In August 2021, a mysterious tsunami rippled throughout the world, traveling an astonishing 10,000 kilometers (6,000 miles) across three different oceans. Seismologists were instantly puzzled – there was a magnitude 7.5 earthquake in the south Atlantic directly prior to the tsunami, but at 47 kilometers (29 miles) below the surface, it was far too deep in the ocean to generate such a wave. The scientists discovered that the huge rupture, spanning 400 kilometers (250 miles), should have resulted in a more significant earthquake, and the mystery has remained ever since. 

Now, new research published in the journal Geophysical Research Letters suggests the causative earthquake that occurred near the South Sandwich Islands was not just one event, but actually five individual sub-quakes in a row, hiding a shallower magnitude 8.2 earthquake that resulted in the far-reaching tsunami. Such a result suggests global earthquake monitoring may require improvements to be able to identify more complex quake interactions. 

Identifying deep-ocean earthquakes is no easy task, and current seismological techniques only take brief snapshots of the event. However, this does not always provide researchers with the full story, so Zhe Jia, Zhongwen Zhan, and Hiroo Kanamori delved into longer, 500-second snippets of the data to look for anything that could explain the tsunami. It was only then that they discovered a 200-second magnitude 8.2 quake that struck just 15 kilometers (9 miles) below the surface, much shallower than the single event was believed to be. At this depth and magnitude, conditions were perfect to send a significant tsunami rippling across the surface. Yet, in the chaotic rumble of multiple other quake events, it passed by undetected. 

“The third event is special because it was huge, and it was silent,” Jia said in a statement. “In the data we normally look at, it was almost invisible.” 

In this brief 200-second period, 70 percent of the total energy from all the quakes combined was released, sending a tsunami that traveled across the Atlantic, Pacific, and Indian oceans. This tsunami was unproblematic, but the scientists are calling on seismologists to learn from the study to improve predictions of the fallout of earthquakes. 

“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. 

“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, geologist at the Earth Observatory of Singapore, who was not involved in the study.