South Georgia Island, the largest island along the subduction trench. Credit: European Space Agency (modified Copernicus Sentinel data)
- A 47 km-deep, magnitude 7.5 earthquake that struck the south Atlantic in 2021 and caused a global tsunami was actually a sequence of five earthquakes.
- A shallow, "almost invisible" magnitude 8.2 quake accounted for 70% of the energy released during the event.
- Global earthquake monitoring needs to improve to understand and mitigate such complex earthquakes and their associated hazards.
Predicting hazards for complex earthquakes can be difficult, as the South Sandwich Islands quake demonstrates. The USGS initially reported the magnitude 7.5 quake and only added the 8.2 event the following day, as the surprise tsunami lapped on shores up to 10,000 kilometers away from its point of origin.
"We need to rethink our way to mitigate earthquake-tsunami hazards. To do that, we need to rapidly and accurately characterize the true size of big earthquakes, as well as their physical processes," Jia said.
Because this type of earthquake can result in unexpected tsunami, it's critical to improve our predictions. "With these complex earthquakes, the earthquake happens and we think, 'Oh, that wasn't so big, we don't have to worry.' And then the tsunami hits and causes a lot of damage," said Judith Hubbard, a geologist at the Earth Observatory of Singapore who was not involved in the study. "This study is a great example of how we can understand how these events work, and how we can detect them faster so we can have more warning in the future."
A magnitude 8.2 earthquake was "hidden" within a magnitude 7.5 earthquake in 2021, sending a mysterious tsunami around the world, according to a new study in Geophysical Research Letters.
Credit: Zhe Jia and AGU
Sneaky seismic signals
When an earthquake hits, it sends waves of vibration through the Earth. The global network of earthquake monitors uses those seismic waves to pinpoint the time, location, depth and magnitude of an earthquake. Common monitoring often focuses on short- and medium-periods of waves, Jia said, and longer periods can be left out. But even incorporating long periods into monitoring, on its own, isn't enough to catch complex earthquakes with messy seismic signals.
"It's hard to find the second earthquake because it's buried in the first one," Jia said. "It's very seldom complex earthquakes like this are observed. … And if we don't use the right dataset, we cannot really see what was hidden inside."
A simple earthquake can easily be pinpointed and described, Jia said. But a messy one needs to be carefully broken down into its constituent parts, to find out what unique combination of simpler earthquakes built up the complex one.
Jia and his colleagues developed an algorithm to tease apart the seismic signals during those messy earthquakes. By "decomposing" complex earthquake signals into simpler forms, using waves over different periods (varying from 20 to 500 seconds long), the algorithm can identify the location and properties of different sub-earthquakes. It's akin to someone with perfect pitch hearing five dissonant notes struck at once, yet being able to identify each individual note.
"I think a lot of people are daunted by trying to work on events like this," said Hubbard. "That somebody was willing to really dig into the data to figure it out is really useful."
Both Jia and Hubbard noted a long-term goal is to automate the detection of such complex earthquakes, as we can for simple earthquakes. For the 2021 quake, the tsunami was small by the time it reached shores, and most of the permanent residents of the remote, volcanic islands are penguins. But complex earthquakes can pose significant hazards if they generate larger tsunami or strike in a densely populated region.
