Earthquakes come and go, often leaving a trail of devastation after they wake up. What they don’t usually do, fortunately, is to go back immediately and come back for it. Except… it looks like they can, in extremely rare cases.
In a new study, scientists found evidence of an unusual and almost unprecedented ‘boomerang’ earthquake that shook the deep sea floor below the Atlantic in 2016.
This earthquake – known as the “backward propagating ultrasonic fault” – occurred along the Romanche fault zone, located near the equator, near the line between the east coast of Brazil and the west coast of Africa. .
The fault zone, a fault that runs about 900 km between the South American and African tectonic plates ̵1; adjacent to the Mid-Atlantic Range – created a magnitude 7.1 quake in August. 2016, discovered by submarine seismographs in the area, as well as by remote observation stations.
Analysis of the signals revealed that this was not a common earthquake, but rather a strange earthquake going one way, before turning and turning again – and at a significantly increased rate.
“While scientists discover that such reverse fault mechanisms could occur from theoretical models, our new study provides some of the clearest evidence for the mysterious mechanism. This happened in a real error, “said researcher and seismologist Stephen Hicks from Imperial College London.
According to an analysis of seismic data, the 2016 earthquake has two distinct phases.
First, the fault propagates upwards and to the east in the direction of a weakness, where the fault zone is adjacent to the Mid-Atlantic Hill. Then, in a sudden surface interval, an “abnormal backward propagation to the west” took place, with the vibrations returning to the center of the fracture and at the “ultrasonic” speed greatly accelerated. up to 6 km / sec (3.7 miles per second).
“Even though the fracture structure looks simple, the way the earthquake evolved, it did not, and this is in stark contrast to how we predicted the earthquake before we started the data analysis,” Hicks said. material, ”said Hicks.
While the team’s explanations of how the boomerang came back is still speculative, the researchers hypothesize that the first deep phase of the quake released enough fracture energy to capture. Early fault reversal in shallower terrain, in the underwater west.
The authors explain in their paper: “Both patches are pre-stressed enough to induce seismic failure or a deeper SE1 fracture that immediately increases the static stress. That means that the shallow SE2 part of the error is broken.
While back-propagating earthquakes have been previously studied by seismologists, the evidence for them is still very little today, with the phenomenon mainly seen in theoretical models.
To detect something like this in the real world – in the middle of the ocean – is the first of its kind, let alone a boomerang that turns back at supersonic speeds.
“To my knowledge, this is the first time it has been reported,” said geophysicist Yoshihiro Kaneko from GNS Science in New Zealand, who was not part of the research team. National Geographic.
The findings are reported in Natural geoscience.