Major 2020 Alaska earthquake triggered larger 2021 event | Polarjournal
(Map: UAF)

Megathrust earthquakes, as the name suggests, are big. Since 1900, all earthquakes of magnitude 9.0 or greater have been megathrust earthquakes. Now, a study of two powerful earthquakes in adjacent areas off the Alaska Peninsula in 2020 and 2021 appears to show connection between the two, while also suggesting that they may have been have been part of a series of earthquakes taking place over the past eight decades that appears to have made the region even more prone to earthquakes.

The research, published in Science Advances, concludes that the two deep earthquakes on the Aleutian-Alaska megathrust fault, where the Pacific plate is sliding beneath the North American plate, may have brought shallow portions of the fault closer to failure. “One large earthquake increases the stress on the neighbouring part of the megathrust fault. This patch then ruptures and increases the stress on the next patch in the fault, like delayed dominos,” Ronni Grapenthin, of the University of Alaska Fairbanks, said. “And that’s what we’re seeing here.”

The first of the two earthquakes, known as the Simeonof Event, took place on 21 July 2020. It registered at magnitude 7.8, struck near the Shumagin Islands south of the Alaska Peninsula and ruptured westward. The second quake, the Chignik Event, took place on 28 July of last year. It registered at magnitude 8.2, was located south of the Alaska Peninsula and north-east of the Simeonof quake and ruptured eastward.

The Chignik Event had piqued scientific curiosity not only due to its size but also due to its proximity to the Simeonof Event. Just 54km separates the locations where each originated. The Simeonof event occurred in a region of the plate interface previously identified by researchers as a “seismic gap”, an area that has produced large earthquakes in the past but has since gone quiet seismically speaking. 

(Map: UAF)

The Shumagin Gap, however, is in a known band of historical ruptures. The 3,000km subduction zone, where the Pacific tectonic plate slides under the North American plate, starts at the tip of the Aleutian Islands. It continues along the south side of the islands and the Alaska Peninsula, curves upward across the Kenai Peninsula and encompasses the Anchorage area and Prince William Sound. 

Since the 1980s, scientist had been saying that the Shumagin Gap was due for an earthquake. The fact that a large earthquake had not taken place at this section of the subduction zone had since 1938 and all of a sudden ruptured twice over the span of a little more than a year invited questions of whether the two events were related. According to the findings of the paper, it appears to they were.

“This could be a case study to understand how adjacent earthquake patches could be activated by a significant release of energy that has accumulated through plate motion,” Revathy M Parameswaran, of the University of Alaska Fairbanks, said. Modeling the Simeonof quake’s stress buildup shows that the focus of the Chignik quake is embedded in an area of increased stress change, consistent with what scientists know about how earthquakes are triggered. 

The work also indicated some notable areas of “very high” stress loading along the fault, especially in the shallower regions of the model fault plane. That area didn’t rupture during this earthquake. The two earthquakes may be part of an 80-year cascade of large subduction earthquakes along this major plate boundary and that the cascade has now concluded, with the most recent large event prior to the 2020 Simeonof quake being the 1965 magnitude 8.7 earthquake off the Rat Islands in the Aleutian Chain.

Prior to 1965, five earthquakes of magnitude 8 or greater occurred within 30 years from the farthest Aleutian Islands to south-central Alaska “In the concept of cascades, the entire Aleutian-Alaska megathrust has now ruptured and released most of the stress that has accumulated since the onset of that most recent cascade,” Dr Grapenthin said.

Featured image: USGS

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