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Rare ‘boomerang’ earthquake is tracked across the Atlantic ocean

A rare type of earthquake known as a ‘boomerang’ has been tracked in the ocean for the first time, and it could help scientists know how they cause devastation on land.

Scientists from the University of Southampton and Imperial College London studied the path one of these quakes took along a fault under the Atlantic ocean. 

Earthquakes occur when rocks suddenly break on a fault – a boundary between two blocks or plates – and during a large quake the breaking can spread on the fault line. 

In the case of a boomerang earthquake the rupture initially spreads away from the original break but then turns and runs back the other way at higher speeds.

The strength and the duration of the rupture along the fault influences how much ground is shaken up on the surface – and the level of damage to buildings. 

Unlike a normal earthquake that spreads destruction along a single path, a boomerang sends a second faster wave that increases the area of destruction. 

Knowing the mechanisms of how faults rupture and the physics involved will help researchers make better models to predict future earthquakes, the team said. 

While large earthquakes occur on land and have been measured by nearby networks of monitors known as seismometers, these earthquakes often trigger movement along complex networks of faults, like a series of dominoes. 

This makes it difficult to track the mechanisms of how this ‘seismic slip’ occurs.

Under the ocean, many types of fault have simple shapes, so provide the possibility get under the bonnet of the ‘earthquake engine’. 

However, they are far away from the large networks of seismometers on land. 

The team made use of a new network of underwater seismometers to track the Romanche fracture zone, a fault line stretching 560 miles under the Atlantic.   

In 2016, the team recorded a magnitude 7.1 earthquake along the Romanche fracture zone and tracked the rupture along the fault. 

This revealed that initially the rupture travelled in one direction before turning around midway through the earthquake and coming back.

During its ‘boomerang’ return run it broke the ‘seismic sound barrier’, becoming an ultra-fast earthquake, the researchers explained.

Only a handful of these earthquakes have been recorded globally. 

The team believe that the first phase of the rupture was crucial in causing the second, rapidly slipping phase.  

First author of the study Dr Stephen Hicks, from Imperial, said this is the clearest evidence of a boomerang rupture mechanism 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 analyse the data.’

However, the team say that if similar types of reversing or boomerang earthquakes can occur on land, a seismic rupture turning around mid-way through an earthquake could dramatically affect the amount of ground shaking caused.

Given the lack of observational evidence before now, this mechanism has been unaccounted for in earthquake scenario modelling and hazard assessments. 

The detailed tracking of the boomerang earthquake could allow researchers to find similar patterns in other earthquakes.

Doing so would allow them to add new scenarios into their modelling and improve earthquake impact forecasts. 

The findings have been published in the journal Nature Geoscience. 

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