Invisible earthquake provoked a powerful tsunami

(ORDO NEWS) — A new study has found that the mysterious source of the tsunami that swept the entire globe and spread 10,000 kilometers from its epicenter was an “invisible” earthquake.

In August 2021, a huge tsunami hit the North Atlantic, Pacific and Indian Oceans. This was the first time a tsunami had been recorded in three different oceans since the disastrous 2004 Indian Ocean earthquake; at the time, scientists thought it was caused by a magnitude 7.5 earthquake found near the South Sandwich Islands (a British territory in the South Atlantic Ocean).

But not everything was as it seemed. Scientists were baffled to find that the quake’s supposed epicenter was 47 km below the ocean floor, too deep to trigger a tsunami. It also turned out that the gap in the tectonic plate that gave rise to it was located at a depth of almost 400 km – such a gap should have caused a much larger earthquake.

A new study published in the journal Geophysical Research Letters has shown that the quake was actually a sequence of five sub-earthquakes separated in time by just minutes.

And the third of these mini-earthquakes – a smaller, “invisible” earthquake, hidden in the data and missed by monitoring systems at the time – was an earthquake of magnitude 8.2, which caused the tsunami.

“The third event is special because it was huge and yet very quiet,” said Caltech seismologist Zhe Jia. “In the data we usually look at [for earthquake monitoring], it was almost invisible.”

The researchers were able to extract the signal from the third quake from the tangle of seismic waves by breaking the data into longer 500-second chunks and using an algorithm to isolate its component parts.

Only then did a 200-second earthquake manifest, which Jia said accounted for 70% of the energy released during the entire event. The hidden earthquake that tore apart the 200 km long boundary between the two plates occurred just 15 km below the Earth’s surface – the ideal depth for a tsunami to occur.

The researchers say the quake remained hidden because it was a hybrid of two types of oceanic quakes: a “deep rip” type, which occurs as a result of sudden plate shifts, and a “slow tsunamigenic shift,” where plates can tectonically rub against each other for weeks.

Slow earthquakes can release just as much tectonic energy as earthquakes of greater magnitude, but their pace and the fact that they do not generate large seismic tremors often make them difficult to detect.

In fact, most earthquake and tsunami warning systems tend to focus on tracking short to medium periods of seismic waves, leaving longer periods, which are still capable of generating life-threatening tsunamis, hidden within the data.

The researchers want to change this and have set a long-term goal to develop a system that can automatically detect and warn coastal areas of more complex tsunami-producing earthquakes, in much the same way that existing systems do for simpler earthquakes.

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