How a 'Huge, Silent' Earthquake Caused a Mysterious Global Tsunami

Researchers have published an investigation into the mysteries behind an enormous "hidden" earthquake that caused a global tsunami last year.

In a study published in Geophysical Research Letters on February 8, scientists said that a total of five earthquakes, including one massive, "hidden" tremor, created the tsunami after striking close to the South Sandwich Islands on August 12, 2021, in the remote Southern Ocean off the coast of the continent of South America.

The study showed that tsunami-causing earthquakes such as the August 12 incident had complex patterns that could confound how scientists measure major tectonic activity.

The authors suggested that existing methods for mitigating the potential damage of earthquakes and tsunamis could need rethinking to make sure tsunami warnings were reliable.

Researchers described an initial rupture that happened deep below the oceanic crust in between the South American and South Sandwich tectonic plates at a depth of around 29 miles. This earthquake, which measured 7.5 on the Richter Scale, then triggered other quakes, including a much shallower—and much more violent—third earthquake.

This later 8.2 magnitude earthquake generated most of the energy in the rupture series, the researchers said, and had far-reaching effects, with waves recorded thousands of miles from its source.

The paper said that the tsunami caused by the earthquake spread far out across the ocean, reaching the North Atlantic, Indian and Pacific Oceans, where tide gauges measured peaks of several inches over 6,200 miles away in some cases.

Yet for such a huge and globe-spanning event, the earthquake left barely any trace in typical data used by scientists to study such phenomena, puzzling researchers as to what had really happened.

"The third event is special because it was huge, and it was silent," Zhe Jia, a seismologist at the California Institute of Technology, told news website Phys.org. "In the data we normally look at it was almost invisible.

"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."

The study showed that the signature of the larger, shallower earthquake was initially lost in the complex data caused by the multiple series of quakes that took place on August 12.

"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."

Stock image of tsunami warning sign
Stock image of tsunami warning sign. The research showed that complex, multiple rupture earthquakes could confound existing tsunami warning methods. laurence soulez/Getty Images