One rare phenomenon that seismologists have been trying to track for years is an event called a "boomerang earthquake." An instance has been observed, and it is much weirder than first expected.

A research team led by the University of Southampton and Imperial College London had successfully recorded a magnitude 7.1 earthquake back on August 29, 2016. The "boomerang earthquake" ran along the 560-mile long Romanche fracture zone that lies underneath the Atlantic Ocean, between Brazil and Africa, near the equator. Their findings were reported in the latest issue of journal Nature Geoscience.

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When Earthquakes "Boomerang"

Boomerang earthquakes are named as such because of its sudden returning action. Usually, a quake begins along a fault line that separates two parts of the Earth's crust, sending a shockwave in one direction.

However, there are rare instances where this quake abruptly stops and reverses direction. Furthermore, when an earthquake actually boomerangs, it does so with higher speeds and magnitude compared to the original wave.

In the 2016 Atlantic occurrence analyzed by the research team, the quake first traveled in one direction between the African and South American tectonic plates. It then boomeranged back to its point of origin at ultra-fast speeds. The research team even noted that the return quake broke the seismic sound barrier, becoming an earthquake with its own sonic boom of some kind.

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Scientists also commented that large earthquakes, such as those of magnitude 7.0 or higher, are especially difficult to study. Larger earthquakes tend to set off a series of chain reactions, mostly smaller, quakes from affected fault lines in the trajectory of the original earthquake. Furthermore, since this one occurred under the Atlantic Ocean, an underwater network of seismometers needs to be in place.

An Elusive and Largely Theoretical Phenomenon

The research team suggests that the first quake somehow triggered the second, boomerang counterpart. However, they are still quite unsure of how the phenomenon exactly took place. This study provides conclusive evidence of the occurrence of these "boomerang earthquakes," which has remained elusive and mostly theoretical. 

"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," said Dr. Stephen Hicks, lead author of the study and member of the Department of Earth Sciences and Engineering at Imperial College London.

Furthermore, the recorded data from the boomerang earthquake surprised the research team. Dr. Hicks added that though the fault structure appeared simple, the development of the underwater earthquake was far from simple. "This was completely opposite to how we expected the earthquake to look before we started to analyze the data."

The report also recognized that this phenomenon remains mostly unaccounted for, especially for earthquake-related scenario modeling and hazard assessment studies. Successfully tracking other instances of boomerang earthquakes would allow seismologists to improve impact forecasts by having data to model hazards from these events.

An international project between the Natural Environment Research Council from the UK, the National Science Foundation in the US, and the European Research Council was the key to detecting and recording the rare event. The underwater seismometer network was a part of the PI-LAB and EUROLAB experimental projects.