Earthquake series was source of mysterious global tsunami

Hidden magnitude-8.2 earthquake

Seismologists said this week (February 8, 2022) they’ve uncovered the source of a mysterious global tsunami, an unusually large ocean wave, recorded at many coastal sea-level stations around the world. This tsunami happened in August 2021. The scientists said the quake wasn’t a single event, but five, which happened over several minutes. The third quake was a shallower, slower magnitude-8.2 quake that hit just 9 miles (15 km) below the surface. That unusual “hidden” earthquake likely triggered the global wave.

The peer-reviewed journal Geophysical Research Letters published these scientists’ study on Feburary 8.

The first in the series of earthquakes was a magnitude-7.5 earthquake that struck in the south Atlantic Ocean. It was a start to finding the tsunami’s overall source, but had left scientists puzzled. The quake’s epicenter was almost 30 miles (47 km) deep and had a fault rupture nearly 250 miles (400 km) long. It was too deep to cause the tsunami.

That’s when the third, shallow, magnitude-8.2 quake – only 10 mile (15 km) deep – was re-named the primary cause of the tsunami.

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Finding the ‘hidden’ quake

Zhe Jia, the lead study author and a seismologist at the California Institute of Technology, says that the earthquake’s seismic signal was challenging to interpret. It was a complex tangle of quakes with seismic waves that interfered with each other. The scientists had to use a special filter to narrow down the source.

Earthquakes send vibrations throughout the Earth. A network of programs around the globe work together to pinpoint their time, location, depth, and magnitude when earthquakes strike. Jia said in a statement that earthquake monitoring often focuses on short- and medium-period seismic waves. The longer-period waves can be left out. It wasn’t until Jia filtered the record of the waves to include the longer periods, up to 500 seconds, that the hidden quake’s signal became clear.

Jia added:

It’s hard to find the second earthquake because it’s buried in the first one. It’s very seldom complex earthquakes like this are observed … And if we don’t use the right dataset, we cannot really see what was hidden inside.

Bottom line: Scientists said on February 8, 2022, that they’ve learned a magnitude-8.2 quake that hit just 15 kilometers below the surface of the south Atlantic Ocean was the source of a mysterious global tsunami in August 2021.

Read more from EarthSky: Earlier tsunami warnings via magnetic fields

Source: The 2021 South Sandwich Island Mw 8.2 Earthquake: A Slow Event Sandwiched Between Regular Ruptures

Via AGU