A tidal wave spreads around the world from the South Atlantic. It did not cause damage or casualties, but the earthquake that had caused it had not been identified
On August 12, 2021, a tsunami spread from the area of the remote South Sandwich Islands, a well-known seismic area in the South Atlantic between South America and Antarctica. In a few hours the anomalous waves, which reached a maximum height of 75 centimeters on the uninhabited and icy coasts of the Antarctic continent, spread to the oceans all over the world: after 22 hours they entered (weakly) into the Mediterranean with a height of some millimeters, after 25 hours they arrive in Alaska, 12,000 kilometers away. They were detected only thanks to satellites and latest generation measurement systems. The tsunami did not cause damage or casualties, but a mystery remained among scientists: which earthquake had generated them?
Mysterious shock
The earthquake that was recorded that day in the South Sandwiches had a magnitude of 7.5 and a hypocenter at a depth of 47 kilometers, two elements not sufficient to give rise to a tsunami of that magnitude. The rupture, however, occurred along an arc subduction line about 400 kilometers long which, according to geophysical models, should instead have given rise to a much stronger earthquake. So there was something wrong with the data: something else must have happened that hadn’t been recorded by the seismographs. Or, if it was registered, it didn’t appear. It took almost a year of studies and analyzes to be able to unravel the complex of measurements made and to discover what happened in the depths of the earth’s crust between 16.32 and 16.35 on 12 August 2021.
Research
An article published on Geophysical Research Letters
clarifies what happened. With a sophisticated analysis of seismic waves, which filtered the medium and short-term components from the long-term ones (20-500 seconds of frequency), the group led by Zhe Jia, a seismologist at the California Institute of Technology, found that no it was a single earthquake, but no less than five: the most powerful lasted three minutes, had a magnitude of 8.2 and an epicenter just 15 kilometers deep. This earthquake was responsible for 70% of the energy released by the entire event and had power and depth compatible with the subsequent development of the tsunami. Except that it happened within a sequence and with low frequency waves that were hidden by the other four events.
The alarm and the risks
These complex and hidden earthquakes must make us understand that saying “the shock is not strong and deep, so we don’t have to worry” can be misleading, commented Judith Hubbard, geologist at the Earth Observatory in Singapore, because inside it can hide events. like that of the South Sandwich Islands which can generate dangerous tsunamis if we are not able to give the alarm in time.
February 11, 2022 (change February 11, 2022 | 12:33)
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