Earthquakes come and go, leaving a trail of destruction in them. What they do not normally do is happily turn around immediately and return for another pass. Except … it seems like they can, in very rare circumstances.
In a new study, scientists have found evidence of an unusual and nearly unusual earthquake ‘boomerang’ that shook the deep seabed under the Atlantic Ocean in 2016.
This earthquake – called a “rear-end supershear rupture” – took place along the fracture zone of the Romanche, which lies close to the equator, roughly between the east coast of Brazil and the west coast of Africa.
The fraction zone, an error that traveled about 900 kilometers between the South American and African tectonic plates – next to the Mid-Atlantic Ridge – produced a 7.1 magnitude earthquake in August 2016, which was detected by seismometers in the region, such as by remote monitoring stations.
Reconstructed image of the frache zone of the Romanche. (Hicks et al., Nature Geoscience, 2020)
Analysis of the signals reveals that this was not an ordinary shaking, but an early temblor that went one way before turning around and coming back for more – and with a significant increase in speed, no less.
“Although scientists have found that such a mechanism for reversible fracture is possible from theoretical models, our new study provides some of the clearest evidence for this enigmatic mechanism occurring in real debt,” says lead researcher and seismologist Stephen Hicks of Imperial College London.
According to analysis of seismic data, the 2016 quake had two distinct phases.
First, the fracture propagated upward and to the east in the direction of a weak spot where the fracture zone meets the Mid-Atlantic Ridge. Then, in a sudden over-face, an “unusual rearward to west” occurred, with the tremors returning to the center of the fault, and at much faster “supershear” speeds of up to 6 kilometers per second (3.7 miles per second). second).
“Even though the fault structure seems simple, the way the earthquake grew was not, and this was in stark contrast to how we expected the earthquake to look before we started analyzing the data,” Hicks says.
fia GIPHY
While the team’s explanations for how this boomerang reversed remain largely speculative, the researchers hypothesize that the first, deep phase of the quake released sufficient fracture energy to initiate the reversal of the fracture in the shallower, western underwater terrain.
“Either fault patches were sufficiently pre-seismically stressed to promote seismic fault, as the deeper SE1 rupture immediately increased the static stress, causing the shallow SE2 portion of the fault to fail immediately,” the authors explain in their paper.
While backward-propagating earthquakes have been studied by seismologists to date, the evidence for them is sparse, with the phenomenon mostly seen in theoretical modeling.
One like this discovered in the real world – out in the middle of the ocean – is a first-of-its-kind kind of event, let alone a boomerang that returned at superhear speed.
“To the best of my knowledge, this is the first time it has been reported,” said geophysicist Yoshihiro Kaneko of GNS Science in New Zealand, who was not part of the student team, National Geographic.
The findings are reported in Nature Geoscience.
.
