“Almost invisible”: researchers discover mysterious phenomenon underground

Deepak Gupta February 11, 2022
Updated 2022/02/11 at 6:52 AM

Scientists have discovered the source of a mysterious tsunami that swept across the world’s oceans in 2021. The giant wave was triggered by a 7.5 magnitude earthquake near the South Sandwich Islands in the sub-Antarctic South Atlantic. Specifically, the researchers were interested in two oddities. On the one hand, the epicenter of the phenomenon was at a depth of 47 kilometers – until now it was not assumed that a tremor at such a depth would tsunami could trigger. On the other hand, they found a geological fault almost 400 kilometers long at this location. Normally this should have been far less.

Tsunami 2021: “Hidden” earthquake was the reason

The researchers showed that the earthquake was not a single event. Rather, there were five subquakes that lasted several minutes. The third, an 8.2 magnitude quake, struck just 15 kilometers below the surface. This “hidden” earthquake was likely the trigger for the tsunami.

Because the South Sandwich Islands earthquake was so complex, its seismic signal was difficult to interpret. Zhe Jia, a seismologist at the California Institute of Technology, agrees. Together with two colleagues, Jia published his results as part of a study in Geophysical Research Letters.

The signal from the subquake only became clear when the team filtered the waves. The earthquake, which lasted 200 seconds, only became visible after a period of 500 seconds. According to Jia, the tremor accounted for more than 70 percent of the energy released in the quake.

“Great example”

“The third event is special because it was huge and it was silent. In the data we normally [für die Erdbebenüberwachung] look at it, it was almost invisible. We need to rethink the way we mitigate the risk of earthquakes and tsunamis. To do this, we need to quickly and accurately characterize the true size of large earthquakes, as well as their physical processes.”

Zhe Jia (via American Geophysical Union)

This event shows that it can be difficult to predict the risks of complex earthquakes. First, the United States Geological Survey, a scientific agency under the division of the United States Department of the Interior, reported a magnitude 7.5 earthquake. It was only on the following day that the third of the subquakes was added. The reason for this was the tsunami, which hit the coast around 10,000 kilometers from its epicenter at the time.

With these complex events, earthquakes are often underestimated. According to geologist Judith Hubbard, only the major damage caused by the resulting tsunami would provide an indication of the actual magnitude of the quake. The study is “a great example of how we can understand how these events unfold and how we can spot them faster so we can be better warned in the future.”

Sources: “The 2021 South Sandwich Island Mw 8.2 Earthquake: A Slow Event Sandwiched Between Regular Ruptures” (2022, Geophysical Research Letters); American Geophysical Union

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