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Geologists confirm the bizarre ‘boomerang’ earthquake deep below the Atlantic

Earthquakes come and go, often leaving a trail of devastation. Fortunately, they usually don’t turn around right away and come back for another pass. Except … it looks like under extremely rare circumstances they can.

In a new study, scientists have found evidence of an unusual and virtually unprecedented boomerang earthquake that shook the deep sea floor beneath the Atlantic in 2016.

This earthquake – known as the “back propagating supershear rupture” – occurred along the Romanche rupture zone, which is near the equator, approximately halfway between the east coast of Brazil and the west coast of Africa.

The rupture zone, a fault that runs about 900 kilometers between the South American and African tectonic plates ̵

1; next to the mid-Atlantic ridge – caused a 7.1 magnitude earthquake in August 2016, which was also recorded by subsea seismometers in the region Monitoring stations.

010 Boomerang earthquake 1Reconstructed image of the Romanche fracture zone. (Hicks et al., Nature Geoscience, 2020)

Analysis of the signals shows that this was no ordinary tremor, but rather a strange tremor going in one direction before turning and coming back for more – and with a significant increase in speed no less.

“While scientists have found that such a reverse break mechanism is possible from theoretical models, our new study provides some of the clearest evidence of this enigmatic mechanism that occurs in real failure,” says senior researcher and seismologist Stephen Hicks of Imperial College London.

After analyzing the seismic data, the 2016 earthquake had two different phases.

First, the fracture spread upward and eastward toward a weak point where the fracture zone meets the mid-Atlantic ridge. Then in a sudden U-turn there was an “unusual westward rebound” with the tremors going back to the center of the fault and at greatly accelerated “supershear” speeds of up to 6 kilometers per second (3.7 miles per second) .

“Although the fault structure seems simple, the way the earthquake grew wasn’t like it, and this was completely opposite to what the earthquake would be expected to be like before we started analyzing the data,” says Hicks .

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While the team’s explanations of how this boomerang flipped remain speculative for now, the researchers believe that the first deep phase of the quake released enough fracture energy to trigger the fracture reversal in the shallower, western underwater terrain.

“Either both fault fields were preseismically loaded enough to promote seismogenic failure, or the deeper SE1 fracture immediately increased the static stress, which immediately led to the shallow SE2 part of the fault failing,” the authors explain in their paper .

While earthquakes that propagate backwards have previously been studied by seismologists, the evidence for this is so far sparse, with the phenomenon mainly seen in theoretical modeling.

Discovering such an event in the real world – in the middle of the ocean – is a unique event, let alone a boomerang that returned at supershear speed.

“To the best of my knowledge, this is the first time this has been reported,” said Yoshihiro Kaneko, geophysicist of GNS Science in New Zealand, who was not part of the study team National Geographic.

The results are reported in Natural geosciences.

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