The Chignik earthquake in Alaska shows a fragmented subduction zone

What is the relationship between the 8.2-magnitude July earthquake off the Alaska Peninsula and last summer’s 7.8-magnitude Simonov earthquake?

Written by Connor Doruff, Ph.D. Candidate, Michigan State University

Citation: Doihroff, C., 2021, Alaska’s Chenyak earthquake showing fragmented subduction zone, Temblor, http://doi.org/10.32858/temblor.194

On Wednesday, July 29 at 6:15 a.m. UTC (10:15 p.m. local time on July 28), an earthquake measuring 8.2 on the Richter scale struck the southwest coast of Alaska. Located off the coast of the Alaskan Peninsula about 115 kilometers southeast of the small town of Chenyak, the earthquake ruptured part of the Alaskan Aleutian subduction zone, where the Pacific plate is constantly thrusting under North America. This seismically active region absorbs about 6 centimeters per year of convergence between the Pacific and North American plates. Each year, scientists detect thousands of earthquakes along the surface between these two plates, and the Chénieck earthquake was the largest to have occurred in the region — and in the United States — since 1965, just over 50 years ago.

A temporary GPS location can be seen here in the Shumagen Islands. Credit: Jeff Fremueler

Earthquake related?

The Chignik earthquake sparked scientific curiosity not only because of its size, but also because of its proximity to last summer’s 7.8-magnitude Simeonof earthquake that struck on July 22, 2020. Only 54 kilometers separate the sites where each event was nuclear. The Simeonof event occurred in the region of the plate front that the researchers previously identified as a “seismic gap” – an area that produced large earthquakes in the past but has since become “quiet” since. It has been hypothesized since the 1980s (Davies et al., 1981) that this region of the plate interface referred to as the Shumagin Seismic Gap—named for the Shumagin Islands above it—was due to an earthquake. The fact that this part of the subduction zone has not hosted a major earthquake since 1938 and has suddenly ruptured twice over a span of just over a year, calls into question whether the two events are related.

tectonic context

Both events occurred in a region where interseismic coupling—a measure of how well the subduction interface between large earthquakes survives—decreases widely from east to west (Fig. 1). In other words, the plate boundary sticks to the east, and slides more easily to the west.

Interseismic coupling distribution for the Aleutian Trench outside the Alaskan Peninsula from Drooff & Freymueller (2021). Blue colors represent a weakly coupled plate interface that slips frequently in small-scale earthquakes. The red colors represent a plate front that is strongly coupled with the potential for rupture in large-scale earthquakes. The divided spots are numbered from east to west. The Hypocenters (where the earthquake originated) and the early aftershock sequence of the July 22, 2020, 7.8 magnitude earthquake and the July 29, 2021, magnitude 8.2 Chignik earthquake are shown as gray and white circles, respectively, with the circle size scaling by earthquake size. The earthquake rupture spot of the Simeonof earthquake from Xiao et al. (2021, under review) shown as magenta profiles, where each line represents a 0.5 m contour break of slip. The Aleutian Trench is shown as a black dashed line. The rupture area of ​​the 1964 Great Alaska earthquake of magnitude 9.2 is shown in black. Credit: Connor Drov

In the suspended, or “highly coupled” regions, the upper portions extending 10-30 km from the plate interface cannot move easily, while the deeper portions constantly slide, accumulating energy in the shallower portion of the subduction zone. This excess energy, also called the slip deficit, will be released in massive future earthquakes. In contrast, the double zones are poorly defined by a plate interface that slides along a constant creep, with only small magnitude transverse seismicity present. This behavior is observed west of the Shumagen Islands, off the southwest coast of the Alaskan Peninsula.

The eastern part of the subduction zone, under the Kodiak Archipelago and the Kenai Peninsula, was completely torn apart in the 1964 Great Alaska earthquake of magnitude 9.2 and has since returned to complete shutdown; Figure 1 shows the western end of that region. The nature of this transition from completely enclosed to fully creeping behavior over the approximately 500 km along the Aleutian Trench is an area of ​​focus for seismologists.

Recent work using ground-based measurements of GPS displacements indicates that the transition in coupling behavior can be best explained by a series of segmented corrections, each with different degrees of coupling between the Pacific and North American plates (Fig. Last summer’s Simeonof earthquake appears to have occurred primarily in a portion below the area east of the Shumagin Islands, with a broader rupture extending westward into the poorly coupled portion of the interface west of the Shumagins (Xiao et al., 2021).

In contrast, the Chizhnik earthquake appears to have ruptured the more closely bound and more stable westward part of Chirikov Island, and the sequence of early aftershocks appears to have extended eastward along the slope front.

Could the Chinik earthquake be caused by the redistribution of regional pressure due to the Semyonov event last summer? “Almost certainly,” says Jeff Fremueler, a geodesy expert at Michigan State University. “The redistribution of stress from the July 2020 earthquake brought the fault that broke out in 2021 closer to failure,” Freymueller says. “This may be another example of the aggregation of large immersion earthquakes, as we saw off the coast of Sumatra 10 to 15 years ago.”

Pending questions

In any case, the Chignik and Simeonof earthquakes represent new and exciting observations of seismic slip behavior in gradient regions with partially suspended and partial slip properties. While the physical model of seismic slip that occurs along discrete fault sections known as asperities has been widely accepted for some time, it is only recently that scientists have begun to understand and appreciate the relationship that these aquifers have with neighboring elements of a subduction system. It also remains an open question as to why events in such sequences are separated by months or even a few years.

Looking at these timelines, it’s hard to say whether or not this sequence is over, and what future ruptures might look like. Given recurring periods of at least 200 years between large earthquakes such as the 1964 event with a magnitude of 9.2, it is unlikely that we will see a complete rupture in the fully closed fault zone for some time. But as we saw in last summer’s Samanov earthquake, the redistribution of stress as a result of events like this can lead to future earthquakes months or even years into the future.

references

Bilek, S. L., & Lay, T. (2002). Earthquakes and tsunamis are perhaps widespread manifestations of conditional stability friction. Geophysical Research Letters, 29(14), 18-1.

Davies, c. , Sykes, L., House, L, and Jacob, K. (1981). The Shumagin Seismic Gap, Alaska Peninsula: History of Great Earthquakes, Tectonic Setting, and Evidence for High Seismic Potential. Journal of Geophysical Research: Solid Earth, 86 (B5), 3821-3855.

Dixon, TH, and Moore, JC (Editors). (2007). The seismic zone of subduction faults. New York: Columbia University Press.

Doruff, C., & Freimoller, J.T. (2021). New restrictions on sliding deficits in the Aleutian giant and inflation at Mt. Veniaminof, Alaska from repeating GPS measurements. Geophysical Research Letters, 48(4), e2020GL091787.

Xiao, X et al. (2021) Shumagen Deep Gap Filling: Kinetic Rip Model and Slip Budget Analysis for the 2020 Mw 7.8 Simeonof Restricted Earthquake by GNSS, Global Seismic Waveforms, and InSAR Floating Earth and Planetary Science Letters (under review)

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