Research reveals mystery source of 2020 Aleutians tsunami

Research reveals mystery source of 2020 Aleutians tsunami

The source and timing of a tsunami following a magnitude 7.6 earthquake southeast of the Aleutian Islands community of Sand Point in 2020 have puzzled scientists for years. New research offers an explanation.

Work led by University of Alaska Fairbanks professor Ronni Grapenthin indicates that the tsunami was generated not by the large earthquake's horizontal fault movement but by slow movement that began up to eight minutes later.

The secondary movement occurred about 60 miles south of the main earthquake along a small section of the subduction zone, where one tectonic plate slides beneath another.

If this type of movement is common, it could affect how scientists assess the risk of future earthquakes and tsunamis in the region.

The study appears in the latest edition of Seismica. Research associate professor Dmitry Nicolsky and research assistant professor Revathy Parameswaran, both with the UAF Geophysical Institute, are among the research paper's seven co-authors. Grapenthin, the paper's lead author, is affiliated with the UAF Geophysical Institute and the UAF College of Natural Science and Mathematics.

"It's been a lot of detective work to get this done," Grapenthin said. "Some of the standard approaches just wouldn't give us a reasonable answer for what generated the tsunami."

Computer modeling suggests the later movement lifted the seafloor about 16 inches, enough to generate a tsunami that was small but larger than expected from an earthquake involving horizontal fault movement.

During the Oct. 19, 2020, earthquake, the two sides of the fault moved mostly horizontally past each other by nearly 7 feet. Horizontal, or strike-slip, fault movement is less likely to generate a substantial tsunami because it usually doesn't displace enough water.

Grapenthin and his colleagues found through computer modeling that the main earthquake increased stress on some sections of the Alaska-Aleutian megathrust while reducing it on others. The megathrust is the massive fault along the roughly 2,500-mile-long subduction zone where the Pacific plate slides beneath the North American plate.

The researchers conclude that the increased stress triggered movement along the megathrust, lifting the seafloor enough to generate the tsunami. Nearby NOAA deep-ocean buoys recorded tsunami waves about 2 inches high. Waves reaching Hawaii measured about 10 inches.

Megathrust faults can produce some of the world's largest earthquakes and tsunamis. They can also move slowly without generating seismic waves detectable by conventional monitoring.

"We have a large earthquake that sends out lots of energy, and the seismometers go berserk, and then about eight minutes later we suggest this slow movement of the megathrust fault that was not as energetic and went unrecorded by our land-based stations," Grapenthin said. "It was slow for an earthquake but fast enough to make a tsunami."

"They were separate events, but the megathrust slip would likely not have occurred at that time without the main strike-slip earthquake," he said.

The megathrust movement also was too far offshore to produce measurable changes at the land-based stations that monitor ground movement.

Two magnitude 5.7 aftershocks occurred near the megathrust about 11 minutes after the main earthquake. The researchers suggest the aftershocks may be related to the delayed movement.

The findings raise questions about whether similar slow fault movements may contribute to tsunamis following other large earthquakes without being detected by instruments on land.

"Nothing says this shallow delayed slow slip can't happen with other earthquakes," Grapenthin said. "For larger megathrust earthquakes, the evidence might be obscured by much larger tsunamis.

"But with only land-based observations, we might not catch this shallow slip and mis-estimate what the seismic hazard is in the shallow megathrust."

ADDITIONAL CONTACT: Ronni Grapenthin, [email protected]

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