Hidden M8.2 earthquake triggers mysterious 2021 global tsunami

Strange Sounds – February 9, 2022

Scientists have uncovered the source of a mysterious 2021 tsunami that sent waves around the globe. A magnitude 8.2 earthquake was “hidden” within a magnitude 7.5 earthquake in 2021, sending a mysterious tsunami around the world.

In August 2021, a magnitude 7.5 earthquake hit near the South Sandwich Islands, creating a tsunami that rippled around the globe. The epicenter was 47 kilometers below the Earth’s surface — too deep to initiate a tsunami — and the rupture was nearly 400 kilometers long, which should have generated a much larger earthquake.

Seismologists were puzzled and sought to understand what really happened that day in the remote South Atlantic.

A new study revealed the quake wasn’t a single event, but five, a series of sub-quakes spread out over several minutes. The third sub-quake was a shallower, slower magnitude 8.2 quake that hit just 15 kilometers below the surface. That unusual, “hidden” earthquake was likely the trigger of the worldwide tsunami.

The study was published in the AGU journal Geophysical Research Letters, which publishes short-format, high-impact papers with implications that span the Earth and space sciences.

Because the South Sandwich Islands earthquake was complex, with multiple sub-quakes, its seismic signal was difficult to interpret, according to lead study author Zhe Jia, a seismologist at the California Institute of Technology. The magnitude 8.2 quake was hidden within the tangle of seismic waves, which interfered with each other over the course of the event. The hidden quake’s signal wasn’t clear until Jia filtered the waves using a much longer period, up to 500 seconds. Only then did the 200-second-long quake, which Jia said accounted for over 70% of the energy released during the earthquake, become clear.

“The third event is special because it was huge, and it was silent,” Jia said. “In the data we normally look at [for earthquake monitoring], it was almost invisible.”

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.

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