On May 30, a magnitude-6.1 earthquake struck northeast of Anchorage, Alaska, on a subducting microplate.
By Elisabeth Nadin, Ph.D., University of Alaska Fairbanks
Citation: Nadin, A., 2021, Southeast Alaska quake stands alone, Temblor, 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 settling to sleep under the Midnight Sun felt the ground sway. The seismic waves, which began close to 11 p.m. near the tiny town of Chickaloon, about 75 miles (120 km) northeast of Anchorage, were triggered by a magnitude-6.1 earthquake that struck 27 miles (44 km) deep. Shaking reports were submitted from 560 miles (900 km) away and dozens of aftershocks have followed.
Aftershock or new quake?
At first thought to possibly be an aftershock of the November 2018 magnitude-7.1 Anchorage earthquake—triggered by descent of oceanic crust beneath North America—the Chickaloon event was quickly determined to be too far away to be called an aftershock. The next question was whether the Chickaloon quake could have been triggered by the Anchorage quake, but “too much time has passed to consider this a triggered quake,” says Natalia Ruppert, senior scientist with the Alaska Earthquake Center. “There is no relation to the Anchorage earthquake except that the mechanism is similar.”
An earthquake’s focal mechanism is a 3D representation of how the fault slipped during the event. Both the 2018 Anchorage and the 2021 Chickaloon mechanisms indicate that the top of the subducting plate is being stretched as it is pulled into the mantle. Even the direction of stretching was the same for both.
Defining a microplate
The location of the recent Chickaloon event is within a distinct cluster of small-scale earthquakes that may well align with the eastern edge of a small tectonic plate that has gained a lot of attention in the past several years.
We may think of Earth’s surface as divided, with great confidence, into large tectonic plates, but with the advent of GPS instruments, scientists are finding regions of the crust that they think belong to distinct smaller plates called microplates. The Pacific Plate, for example, is associated with subduction beneath Alaska that gives rise to the chain of volcanoes that forms the Aleutian Islands. However, geologists have noted a surprising gap in the volcanic chain between the eastern Aleutians and the Wrangell Mountains. Over the past few decades, scientists have surmised that there is a different plate subducting here, one that is too thick for the regular melting that occurs above a subducting slab, creating magma that rises to the surface. They call it the Yakutat Plate.
The Yakutat Plate is considered an exceptionally thick swath of oceanic crust. And while some scientists may consider it to be indistinct from the Pacific Plate, calling it merely a thicker region of the Pacific Plate, others note that the Yakutat Plate is moving separately and differently. “If they’re moving differently, they are different plates,” says Jeffrey Freymueller, a geodesist at Michigan State University. He has studied plate motion in Alaska for the past 25 years.
Locating the quake
While an earthquake in the downgoing Yakutat slab is not a surprise, says Freymueller, its location helps scientists define the edge of the microplate, which is still a matter of debate. In the absence of good subsurface imaging, earthquake locations are the best indicator of the shapes and boundaries of plates. “What happens at depth is a bit of a mystery—are the Pacific and Yakutat Plates attached somewhere, scissoring, or completely different? We don’t have a really clear picture of it,” Freymueller says.
While the largest earthquakes happen where a subducting plate rubs against the plate above it, the Chickaloon and Anchorage earthquakes both took place much deeper, within the part of downgoing oceanic plate called the slab. Freymueller notes that much greater and more damaging earthquakes than these two events can happen within subducting slabs, which is why it is important to understand their properties better. The biggest earthquake within a slab under Alaska was a magnitude-7.9 near Amchitka Island in 2014, and a magnitude-8.3 quake occurred beneath the Sea of Okhotsk in 2013. “We’ve seen a few examples of quite big events, and we don’t really know how often they can happen,” he says.
Thus, the Chickaloon quake provides more information about the seismic hazard in the region, because we don’t have a good understanding of how frequently these big events happen. One thing we do know, says Freymueller, is that “anywhere that you have these events, you have the possibility of a bigger one.”
References
Eberhart‐Phillips, D., Christensen, D. H., Brocher, T. M., Hansen, R., Ruppert, N. A., Haeussler, P. J., & Abers, G. A. (2006). Imaging the transition from Aleutian subduction to Yakutat collision in central Alaska, with local earthquakes and active source data. Journal of Geophysical Research: Solid Earth, 111(B11).