The cause of the tsunami in August 2021 was a previously unnoticed tremor, seismologists from the California Institute of Technology discovered. They shared their discovery in an article in the journal Geophysical Research Letters.

In August 2021, a tsunami unexpectedly appeared in the South Atlantic Ocean, spreading over more than 10,000 km across the North Atlantic to the Pacific and Indian Oceans. This event was the first since 2004 when a tsunami was recorded in three different oceans.

The epicenter of the August tsunami was at a depth of 47 km under the ocean floor - too deep to cause a tsunami of this magnitude. However, the wave formation was not caused by a single earthquake of magnitude 7.5, as previously thought, but by a series of five aftershocks, one of which, with a magnitude of 8.5, occurred at a depth of only 15 km under the ocean floor. The shock lasted about three minutes, during which it caused a 200 km long tectonic plate to break. This shock alone accounted for more than 70% of the seismic moment of the entire tsunami. It remained unnoticed until seismologists began to analyze observations in periods of 500 seconds - usually work is done with shorter time periods.

“It is difficult to find the second earthquake because it is hidden in the first one,” the researchers explain.

Today, earthquake monitoring systems, as a rule, analyze short and medium periods of seismic wave activity, the authors of the work note. But even longer periods seem to contain important information. This must be taken into account in further observations - if you do not notice the origin of the tsunami in time, it can lead to much more damage.

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