An imperceptible earthquake caused the mysterious August tsunami in 2021

In August of 2021, a massive tsunami hit the North Atlantic, Pacific and Indian Oceans, and this was the first time that a tsunami had been recorded in 3 different oceans since the catastrophic earthquake in the Indian Ocean in 2004.

At the time, scientists believed that it was caused by a 7.5-magnitude earthquake that was detected near the South Sandwich Islands (a British territory in the South Atlantic).

Hidden earthquake

A report published by Live Science on February 11 states that this was not what it seemed. Scientists were puzzled when they discovered that the epicenter of the supposed earthquake was 47 km below the ocean floor, which is too deep to cause an earthquake. tsunami, and that the tectonic plate rupture that caused the quake was 400 km long - this kind of rupture should have caused a much larger earthquake.

Now, the new study, published Feb. 8 in Geophysical Research Letters, reveals that the quake was actually a series of five sub-quakes, separated by only a few minutes in time, and the third of these small quakes — A shallow 'invisible' earthquake hidden in the data and absent from monitoring systems at the time - an 8.2-magnitude earthquake, was responsible for the tsunami.

“The third event is special, because it It was huge and silent, in the data we usually look at [for earthquake monitoring], it was almost invisible."

Hybrid earthquake

The researchers were able to retrieve the third earthquake's signal from the entanglement of seismic waves by cutting the data into longer 500-second chunks and using an algorithm to extract its component parts. the entire event.

The subtle earthquake, which tore a 200-kilometer-long interface between two tectonic plates, occurred 15 kilometers below the Earth's surface, the ideal depth for a tsunami.

The researchers say the earthquake remained hidden because it was a hybrid between two types of ocean earthquakes, the "deep rupture" type caused by sudden plate slipping, and the "slow tsunami-generating slip" caused by the much slower grinding that sometimes lasts for weeks, one plate against the other.

These slow-slide earthquakes can release the same amount of tectonic energy as very powerful earthquakes, but their slow pace, plus no obvious seismic shaking, can often make them difficult to detect.

Undetected earthquake

Indeed, Jia says, most earthquake and tsunami warning systems tend to focus on tracking short to medium periods of seismic waves, leaving waves with longer periods, which can still generate life-threatening tsunamis, buried within the data.

And while the researchers want to change that, they have set a long-term goal of designing a system that can automatically detect and warn coastal areas of more complex earthquakes that cause tsunamis in the same way that current systems warn of simpler earthquakes.

“With these complex earthquakes, the earthquake happens and we think… yeah, it wasn't that big, you don’t have to worry, but then a tsunami comes along and it does a lot of damage,” said Judith Hubbard, a geologist at the Earth Observatory of Singapore (Earth Observatory of Singapore). ), which did not participate in the study.

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