Seismologists have discovered the source of a mysterious tsunami last year that spread around the world from the remote South Atlantic.
In August 2021, a magnitude 7.5 earthquake struck near the South Sandwich Islands, creating a tsunami that spread across the globe. The epicenter was 47 kilometers below the surface of the Earth., too deep to start a tsunami, and the rupture was almost 400 kilometers long, which should have generated a much larger earthquake. Seismologists were puzzled and sought to understand what really happened.
A new study, published in Geophysical Research Letters, reveals that the earthquake was not a single event, but five, a series of sub-earthquakes spread over several minutes. The third sub-earthquake was a shallower, slower magnitude 8.2 quake that struck just 15 kilometers below the surface. That unusual and “hidden” earthquake was probably the trigger for the global tsunami.
Because the South Sandwich Islands earthquake was complex, with multiple sub-earthquakes, its seismic signal was difficult to interpret, according to the study’s lead author, Zhe Jia, a seismologist at the California Institute of Technology. The magnitude 8.2 earthquake was hidden within the tangle of seismic waves, which interfered with each other during the event. The hidden quake signal wasn’t clear until Jia filtered out the waves using a much longer period, up to 500 seconds. Only then did the 200-second-long quake become apparent, which Jia said represented more than 70% of the energy released during the earthquake.
“The third event is special because it was huge and silentJia said in a statement. “In the data that we normally look at [para el monitoreo de terremotos]It was almost invisible.”
Predicting the hazards of complex earthquakes can be difficult, as the South Sandwich Islands earthquake demonstrates. The US Geological Survey initially reported the 7.5 magnitude quake and only added the 8.2 event the following day, when the surprise tsunami hit the coasts up to 10,000 kilometers away from its point of origin.
“We need to rethink how we mitigate earthquake and tsunami hazards. To do so, we need to quickly and accurately characterize the actual size of large earthquakes as well as their physical processes,” Jia said.
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When an earthquake occurs, 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 wavesJia said, and longer periods can be skipped. But even incorporating long monitoring periods alone is not enough to detect complex earthquakes with disordered seismic signals.
“It’s hard to find the second quake because it’s buried in the first one,” Jia said. “It is very rare that complex earthquakes like this are observed… And if we don’t use the right data set, we can’t really see what was hidden inside.”
A simple earthquake can be easily identified and described, Jia said. But a messy one must be carefully broken down into its constituent parts, to discover what unique combination of simpler earthquakes built the complex.
Jia and his colleagues developed an algorithm to separate the seismic signals during those messy earthquakes. By “breaking down” complex earthquake signals in simpler forms, using waves during different periods (varying from 20 to 500 seconds in length), the algorithm can identify the location and properties of different sub-earthquakes. It is similar to someone with perfect hearing hearing five dissonant notes played at once, but being able to identify each individual note.
“I think a lot of people are intimidated trying to work events like this,” Hubbard said. “That someone was willing to dig into the data to find out is really helpful.”
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Both Jia and Hubbard noted that a long-term goal is to automate earthquake detection as complex as we can for simple earthquakes. For the 2021 earthquake, the tsunami was small when it reached the shores, and most of the permanent residents of the remote volcanic islands are penguins. But complex earthquakes can pose significant dangers if they generate a larger tsunami or hit a densely populated region. (I)
Source: Eluniverso
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