Scientists have discovered the source of the mysterious 2021 tsunami that sent waves around the world. In August 2021, a magnitude 7.5 earthquake near the South Sandwich Islands triggered a tsunami that swept across the globe. The epicenter was reported 47 kilometers below the earth’s surface – deep enough for current scientific and technical means to create a tsunami.

Seismologists were shocked to find out what happened that day in the far south Atlantic.

Now, a new study shows that an earthquake is not an event, but a series of five aftershocks that spread in a matter of minutes. The third quake was a magnitude 8.2 earthquake just 15 km below the surface. This unusual and “hidden” earthquake could be the triggering point of a global tsunami.

The research is published in the AGU Journal of Geophysical Research Letters, which publishes short and highly influential papers on Earth and space science.

Caltex seismologist She Jia, the lead author of the study, commented that the earthquake in the South Sandwich Islands is complex and has multiple sub-seismic, making it difficult to interpret its seismic signal. The quake, which measured 8.2 on the Richter scale, was obscured by a series of seismic waves that interacted with each other. The secret seismic signal was not clear until Jia filtered the waves for 500 seconds. Only then, Jia said, would an earthquake last more than 70 percent of the energy emitted during an earthquake that lasted 200 seconds.

“The third event is special because it’s so big and quiet,” Jiah said.

It is difficult to predict the dangers of complex earthquakes as shown by the earthquake on the South Sandwich Islands. U.S. The Geological Survey first reported an earthquake measuring 7.5 on the Richter scale, followed by a magnitude 8.2 the next day.

“We need to rethink how to mitigate earthquake-tsunami hazards,” Jia said.

Since such earthquakes can lead to unpredictable tsunamis, it is important to improve the forecast.

Hidden seismic signal

When an earthquake occurs, it sends vibration waves across the earth. These seismic waves are used by the Global Seismic Monitoring Network to determine the time, location, depth and magnitude of earthquakes. Normal observation focuses on short and medium waves, while long-wavelengths may be lost, Jiah said. But even long-term surveillance is not sufficient to capture seismic signals that interfere with complex earthquakes.

“It’s hard to find a second earthquake, because it’s buried in the first,” Jiah said.

Jiah pointed out that simple earthquakes can be easily detected and described. But a chaotic earthquake needs to be carefully subdivided so that a unique combination of simple and complex earthquakes can be found.

In response, Jia and colleagues developed an algorithm to separate seismic signals from these random earthquakes. The algorithm can convert complex seismic signals into simple shapes, using waves of different duration (20 seconds to 500 seconds in length), to identify the positions and characteristics of different seismics. It’s like listening to five dissenting notes at the same time, but recognizing each individual note.

“I think it scares a lot of people trying to deal with things like this. It’s really useful that someone is actually willing to check the data to find out,” said Judith Hubbard, a geologist at the Earth Observatory in Singapore. Not involved in the study.

Jiah and Hubbard agree that the long-term goal is to automatically detect such complex earthquakes, as we can detect simple earthquakes. By the time the 2021 earthquake hit the coast, the tsunami was small, and most of the permanent inhabitants of this remote volcanic island were penguins. But complex earthquakes can cause significant damage if they cause large tsunamis or occur in densely populated areas.