Uncovering the secrets of a "mysterious tsunami" that spread around the world in 2021

In August last year, a sudden tsunami in the South Atlantic Ocean spread to distances of more than 10,000 km, spreading across the North Atlantic, Pacific and Indian Oceans.

It was the first time a tsunami had been recorded in three different oceans since the 2004 Indian Ocean earthquake, and scientists have only now discovered how the waves occur.

The epicenter in August was measured 47 km below the ocean floor, which is too deep to cause a large tsunami, even one with relatively small waves 15 to 75 cm (6 to 30 in) high.

But as it turns out, this tsunami wasn't the product of a single 7.5-magnitude earthquake.

A new look at the seismic data suggests that it was actually a series of five sub-quakes, and in the middle of it, hid a much larger and shallower rumble that may have caused the global tsunami.

This third "invisible" earthquake struck only 15 kilometers below the surface of the earth, with a magnitude of 8.2.

But in the midst of an earthquake, the monitoring systems got it all wrong.

"The third event is special because it was huge and was silent. In the data we usually look at [for earthquake monitoring], it was almost invisible," explains seismologist Zhe Jia of Caltech.

After chopping the seismic data into periods longer than 500 seconds, Jia and colleagues were able to detect the presence of an unprecedentedly slow, shallow earthquake.

And among groups of other regular ruptures, they found a 3-minute rumble that ruptured a 200-kilometre section of the plate front.

Altogether, this single event accounted for more than 70% of the total recorded seismic moment.

Thus, the researchers conclude: "The South Sandwich Island earthquake appears to be a combination of deep rupture and slow tsunami slip; this explains the somewhat unusual combination of relatively large depth and globally observed tsunamis."

The results indicate that our earthquake and tsunami warning systems need updating.

If we are to warn coastal communities of similar events, our systems need to read between earthquake lines to see larger earthquakes.

Earthquake monitoring systems tend to focus on short and medium periods of seismic waves, but longer periods also appear to contain important information.

"It's hard to find the second earthquake because it's buried in the first," Jia says. "Complex earthquakes like this are rarely observed, and if we don't use the right data set, we can't really see what was hidden inside."

Geologist Judith Hubbard, who works at the Earth Observatory in Singapore and was not involved in the current research, says she is grateful that others are digging into unexpected tsunami data to better understand where it came from.

And with these complex earthquakes, earthquake happens and we're thinking, if it wasn't that big, you don't have to worry.

And then a tsunami hits and causes a lot of damage.

This study is a great example of understanding how these events work, and how we can detect them faster so that we can have more warning in the future.

The study was published in Geophysical Research Letters.

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