A powerful offshore earthquake triggered a tsunami warning for communities along a 700-mile (1,100-kilometer) stretch of Alaska’s southern coast on Wednesday, July 16. Fortunately, the wave never came, and ground shaking caused minimal damage, but another large quake could strike this area in the near future.
Since 2020, five earthquakes ranging from magnitude 7.2 to 8.2 have struck the southern coast of Alaska. It’s not unusual for seismic activity to occur in this part of the state, as it runs along an active tectonic plate boundary called the Aleutian subduction zone. Still, seeing five large quakes within close range of each other in just five years has captured the attention of seismologists like Michael West, Alaska’s state seismologist and director of the Alaska Earthquake Center. West told Gizmodo that the southern coast appears to be experiencing an earthquake sequence. While it’s possible that Wednesday’s quake was the last in this sequence, it’s also possible that more large earthquakes—or even one huge one—could strike within the next few years, he said.
“Five earthquakes is enough to be statistically significant,” West said. “This area is clearly undergoing a period of strain release while other areas of this particular boundary are—at the moment—a bit more quiet.”
Earthquakes occur when accumulated stress along the border between two converging tectonic plates suddenly releases, causing them to slip past each other. Sometimes, one earthquake is enough to relieve the stress on a particular section, but not always. It can take multiple quakes spanning several years to release a significant buildup of stress, which is probably happening on Alaska’s southern coast.
Wednesday’s quake occurred southeast of Sand Point, a small town in the Aleutian Islands. According to the U.S. Geological Survey, it resulted from strike-slip faulting—when two tectonic plates slip horizontally past each other—in the Aleutian subduction zone. Seismologists have been keeping a close eye on this part of the plate boundary since the 1980s, when they identified it as an area of accumulating tectonic stress, West said. It took another 40 years for that built-up stress to finally result in seismic activity.
“Starting in 2020, it was like, ‘Okay, now it’s time for this particular segment to do its thing,’” West said.
It’s fortunate—and “remarkable”—that none of the large quakes that have struck this area since 2020 have produced a tsunami, he added. Seismologists know the Aleutian subduction zone is capable of triggering devastating, Pacific-wide tsunamis. In 1946, for example, an 8.6 magnitude earthquake in this plate boundary caused a tsunami that traveled all the way to the shores of Antarctica and killed more than 150 people in Hawaii. The epicenter of that quake was located just 100 miles away from that of Wednesday’s quake, West said.
The earthquake sequence currently unfolding in this part of the Aleutian subduction zone could lead to a few different scenarios, he explained. If Wednesday’s quake released all the accumulated stress in this segment, seismic activity could stall out and remain quiet for decades. Alternatively, it could take several more magnitude 7 to 8 quakes—or a single magnitude 9—to release all the stress.
“The societal consequences are very, very different for those two different paths,” West said. Unlike the relatively inconsequential quakes Alaska’s southern coast has experienced in the last five years, a magnitude 9 would be highly likely to produce a dangerous tsunami and damaging ground shaking. Fortunately, “there are, and long have been, very strong preparedness efforts underway in these communities,” West said. “Every community near here has been studied for its tsunami inundation potential.”
That said, “we can always do more education,” he added. “We’ve always got training to do to help people understand how to use that information, and how to do it quickly. You don’t have much time [during] events like this.”
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