peninsula and Aleutian Chain faced the greatest likelihood of belongings falling off the shelves, the earthquake was felt as far away as Anchorage and the Matanuska-Susitna Borough.

Peter Haeussler, a research geologist with the U.S. Geological Survey, said the event is what’s called a megathrust earthquake. It’s the kind that occurs on the interface of one tectonic plate — in this case, the Pacific plate — subducting or sliding down underneath an overlying plate — in this case, the North American plate.

The term for a megathrust earthquake actually came out of studies of 1964′s Good Friday quake, Haeussler said.

“In Southern Alaska, on a whole, we have what we call a subduction zone,” Haeussler said. “Which is where one plate goes down underneath another. And so, for example, the 1964 earthquake was a really big version of one of these subduction zone megathrust earthquakes.”

Wednesday’s earthquake was like that, Haeussler said, it just wasn’t quite as big as the Good Friday earthquake.

Haeussler also referenced the 2018 magnitude 7.1 earthquake that damaged buildings and ruptured roads in Anchorage and the Mat-Su. As subducting plates get pulled down back into the earth’s mantle, Haeussler said they can create large earthquakes that tend to be more shallow. Other times, the bending of the slab itself as it gets pulled down is what makes an earthquake. That was the case for the 2018 quake, he said.

“So although that 2018 earthquake and this earthquake are in the same kind of geologic environment, they’re actually quite a bit different in how they actually formed,” he said.

Certain kinds of earthquakes are also more likely to trigger tsunamis than others, Haeussler said. When two plates move side to side, that’s called a strike-slip earthquake. Those are less likely to generate waves.

“In particular, these megathrust earthquakes make the largest tsunamis happen in the world,” Haeussler said. “So in the same way that the 1964 earthquake triggered a tsunami that, you know, traveled all the way across the Pacific, and the 2011 Tohoku earthquake in Japan triggered a giant tsunami, or the ... 2004 Indonesian earthquake triggered a tsunami — this flavor of earthquake can be a really effective tsunami generator.”

The reason Wednesday’s quake didn’t create a large wave is most likely because of its depth, Haeussler said. It started pretty deep, and ended not as shallow as other earthquakes, he said.

“If it had gone a lot shallower, it could have made a much larger tsunami that would have impacted those, particularly the coastlines nearby much more highly,” Haeussler said.

Another thing to note about the magnitude 8.2 earthquake, according to Haeussler, is that it appears to have re-ruptured an area that was impacted back in 1938. The 1964 quake, he explained, ruptured an area that went to the southwest end of Kodiak. Beyond that is a region that holds the Semidi Islands.

Back in 1938, there was an earthquake there that was about a magnitude 8. At the time, there didn’t seem to be a lot of slip within the area of that quake, Haeussler said. Wednesday’s earthquake appears to have re-ruptured part of the rupture area of the 1938 quake.

“So there’s another large chunk of the 1938 rupture area that has the potential to slip in the future, and we really don’t have a good idea of when that might occur,” Haeussler said. “We do have some evidence that that area is quite highly locked right now, and there’s no doubt that stress is building within that particular area for a future large earthquake.”

Another aspect of responding to earthquakes is the system by which people are alerted to potential threats, like tsunamis. When sirens went off in coastal towns Wednesday night, Zidek said some people who did not live in the affected areas also got an alert on their cellphones, including some in Anchorage.

“In some cases, people are close enough to those tsunami warning areas that the cell towers that service those warning areas also service other areas that are not vulnerable to tsunamis,” he explained.

“It’s always our practice to over-alert people because we don’t want to have someone within that tsunami area not receive the message,” he continued.

Correction: This article has been updated to correct the magnitude of the 2018 Anchorage earthquake to 7.1

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