The 2020 Alaska earthquake caused the nearby 2021 earthquake

A study of two powerful earthquakes in adjacent areas off the Alaskan Peninsula in 2020 and 2021 shows a link between the two. It also suggests that it may be part of an 80-year successive rupture along the fault.

The research was published today by Science Advances in a research paper led by University of Alaska Fairbanks Institute of Geophysics Associate Professor Ronnie Grabenthen and Michigan State University Associate Professor Julie Elliott. Postdoctoral researcher at UAF Revathy M. Parameswaran is among the four co-authors.

The researchers also concluded that two deep earthquakes on the Aleutian-Alaska giant fault, where the Pacific plate slides under the North American plate, may have brought shallow parts of the fault closer to breaking.

They add that their research will help scientists better understand the stress transmission and earthquakes that occur in the region.

“One large earthquake increases pressure on the adjacent segment of the mega-bottom fault. Then that patch ruptures and pressure increases on the next patch in the fault, like lagging dominoes,” Grapentin said. “And that’s what we see here.”

The first of the two major earthquakes, known as the Semyonov event, occurred on July 21, 2020. Recording a magnitude of 7.8 on the Richter scale, it struck near the Shumagin Islands south of the Alaskan Peninsula and ruptured westward.

The second earthquake, the Chignik event, occurred a little over a year later on July 28, 2021. It was recorded as 8.2 on the Richter scale, located south of the Alaskan Peninsula and northeast from the Semyonov earthquake and ruptured towards the east.

The two earthquakes and their aftershocks occurred at the Shumajin Gap, a spot near the Shumagin Islands in a well-known group of historical ruptures. The 1,900-mile-long subduction zone, where the Pacific tectonic plate slides under the North American plate, begins at the tip of the Aleutian Islands. It continues along the southern side of the islands and the Alaskan Peninsula, curves upwards across the Kenai Peninsula and includes the Anchorage District and Prince William Sound.

No major earthquakes were recorded in the Shumagin Gap, an area about 100 miles long in the subduction zone, until the 2020 and 2021 earthquakes.

“This could be a case study to understand how neighboring earthquake patches can be activated by a significant release of energy accumulated through plate movement,” Parameswaran said.

The researchers studied the data to assess the impact of stress changes from the 2020 Semyonov earthquake, particularly as they may relate to the site of the Chinyk earthquake rupture.

Modeling of stress build-up from the Simonov earthquake shows that the Chignik hypocenter, the site within Earth at which earthquake rupture begins, is incorporated into a region experiencing increased stress change, consistent with what scientists know about how earthquakes occur.

The work also indicated some notable areas of “too high” fault stress loading, particularly in the shallower regions of the typical fault plane. This area was not torn apart during this earthquake, according to the research paper.

The researchers added that the two earthquakes may be part of a series of large 80-year immersion earthquakes along this main plate boundary and that the waterfall is now over, as the most recent major event was before the 2020 magnitude 8.7 Simonov earthquake in 1965. The earthquake off the Rat Islands in Aleutian series.

Prior to 1965, five earthquakes of magnitude 8 or greater occurred within 30 years from the most remote Aleutian Islands to south-central Alaska.

“In the Falls concept, the entire Aleutian-Alaska giant structure has now ruptured and released most of the pressure that has built up since the beginning of that last streak,” said Grapenthin.

Story source:

Materials provided by University of Alaska Fairbanks. Original by Rod Boyce. Note: Content can be modified according to style and length.

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