Two earthquakes struck the waters off the Alaska Peninsula almost eactly a year apart. The magnitude 7.8 Simeonof earthquake of 2020 is shown in black. The magnitude 8.2 Chignik earthquake of 2021 is shown in purple. Red lines show contours of other major earthquakes in the past.
Shortly after 10 p.m. on July 22, 2020, emergency sirens blared in coastal communities from the Aleutians to the Kenai Peninsula, alerting residents to evacuate to higher ground because a magnitude 7.8 earthquake could be followed by a dangerous tsunami.
Almost exactly a year later, shortly after 10 p.m. on July 28, 2021, coastal residents had to do the very same thing, this time for an earthquake measured at magnitude 8.2, making it the most powerful in the United States since 1965.
Now scientists know that the 2020 and 2021 earthquakes off the Alaska Peninsula had even more in common than evacuation deja vu. They were seismically related, with the first triggering the second, according to a newly published study in the journal Science Advances.
And the dual earthquake action deep underground appears to have increased the risks of future tsunamis in that region, the study said.
When it comes to continuing hazards, the good news in the study is that the linked earthquakes relieved some of the stress in the deeper part of the subduction zone, the area where Earth’s Pacific plate is slipping beneath the North American plate. That relieved stress probably lessened the risk of more huge earthquakes, magnitude 8.5 or even more powerful, deep in that area of plate overlap.
The bad news is that they transferred stress to the shallower part of the subduction zone, where earthquakes are more likely to cause tsunamis.
“The two larger, deeper earthquakes have increased the stress, so have further loaded the spring of the shallower part of the subduction zone,” said study co-author Ronni Grapenthin of the University of Alaska Fairbanks’ Geophysical Institute.
Scientists see change after earthquakes in 2020 and 2021
The study, led by Julie Elliott of Michigan State University, used data gathered by high-precision GPS and other means about land positions and changes caused by the 2020 Simeonof quake and the 2021 Chignik quake. It measured the effects underground, in the Aleutian-Alaska megathrust fault, the name for the place where Earth’s Pacific Plate is sliding under the North American Plate. It found that in the epicenter of the second quake, located about 12 to 18 miles below the seafloor, the slip was close to 20 feet.
The two quakes happened in part of an area known as the “Shumagin Gap,” an approximately 100-mile stretch that was notorious for having had no big earthquakes, even though there have been plenty of big quakes rupturing over the past decades elsewhere along the same megathrust fault that extends from the Aleutian Islands to Prince William Sound.
Now these 2020 and 2021 quakes have finally occurred within a section of that gap. Those quakes “may actually be the trailing effects of that long cascade of megathrust earthquakes,” signaling the end of an 80-year series of huge shakers, Grapethin said.
However, there is probably an increased likelihood of less-powerful earthquakes – perhaps in the range of magnitude 7 to 8 – closer to the seafloor, the study found. The shallower the quake, the more likely it lifts up the seafloor and the more likely a tsunami happens, he said. The big concern is a quake that actually breaks through that surface.
“Vertical defeat of the seafloor is what moves the water that’s sitting on top of it,” Grapethin said.
That does not mean that shallow earthquakes and resulting tsunamis are inevitable, he said. The increased pressure may be relieved another way, in more gradual creeping, he said.
While the new study gives more information about seismic hazards, with additional information to come as research continues, there is no way to predict an earthquake. Scientists can detail past events, current hazards and conditions like ground stress, “but we cannot say tomorrow at 5 o’clock there’s going to be a magnitude 9 here,” Grapethin said.
For the general public, the lesson of the study is to be prepared for earthquakes, tsunamis and the dangers they cause, he said.
“Earthquakes are part of where we live,” he said.
A Vigilant Community
Such knowledge appears to be already deeply ingrained in Alaska coastal communities.
In Sand Point, for example, there are siren tests every Friday, with the noise serving as a regular safety reminder, said Jordan Keeler, city administrator for the island community just off the tip of the Alaska Peninsula. “It always gets my attention,” Keeler said.
Sand Point, home to about 1,300 people, was one of the many sites where residents made the emergency trek uphill on those July nights in 2020 and 2021. They gathered at the school, which is the designated meeting spot, Keeler said. Trident Seafoods, which operates a processing plant in Sand Point, also has earthquake and tsunami protocols, he said.
“It’s well known that it’s a seismically active region,” Keeler said. “I think people are definitely cognizant, both very cognizant of earthquakes and tsunamis.”
He does have a recommendation for safety improvements. As people shift away from traditional radio and landline service and toward mobile phones, continued modernization of hazard communication to match “would be nice,” he said. Still, the old-school communication technology has its uses, he said. “We still rely on the messages from the Tsunami Warning Center,” he said.
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