A slowly oscillating wail cut through the sunlit summer night outside Kasey Aderhold's house in Homer, Alaska. The same siren had sounded just hours before as a test of the tsunami warning system—but this time the danger was real.
Some 380 miles to the west, off the coast of the Alaskan Peninsula, a magnitude 8.2 earthquake had just ripped through this part of the planet—the biggest temblor the U.S. has seen in more than 50 years. The event, which struck just after 10 p.m. local time on July 28, jolted nearby shores and sent landslides rushing down the steep coastal mountains.
Luckily, the quake caused little damage. It struck far from dense population centers and only caused rough seas. But such a large event would be devastating elsewhere. A magnitude 8.2 earthquake is nearly as large as the biggest earthquake thought possible along the San Andreas fault.
"The ‘big one’ that we talk about in Southern California, it's like that," says Wendy Bohon, an earthquake geologist at the Incorporated Research Institutions for Seismology (IRIS), an assemblage of research universities that collects, curates, and distributes U.S. seismic data.
This intense shaking had dissipated to gentle rocking by the time it reached Aderhold, an earthquake seismologist also at IRIS. She looked up to see her hanging stained glass lamp—a remnant from the house's past owners—slowly swinging side to side "like a pendulum," she says.
The quake now serves as a powerful reminder of the restlessness of our planet's surface—and it presents an exciting opportunity to peer deeper at our planet’s inner workings. The event was so strong, it sent out seismic waves that lit up detectors around the world, even disturbing groundwater levels in Washington County, Maryland.
Because the waves move differently through rocks with different temperatures and compositions, scientists can use these seismic rattles and shakes like planetary x-rays to map out Earth's innards. Additionally, comparing this quake to past events can help scientists better understand the region's potential to produce future shakes.
"Every earthquake we're recording is going to teach us something new," Aderhold says. "It's an excitement to see that data."
A shaky history
The powerful earthquake struck off the southern coast of Perryville, on the Alaskan Peninsula. Here, the land juts out from North America in a thin curving spit that peters out into a series of islands, like beads falling off a string. This sweep of land and the quake that recently rocked it both come from the same source: a subterranean battle between tectonic plates.
The Pacific plate is slowly shoving its way beneath the overlying North American plate, shifting northward roughly 2.5 inches each year in the zone where the new quake struck. The process, known as subduction, can raise mountains and is responsible for the volcanoes that built the Aleutian Islands. But the pair of tectonic plates don't smoothly slide by each other, and each slow shift builds stresses along the fault until it hits a breaking point and the land suddenly shifts in a ground-rattling quake.
Just such an event happened during last night's temblor, which scientists think fractured right at the subsurface juncture between the Pacific and North American plates.
This tectonic battle means that earthquakes in Alaska come as no big surprise. On average, a quake rumbles detectors of the Alaskan Earthquake Center every 15 minutes, which translates to tens of thousands of temblors each year.
The last time a larger earthquake struck in the United States was also in Alaska, when a magnitude 8.7 quake hit near the Aleutian's Rat Island in 1965. This was just a year after a powerful magnitude 9.2 temblor rocked the region, the second-largest earthquake ever recorded anywhere in the world.
The latest event is particularly intriguing to scientists because it struck just a few dozen miles east of two large temblors that gripped the region in 2020: a magnitude 7.8 on July 22 and a magnitude 7.6 on October 19.
While the numerical differences sound small, they translate to huge amounts of energy. Bohon explains it using pasta: If one spaghetti noodle is the energy released by a magnitude 5 quake, then 900 noodles would represent a magnitude 7, and 25,000 noodles would be a magnitude 8.
The position of these three large events is likely no coincidence: While an earthquake may release stress in one area, it can layer on stress in nearby zones, increasing their potential for future quakes.
"Every earthquake makes other earthquakes more likely," Bohon says. The latest quake struck within 25 miles of another magnitude 8.2 that rattled the region in November 1938. Scientists are still analyzing the event, but it’s possible the zone where these two quakes tore through the subsurface may even overlap.
These quakes both sit adjacent to a curious area known as the Shumagin Gap. This so-called seismic gap is an area along the subduction zone that hasn’t had a big earthquake in a relatively long time—the Shumagin Gap has largely laid quiet over the last century. The July 2020 temblor broke part of the gap, surprising some scientists because they thought the plates in this region were slowly sliding past one another and not building up enough stress to trigger a big quake.
The latest quake doesn't appear to have reached into the quiet zone. But it may help provide further clues into the subterranean forces at play. Many questions remain about the Shumagin Gap and the reasons why it might differ from other stretches along the Alaskan coast.
"Does it have something to do with how the stress and strain are building up? Does it have something to do with the properties of the rocks in the Shumagin Gap?" Bohon wonders.
"As we have more and more earthquakes, we can figure out how the plate behaves through time," she adds.
Big shakes but little waves
The release of big tectonic jolts has the potential to unleash another devastating force: tsunamis. As the Pacific plate shoves its way under the Alaskan coast, the overlying North American plate bunches up. But when a fault unleashes an earthquake, the land suddenly shifts and the upper plate springs back, shoving the nearby ocean waters like the mechanism in a wave pool.
Right after the July 28 quake, warning systems sent out tsunami alerts for places as far away as the Hawaiian islands, and more waves than usual began crashing on nearby shores, but no towering walls of water materialized.
That’s because of the latest quake's depth, starting at least 20 miles underground. The largest movements along a plate happen in the subterranean zone where the break begins, known as the hypocenter. So if a quake breaks near the surface, this translates to lots of motion in the ocean. But if the quake is deep, like the recent magnitude 8.2, that motion dissipates by the time the temblor reaches shallower parts of the plate.
For now, the risk of a tsunami has long since passed, and while aftershocks will continue to rattle the region, they will all likely be comparatively small. The chance of a quake magnitude 7 quake or greater in this part of Alaska over the next week is less than 4 percent, according to the USGS.