Earthquakes usually come and go, and they often leave traces of devastation. However, what earthquakes do not usually do is turn around again. But it appears that it can do so under very rare circumstances!
In a new study, geologists have found evidence of a rare and unprecedented aftershock that shook the bottom of the Atlantic in 2016.
This tremor is called “back-propagating supershear rupture”, which is an earthquake that bounces backward spreading and refraction occurs at a speed higher than the velocity of the shear waves (surface waves), so the result is the plane penetrates the sound wall and exceeds its speed Sound, which leads to a sonic explosion.
This tremor occurred along the Romanche fracture zone (rift zone) near the equator between the west coast of Africa and the east coast of Brazil .
In this rift zone, a earthquake of magnitude 7.1 occurred in August 2016. The fault extends in that region for about 900 km between the African tectonic plate and the South American tectonic plate; Close to the Mid-Atlantic Mountains. The earthquake was detected by seismometers installed under the ocean in the area, as well as monitoring stations far from the area.
The analysis of the seismic waves indicated that it was an unusual tremor, it was a strange earthquake that happened in one direction, but quickly turned and returned again more quickly than before.
Lead researcher and seismologist Stephen Hicks of Imperial College London said: “While scientists have found that this reversible shock mechanism is possible from theoretical models, our new study provides some clear evidence for this ambiguous mechanism in the fault. real”.
According to the analysis of seismic data, the 2016 earthquake marked two different phases.
First, the quake moved up and east towards a weak point where the fault zone meets the Mid-Atlantic Mountains. Then, in a sudden shift, the seismic wave headed and spread backward to the west, where the tremors returned to the center of the rift zone at a breakneck speed of 6 kilometers per second (3.7 miles per second).
“Although the fault synthesis (cracking) appears simple, the way in which the earthquake happened is not,” Hicks said. “This was totally against our expectations of what the earthquake would look like before we started analyzing the data.”
Although the researchers’ explanations are still speculative, they assume that the first deep stage of the earthquake released enough fault energy to reverse the fracture direction toward the shallow, underwater terrain.
“Either both parts of the fault were exposed to sufficient pressure before the earthquake to release sufficient seismic energy, or the seismic wave of deep refraction of the first side immediately increased the pressure on the surface part of the fault, leading to the release of the energy of the fault,” the researchers said in their paper.
Although seismologists and experts have previously studied reversal earthquakes (back-propagation of a full shear rupture), evidence for them is still sparse, often limited to theoretical simulations.
The observation of an earthquake like this, in the middle of the ocean, is the first of its kind, not to mention the force of seismic recoil that occurred at high speed (broke the sound wall).
“As far as I know, it is the first time that an earthquake like this has been reported,” said geophysicist Yoshihiro Kaneko of the GNS Science Research Institute in New Zealand (who was not part of the research team).