Southeast Alaska earthquake stands alone

On May 30, a magnitude 6.1 earthquake struck northeast of Anchorage, Alaska, over a small submerged plate.

Written by Elizabeth Nadine, Ph.D., University of Alaska Fairbanks

Quote: Nadine, A, 2021, Southeast Alaska earthquake standing alone, Templor, http://doi.org/10.32858/temblor.179

On the night of May 30, from the Pacific coast all the way to the Yukon Flats, Alaskans who settled to sleep under the midnight sun felt the impact of the earth. The seismic waves, which began around 11 p.m. near the small town of Chiklon, about 75 miles (120 km) northeast of Anchorage, were caused by a 6.1-magnitude earthquake that struck 27 miles (44 km) deep. Seismic reports were made from a distance of 560 miles (900 km) and dozens of aftershocks followed.

On May 30, a 6.1-magnitude earthquake occurred near the Matanuska Glacier, as seen here from the Glenn Highway. Credit: Frank K. From Anchorage, Alaska, USA, CC BY 2.0, via Commons Wikimedia

Aftershock or a new earthquake?

Initially thought to have been a possible aftermath of the November 2018 magnitude 7.1 Anchorage earthquake – which was caused by subsidence of oceanic crust under North America – the Chicalon event was soon determined to be too distant to be described as an earthquake. The next question was whether the Chicalon earthquake was caused by the Anchorage earthquake, but “it’s been too long for this earthquake to be considered an earthquake,” says Natalia Ruppert, chief scientist at the Alaska Seismological Center. “Nothing related to the Anchorage earthquake, but the mechanism is similar.”

The focal mechanism of an earthquake is a three-dimensional representation of how a fault slides during an event. Both the 2018 Anchorage and 2021 Chickaloon mechanics indicate that the top of the skid plate is tightened as it is pulled into the sash. Even the direction of stretching was the same for both.

The location of the Chicalon earthquake, along with other earthquakes in the area recorded by the US Geological Survey last month.

Microplate identification

The site of the recent Chickaloon event falls within a distinct group of small-scale earthquakes that may align well with the eastern edge of a small tectonic plate that has gained a lot of attention in the past several years.

The location of this earthquake in relation to all earthquakes that occurred in 2020. Credit: Alaska Earthquake Center, University of Alaska, Fairbanks

We might think of the Earth’s surface as being divided, with great confidence, into large tectonic plates, but with the advent of GPS instruments, scientists have found regions of the crust that they believe belong to distinct small plates called microplates. The Pacific Plate, for example, is associated with the subduction under Alaska that gives rise to a chain of volcanoes that make up the Aleutian Islands. However, geologists have noticed a sudden gap in the volcanic chain between the Eastern Aleutians and the Wrangel Mountains. Over the past few decades, scientists have thought that a different plate is subsiding here, which is too thick for the regular melting that takes place over a sloping plate, creating the magma that rises to the surface. They call it a sapphire dish.

The sapphire plate is an exceptionally dense patch of oceanic crust. And although some scholars may consider it indistinguishable from the Pacific Plate, and describe it as merely a region thicker than the Pacific Plate, others have noted that the Yakut Plate moves separately and differently. “If they’re moving differently, they’re different,” says Jeffrey Freimüller, a geodesy expert at Michigan State University. He has studied plate movement in Alaska over the past twenty-five years.

Determine the location of the earthquake

While the earthquake in the deteriorating Yakut slab isn’t surprising, Freymueller says, its location helps scientists pinpoint the plate’s exact edge, which is still a matter of debate. In the absence of good subsurface imaging, earthquake locations are the best indication of the shapes and boundaries of the plates. “What’s going on in the deep is a bit of a mystery — are the Pacific and Yakut plates connected somewhere, scissors, or completely different? We don’t have a really clear picture of that,” Freymueller says.

While the largest earthquakes occur when a submerged plate rubs against the plate above it, both the Chicalon and Anchorage earthquakes occur much deeper, within a portion of the descending oceanic plate called the plate. Freymueller notes that earthquakes larger and more damaging than these two events can occur within merging plates, which is why it is important to better understand their characteristics. The largest earthquake within a 7.9-magnitude sub-Alaskan plate was near Amchitka Island in 2014, and an 8.3-magnitude earthquake occurred under the Sea of ​​Okhotsk in 2013. “We’ve seen some examples of very large events, and we don’t say ‘I don’t really know how often in which it may occur.

Thus, the Chicalon earthquake provides more information about the seismic hazard in the area, because we do not have a good understanding of how frequently these large events occur. One thing we do know, says Freymueller, is that “wherever these events are located, you have the probability of a greater event occurring.”

references

Eberhart-Phillips, D., Christensen, D. H., Brocher, T. M., Hansen, R., Ruppert, N. A., Haeussler, PJ, Abers, GA (2006). Depicting the transition from the Aleutian subduction to a sapphire collision in central Alaska, with local seismicity and energetic source data. Journal of Geophysical Research: Solid Earth, 111 (B11).

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