For the First Time, Scientists Find Evidence of a ‘Boomerang Earthquake’
In a first, scientists have confirmed a “back-propagating supershear rupture” — in simpler terms, a ‘boomerang earthquake.’
Published in Nature Geoscience on Tuesday, the study was conducted by an international team of experts from the UK, the US, Germany, and Japan, among others. Recorded on August 29, 2016, a magnitude 7.1 earthquake ran along the Romanche fracture zone, a 560-mile-long fault line under the Atlantic Ocean near the Equator, between Brazil and Africa. But, instead of the temblor heading in one direction, it came back with a significant increase in speed. Using local ocean-bottom seismometers and teleseismic data of the earthquake and its aftershocks, the researchers have concluded that this presented clear evidence of a ‘boomerang earthquake.’
The earthquake initially headed eastward, but then turned back west — unlike a normal earthquake that spreads destruction along a single path, a ‘boomerang earthquake’ sends another second wave. “Whilst scientists have found that such a reversing rupture mechanism is possible from theoretical models, our new study provides some of the clearest evidence for this enigmatic mechanism occurring in a real fault… Even though the fault structure seems simple, the way the earthquake grew was not, and this was completely opposite to how we expected the earthquake to look before we started to analyze the data,” Dr. Stephen Hicks, an earthquake seismologist at the Imperial College London, and lead author of the study, said, adding: “This was a weird sort of configuration to see.”
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However, scientists aren’t sure exactly how this happened. At the moment, they’re speculating that the first phase released sufficient energy to trigger the rupture reversal. In addition, when, and how often, ‘boomerang earthquakes’ happen, also remain unanswered.
Until now, the lack of observational evidence for ‘boomerang earthquakes’ had prevented the phenomenon from being accounted for in earthquake scenario-modelling and hazard assessments. “Studies like this help us understand how past earthquakes ruptured, how future earthquakes may rupture, and how that relates to the potential impact for faults near populated areas,” Kasey Aderhold, a seismologist with the Incorporated Research Institutions for Seismology, who wasn’t involved in authoring the study, told National Geographic. Given the colossal risks that earthquakes pose to lives and livelihoods, gaining further insights into the phenomenon of ‘boomerang earthquakes’ may help scientists assess potential hazards better, and and implement more efficient warning systems for them in future.
“To the best of my knowledge, this is the first time it has been reported… [But] this might be actually more common than we think,” said Professor Yoshihiro Kaneko from the Department of Geophysics at Kyoto University, and a former research seismologist at GNS Science, New Zealand, who was not involved in the study. “The more and more we look at earthquakes in more detail, of course we see stranger things,” Dr. Hicks concluded.