The Fault in the Facts: Can one earthquake cause another?

ANCHORAGE, Alaska (KTUU) - In this installment of the Fault in the Facts, we will answer the question ‘Can one earthquake cause another?’ by explaining new research on two large quakes off the Alaska Peninsula in 2020 and 2021.

The magnitude 7.8 Simeonof Event struck on July 22, 2020. The quake’s epicenter was about 61.8 miles southeast of Perryville on the Alaska Peninsula, according to the United States Geological Survey (USGS).

The Chignik Event happened in the same area a year later, nearly to the day. The magnitude 8.2 hit about one-tenth of a mile further from Perryville at 61.9 miles southeast from the town, according to USGS. It shook Alaska on July 29, 2021.

These events were each the largest quakes in the world for their respective year, according to the Alaska Earthquake Center. And the Chignik Event was the largest quake in the United States in more than 50 years.

Ronni Grapenthin is an associate professor of geodesy at the University of Alaska Fairbanks who has been researching for 17 years. He worked on the new research that suggests the Simeonof and Chignik Events are connected and may be a part of an 80-year sequence of earthquakes.

The events struck in an area of the Aleutian Arc called the Shumagin Gap, where no large earthquakes have been recorded in the last 100 years.

They happened on a type of fault that’s called a subduction zone. In this case, the Pacific Plate is being pushed underneath — or is subducting under — the North American Plate. That motion causes pressure on the rocks, and when it is released, or ruptures, an earthquake happens.

Grapenthin said that decades ago, scientists learned that subduction zones around the world usually rupture as large earthquakes.

“They rupture one portion, and then another portion, and perhaps something a little bit further down here and later, they fill each other in kind of like a jigsaw puzzle,” he said.

The fault line where the Simeonof and Chignik Events happened had other significant earthquakes in the last 80 years, including the 9.2 Great Alaska Earthquake in 1964. It ruptured the fault line from the Prince William Sound area to the west of Kodiak Island.

There was also an 8.6 magnitude quake west of the Shumagin Gap in 1946.

“But in between there, there was this area where we would have expected a large earthquake,” Grapenthin said.

But the area has only had smaller quakes recorded in the last 100 years or so. He said the lack of large quakes is what earned the area the title of the Shumagin Seismic Gap.

However, the Simeonof and Chignik Events did rupture at least part of the gap.

Can one earthquake cause another?

Grapenthin said the new research shows that one earthquake can cause another.

He said during the first quake, the Simeonof Event, some slip was released.

“The surrounding areas that don’t slip, they experience a higher stress,” he said.

Grapenthin said the Simeonof Earthquake increased the stress in the area where the Chignik Earthquake ended up happening, and in turn, moved the area toward rupture.

“These earthquakes are right next to each other. We see higher stresses or stress increases in the area of the Chignik Earthquake due to the Simeonof Earthquake,” he said.

The two quake’s rupture areas don’t overlap very much, so Grapenthin said they are kind of complementary to one another.

“Certainly, the Simeonof Earthquake impacted or helped advance the Chignik Earthquake,” he said.

“That doesn’t mean that the Chignik Earthquake wouldn’t have happened without the Simeonof Earthquake, but it means that it happened somewhat sooner due to that, and that establishes a link between those two earthquakes,” Grapenthin said.

He said both earthquakes increased the stresses in the region and there’s an increased seismic hazard, however, they found post-seismic relaxation. He said that means “a lot of that increased stress is actually released as kind of a slow earthquake over the course of weeks to months,” he said.

The researchers are still trying to understand whether another part of the fault is more likely to have an earthquake and create a tsunami, or if the post-seismic relaxation releases most of the stress. If the post-seismic relaxation did release most of the stress, he said it would explain why researchers haven’t seen substantial tsunami deposits in that area from past large earthquakes.

Is the sequence of earthquakes just starting?

The earthquakes in the area over the past 80 years have been grouped together in a sequence of large earthquakes.

Grapenthin said the researchers investigated whether the sequence is just starting or closer to the end.

“We came to the conclusion that, based on how the stresses have evolved, that it’s more likely that those two events are kind of at the tail end of that 80-yearlong cascade of earthquakes that ruptured the entire Aleutian Arc, rather than being the onset of a of a new cascade,” he said.

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