The source of a mysterious global tsunami has been found near Antarctica

When a mysterious series of earthquakes erupted from the uninhabited South Sandwich Islands, scientists around the world found themselves puzzled.

An earthquake measuring 7.5 on the Richter scale initially struck Under the Islands, a British Overseas Territory in the frigid waters of the South Atlantic. Three minutes later, an 8.1-magnitude earthquake struck the region again.

These vibrations, which occurred on August 12, 2021, were not strange on their own, because the islands are located on top of a combative meeting of tectonic plates. The strange part is that they were followed by a tsunami powerful enough to appear on distant shores along the Atlantic, Pacific and Indian oceans. Although the swell was not devastating, it was the first since the 2004 tsunami to be recorded in three different oceans.

Historic whaling stations on South Georgia Island, such as those at Grytviken, have been abandoned, but the British Antarctic Survey maintains a research station at King Edward Point. Meanwhile, the South Sandwich Islands are currently uninhabited. There is no geophysical monitoring station on these islands nor seismographs on the ocean floor, which makes studying earthquakes there difficult.

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While certain types of earthquakes are known to cause tsunamis, initial depth estimates for these earthquakes indicated that they were too deep to sufficiently bend the sea floor and push a massive amount of water out. “It was a huge mystery, and a huge challenge for the seismology community,” says Zhi Jia, a graduate student in geophysics at Caltech.

After spending months solving this mysterious earthquake sequence, Jia and his colleagues think they’ve figured out what happened. In a study published this month in Geophysical Research Letters, the team concluded that there were actually five large faults that day, components of one powerful earthquake that occurred within a few minutes of each other. Previously buried in loud seismic signals, one of these ruptures was strong and shallow enough to trigger a multi-ocean tsunami.

By deciphering this strange seismic event, Earth scientists can develop a better understanding of how earthquakes generate tsunamis. “We rely heavily on initial seismic estimates to guess whether the earthquake triggered a tsunami,” says Judith Hubbard, a structural geologist at Earth Observatory in Singapore who was not involved in the study.

“I think the event tells us more that our tsunami detection systems may not be good enough.”

The series of individual earthquakes has also made researchers wonder whether we will fully capture the many complexities of our planet. “As time goes on, and we see more earthquakes, they tend to get more exotic and complex,” says Stephen Hicks, a seismologist at Imperial College London who was not involved in the study.

Glaciers are rolling off the coast of the remote South Georgia island. When the earthquake struck nearby in August 2021, it sent tsunami waves to distant shores along the Atlantic, Pacific, and Indian Oceans.

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Falling panels, raising eyebrows

Below the isolated South Sandwich Islands, the South American tectonic plate is sinking beneath the South Sandwich Plate at a modest velocity of 2.8 inches per year. The uneven grinding between these two plates causes the stress to increase over time. Sometimes this pressure is released in earthquakes – including the kind that can generate tsunamis.

These large coastal waves usually form when something displaces a lot of water. In this case, Hubbard says, “The thing that causes a tsunami is the deformation of the sea floor.” But the earthquake here has to be shallow enough for a tsunami like this to happen.

The 7.5-magnitude earthquake occurred on August 12 at a depth of 29 miles, so “it was very unlikely to have triggered the global tsunami that we later observed,” says Jia. The magnitude 8.1 event immediately afterward was a slightly more suspicious event, with a depth of less than 14 miles.

But the seismic signals from this earthquake sequence were dangerously chaotic. The two earthquakes hit each other in very rapid succession. The data is a bit like recording a person speaking when someone else starts yelling over them, making a lot of unfathomable noise. Automated systems struggled to produce consistent values ​​for the magnitude, location, and depth of the second earthquake.

“We all thought we were missing something,” Jia says.

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Left: Grytviken Church, also called Whalers Church, is a Norwegian Anglican church built on the island of South Georgia in 1913.

Right: A cracked and empty penguin egg on the island of South Georgia.

