Does Alaska’s magnitude-7.8 Simeonof earthquake finally close a seismic gap?

A magnitude-7.8 quake struck near the mostly uninhabited Shumagin Islands on July 22, 2020, potentially closing a large seismic gap along the Aleutian trench.
 

By Elisabeth Nadin, Associate Professor, University of Alaska Fairbanks
 

Citation: Nadin, E., 2020, Does Alaska’s magnitude-7.8 Simeonof earthquake finally close a seismic gap?, Temblor, http://doi.org/10.32858/temblor.106
 

Scientists install a seismometer on Chernabura Island in the Shumagin Islands. Credit: Natalia Ruppert

 

Alaska’s southern subduction zone is no stranger to large-magnitude earthquakes, but the magnitude-7.8 on July 22, 2020 will keep many scientists scratching their heads for a while to come. “To me, this was an unusual earthquake,” says seismologist Natalia Ruppert of the Geophysical Institute at University of Alaska Fairbanks. The event, named the Simeonof earthquake, ruptured a portion of the Aleutian subduction zone that has not hosted an earthquake in a long time. The Shumagin Gap, as it is known, is a seismic gap along the Alaska Peninsula, more than 600 miles (400 kilometers) southwest of Kodiak Island. In the days following the main event, aftershocks as large as magnitude-6.1 and as numerous as 100 per day have spanned the gap.

 

Map showing the location of the July 21, 2020 magnitude-7.8 Simeonof earthquake on the Alaska Peninsula. The star shows the epicenter, and the reddish ovals delineate areas that ruptured in past earthquakes, shown with the magnitude and date. In the top left is a cross section, or 3D view, representing a rupture patch between the two plates (not this earthquake specfically). As the Pacific plate descends beneath the North American plate, earthquakes occur when strong friction built up between the plates ultimately ruptures, but the entire interface does not usually slip. Around the rupture patch, the fault plane remains locked, or unmoving, building stress toward the next earthquake. Credit: Alaska Earthquake Center

 

A seismic gap is a region along a subduction zone that has not ruptured in a long time. These gaps are often monitored with some trepidation, as a large gap in seismicity could mean that stress is building and may one day be released in a major earthquake. Shumagin Gap, however, was thought to be incapable of large-magnitude earthquakes because scientists postulated that it was silently slipping instead of locked. “I don’t think anyone expected a major earthquake in that gap,” says Ruppert.

 

A 100-year earthquake gap

There hadn’t been a large-magnitude quake in the Shumagin region for more than 100 years, which is odd for an area thought to have a 50-90 year recurrence interval (Davies et al., 1981). Because of this, scientists had begun to speculate that this section of the subduction zone was slipping smoothly,without a build-up of stress that would ultimately lead to rupture. The Shumagin Gap occupied a 125 mile-wide (200-kilometer) length between a section of the subduction zone that ruptured in 1938 and one that ruptured near Unimak Island in 1946. Aftershocks from the Simeonof earthquake now span that entire region.

 

Map showing the distribution of aftershocks in the two weeks following the magnitude-7.8 main event, as well as the rupture patches of historical earthquakes in the surrounding area. A magnitude-6.1 occurred 4 minutes after, and just west of, the main shock. The number of aftershocks increased daily. Another magnitude-6.1 in the western Shumagin Islands took place on July 28. (See https://earthquake.alaska.edu/earthquakes for an up-to-date, interactive map of earthquakes in the region.) One question scientists will address is whether the aftershocks map out a continuous rupture across the Shumagin Gap. Credit: Alaska Earthquake Center

 

A moderate-depth quake

The July 22 main shock and subsequent aftershocks are so far restricted to the lower portion of the megathrust zone. Since the trench is deep underwater, it’s not possible to determine if the rupture reached all the way to the surface, but the lack of aftershocks at shallow levels suggests it did not. This observation leads scientists to speculate that the upper portion of the subduction zone is still silently slipping, or gearing up for another large earthquake.

“The largest slip [along the fault] was about 3–4 meters (10-14 feet), which is not very large for a magnitude-7.8,” says Ruppert. The quake ruptured a 60 by 120 mile-wide (100 by 200 kilometer) section of the subduction zone. Despite how large the earthquake was, there was no tsunami generated, probably because of its depth and limited amount of slip.

 

Time (and data) will tell

Over the next few months to years, scientists will continue to refine their understanding of exactly which portions of the Shumagin Gap slipped in this quake, and what this implies for the surrounding region. While they have a clear picture of the depth at which the fault ruptured, and how much slip occurred, exactly how the slip was distributed along the fault during the earthquake, and whether aftershocks across the gap are part of the slip system of the earthquake, is less clear. The question remains whether rupture was restricted to a small-ish patch, or if the aftershocks are actually mapping out slip across the entire gap.

Preliminary data from instruments in the area suggest that displacements were far greater in the eastern Shumagin Islands than in the western. “Where the slip actually occurred is an important question that is not yet resolved,” comments Jeffrey Freymueller, a geodesist at Michigan State University who began studying the region over a decade ago (see Fournier and Freymueller, 2008). Seismic and GPS data from instruments in the area will be used to estimate net slip and come up with total displacement models. The Alaska Community Experiment , for example, includes ocean-bottom seismometers that captured the aftershock sequence of the 2018 Offshore Kodiak earthquake as well as seismicity leading up to the Simeonof quake. While the seismometers were removed in fall 2019, ocean-bottom geodesy sensors in the region will be able to highlight ocean-floor movement that occurred during and after the quake. Assessing the horizontal to vertical motion of a GPS station on nearby Chernabura Island, right in the middle of the Shumagin Islands, and comparing it to slip measured from satellite data will help scientists create “a real slip model,” says Freymueller.

Freymueller suggests that comparing different types of data will give better estimates of surface displacements that will provide further insight into the nature of slip. For now, he thinks that larger surface displacements in the easternmost Shumagin islands than in the western islands indicates that the greatest amount of slip along the subduction zone occurred in the east. Separating the aftershocks into eastern and western clusters supports this idea. Data modeling will provide better estimates of where the rupture started and how and where it propagated, as well as which patches are locked and which are slipping.

“This event will definitely create more questions and discussions,” says Ruppert. “There’s still definitely the possibility of another magnitude-8.0 earthquake there, just looking at the upper portion of the gap that remained unruptured.”

 

Further Reading

Davies, J., Sykes, L., House, L., & Jacob, K. (1981). 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.

Fournier, T. J., & Freymueller, J. T. (2007). Transition from locked to creeping subduction in the Shumagin region, Alaska. Geophysical Research Letters, 34(6).
 
 

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