Hidden subantarctic earthquake caused global tsunami in 2021 | Polarjournal
The South Sandwich Islands are a chain of volcanic islands in the middle of the South Atlantic Ocean, at the end of the Scotia Arc. On the one hand, the islands are home to millions of penguins, but on the other hand, they are also in the midst of numerous earthquakes in the region, which sometimes cause a tsunami. Picture: Michael Wenger

The subantarctic region of the South Sandwich Islands belongs to a seismically very active zone. The islands are all volcanic and some are still very active. Earthquake monitors in the wider area regularly record tremors and earthquakes in the region, which also trigger tsunami warnings at times. Last August, a tsunami ended up being recorded around the globe. The real reason for this remained unknown for a long time. But now researchers have found an explanation.

Seismologists Zhe Jia, Zhongwen Zhan and Hiroo Kanamoori of the California Institute of Technology (CalTec), analyzing earthquake data from the South Sandwich Islands, discovered that not one earthquake but a series of five had occurred that day. The strongest of these, with a magnitude of 8.2, occurred at a depth of just 15 kilometers, which had triggered the recorded tsunami. “The quake recorded at that time near the South Sandwich Islands was complex and consisted of several sub-earthquakes,” explains Jia, the study’s lead author. “This made it difficult to interpret the seismic signal.” The three scientists published the results of their work in the new issue of the journal Geophysical Research Letters.

The Scotia Arc is an insertion of the Scotia Plate between South America and Antarctica. South Georgia and the South Sandwich Islands are at the edge of the arc, the latter having originated as volcanic islands. The quake responsible for the tsunami was so slow, despite its magnitude, that it was almost invisible in the data. Map: Michael Wenger via Google Earth / Graphic: Jia et al. (2022)

In August 2021, researchers recorded an earthquake near the South Sandwich Islands at a depth of 47 kilometers with a magnitude of 7.5. Until now, the assumption was that quakes at this depth do not cause tsunamis. Experts were all the more surprised when tsunami waves hit beaches more than 10,000 kilometers away in the Pacific and Indian oceans and even the North Atlantic, without warning. Jia and his colleagues analyzed the data from that time and discovered a hidden quake in a series of five that had made up the quake at that time. To do this, they had to filter the earthquake waves specifically by the length of the waves. They found a signal lasting about 200 seconds in a third quake that, according to their calculations, was responsible for about 70% of the quakes’ total energy “The third event is special because it was huge and it was silent,” Jia continues. “In the data we normally look at, it was almost invisible.” With the help of a newly developed algorithm, the researchers were only able to discover the earthquake hidden among the four other quakes.

“But we need to rethink our way to mitigate earthquake-tsunami hazards”

Zhe Jia, Seismologist CalTec

Typically, earthquakes are analyzed for waves that are between 20 and 500 seconds long. The researchers distinguish here between “simple” and “messy” quakes. In the latter, longer waves, i.e. “slower”, earthquakes can hide between such shorter signals and are thus not registered. However, the work of Jia and his colleagues goes even further, because the story can become even more complicated. “It is rare to observe complex earthquakes like this,” he said. “It’s hard to find the second earthquake because it’s buried in the first one. And if we don’t use the right dataset, we cannot really see what was hidden inside.” As a result, potential tsunamis can also be missed.

In the case of the series of earthquakes last August, the outcome was mild. This is because the waves were relatively small — around 20 centimeters — when they hit coastal areas. “But we need to rethink our way to mitigate earthquake-tsunami hazards,” Jia says of the study’s findings. “To do that, we need to rapidly and accurately characterize the true size of big earthquakes, as well as their physical processes.” Without these data, a next quake could result in a tsunami that would surprise more than just the penguins on islands surrounding the Scotia Arc, such as South Georgia and the South Orkneys.

Dr Michael Wenger, PolarJournal

Link to the study: Jia, Z., Zhan, Z., & Kanamori, H. (2022). The 2021 South Sandwich Island Mw 8.2 earthquake: A slow event sandwiched between regular ruptures. Geophysical Research Letters, 49; https://doi.org/10.1029/2021GL097104

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