Earthquakes come and go, often leaving traces of destruction on their doorstep. What they usually do not do, fortunately, is to go back right away and come back for another switch. Except … it seems they can, in extremely rare circumstances.
In a new study, scientists have found evidence of an unusual and virtually unprecedented ‘boomerang’ earthquake that shook the deep sea water under the Atlantic Ocean in 2016.
The quake – dubbed a “backward-breaking supershear collapse” – struck along the Romanche fracture zone, which lies near the equator, roughly midway between the east coast of Brazil and the west coast of Africa.
The fracture zone, a fault that runs for about 900 kilometers between the tectonic plates of South America and Africa ̵1; adjacent to the Mid-Atlantic Ridge – produced a magnitude 7.1 earthquake in August 2016, which was detected by underwater seismometers in the region, as and from remote monitoring stations.
Signal analysis reveals that this was not an ordinary earthquake, but a strange temptation that went a long way, before turning back and forth for more – and with a considerable increase in speed, no less.
“While scientists have discovered that such a reversible decay mechanism is possible from theoretical models, our new study provides some of the clearest evidence for this enigmatic mechanism occurring in a real fault,” says lead researcher and seismologist Stephen Hicks from Imperial College London.
According to seismic data analysis, the 2016 earthquake had two distinct phases.
First, the fissure spreads upward and eastward toward a weak point, where the fracture zone meets the Mid-Atlantic Backbone. Then, in an unexpected face, an “unusual back-west spread” took place, with vibrations going back to the center of the fault, and “supershear” speeds accelerating very fast to 6 kilometers per second (3.7 miles per second). ).
“Although the structure of the fault seems simple, the way the earthquake grew was not and it was completely the opposite of how we expected the earthquake to look before we started analyzing the data,” says Hicks.
While the team’s explanations for how this boomerang reversed remain speculative for now, researchers hypothesize that the first, deep phase of the quake released enough fracture energy to begin reversing the rupture in shallow, westerly underwater terrain. .
“Either both fault faults were sufficiently stressed preseismically to promote seismogenic failure or deeper SE1 rupture immediately increased static stress, immediately causing the shallow SE2 portion of the fault to fail,” the authors explain in their paper. them.
While back-spreading earthquakes have been studied by seismologists before, so far, evidence for them is scarce, with the phenomenon seen primarily in theoretical modeling.
To discover one in the real world – out in the middle of the ocean – is an event of the first kind, let alone a boomerang that returned at supershear speed.
“To the best of my knowledge, this is the first time it has been reported,” geophysicist Yoshihiro Kaneko of GNS Science in New Zealand, who was not part of the study team, told. National Geographic.
Findings are reported in Natural geosciences.