Earthquick sandwich

Over the next few months, Jia and his team untangled the chaotic web of seismic waves that day, removing interference and identifying individual ruptures within the chaos. In the end, they discovered that there were not two major rips in this event, but five, all of which struck within just 260 seconds.

“In the first sense, this is one earthquake,” Hicks says. It was complicated — and powerful. “It tore off the greater part of this subduction zone.”

The first two earthquakes, both with a magnitude of 7.2, lasted only 23 and 19 seconds, respectively. These two ruptures, when combined, produced what was previously thought to be a single event with a force of 7.5. The last two earthquakes—magnitudes 7.6 and 7.7, respectively—were also short-lived earthquakes.

However, the third rupture in the rock, trapped between the other four, was notable. Recorded as a magnitude 8.2 event, it was extremely powerful, unleashing most of the entire five-part earthquake’s energy. It was also slow, taking 180 seconds to appear. This was partly why it stayed hidden for so long: The team wasn’t using the right kind of seismic detector.

Seismic wave forms of slow earthquakes are different from the kind you usually get with sudden ruptures. When Jia altered filters on data recorded by a global network of seismographs to look for much slower ruptures, the event suddenly jumped.

In addition to being stronger and longer lasting, this earthquake was very shallow, only 8.5 miles deep – fully capable of triggering a global tsunami.

King penguins, the second largest species of penguin in the world, congregate in the tens of thousands on the island of South Georgia. The animals there were too far from feeling the August 2021 earthquake.

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Strangeness shakes the world

Earthquakes are rarely as direct as they first appear. Sometimes, as with the 2016 Kaikoura earthquake in New Zealand, this complexity is easy to see. Like Rube Goldberg’s rocky machine, the quake showed a rip that jumped between dozens of different faults. But the mysteries of this earthquake can be solved more easily.

“In this case, you’re talking about surface rupture,” says Casey Aderhold, a seismologist at the Incorporated Research Institute of Seismology who was not involved in the new study. “It’s tearing up all these different flaws, and it’s at the top. We can just walk around on them and look at them.”

Not so for the strange earthquake that struck the South Sandwich Islands in 2021. “It’s deep, it’s under the ocean, it’s way too far in. You can’t touch it or walk around it,” says Aderhold. There is no geophysical monitoring station on these islands, nor are there Seismometers on the ocean floor listen to the screeching subduction zone.

Researchers have only been able to decipher geological processes thanks to the Global Seismological Network – an open-access, planet-wide network of 152 seismometers operated jointly by the US Geological Survey and the National Science Foundation. “It’s really important,” Jia says. Without node recording of seismic vibrations coming from distant earthquakes, it would be impossible to decipher strange events like this.

As with most complex earthquakes, there are still some mysteries. The 8.2-magnitude rupture likely caused the tsunami, but details are not clear yet.

“A lot of tsunamis actually involve undersea landslides, which can be triggered by earthquakes,” says Robert Larter, a marine geophysicist at the British Antarctic Survey. “I personally wonder if that’s the case here.” The only way to check is to look at the sea floor – perhaps with a sonar-equipped boat, or with robotic submarines – and compare its appearance with ancient bathymetric maps.

Geologists also wonder if the subduction zone under the South Sandwich Islands could continue to trigger strong earthquakes. The split last year was wide and strong. “Does this mean that this probably won’t be torn apart in such a big event for the last, 500, 1,000 years?” Hicks asks. The lack of hardware in the area makes this a “really difficult question to answer”.

The good news is that the discovery of this quintuple earthquake will enhance scientists’ ability to see similar events coming next time. “This provides an opportunity to detect these types of slow rupture events from seismic data, and with that we can fire warnings faster and more accurately,” Hubbard says.

The bad news is that this wouldn’t be the last time an earthquake tricked scientists into hoping to make up for their most destructive elements. As Larter puts it: “The world of nature is full of surprises.”

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