Earthquakes occur when rocks suddenly explode a fault – a boundary between two blocks or slabs. During large earthquakes, rock breakage can spread to the fault line. Now, geoscientists have recorded a ‘boomerang’ earthquake in equatorial Atlanta, where the rupture initially spreads away from the initial break, but then reverses and drives back to the next path at higher speeds.
While large earthquakes (magnitude 7 or higher) occur on the ground and are measured by nearby networks of seismometers, these earthquakes often cause motion along complex networks of faults, like a domino series.
This makes it difficult to trace the underlying mechanisms of how this ‘seismic slip’ occurs.
Under the ocean, many fault types have simple shapes, so provide the opportunity to get under the cover of the ‘earthquake engine’. However, they are far from large networks of seismometers on the ground.
Dr Stephen Hicks of Imperial College London and the University of Southampton used a new network of underwater seismometers to monitor the Romanche Fracture Zone, a fault line that stretches 900 km (559 miles) below the Atlantic near the equator.
In 2016, researchers recorded a magnitude 7.1 earthquake along the fracture zone and tracked the rupture along the fault.
This revealed that the rupture initially traveled in one direction before turning in the middle of the road in the midst of the earthquake and breaking the ‘sound seismic barrier’, becoming an extremely fast earthquake.
Only a handful of such earthquakes have been recorded globally.
The authors believe that the first phase of rupture was essential in causing the second phase, with sliding speed.
“While scientists have discovered that such a reversible decay 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. Hicks.
“Although the structure of the fault seems simple, the way the earthquake grew was not, and it was the complete opposite of how we expected the earthquake to look before we started analyzing the data.”
“However, if similar types of inverted or boomerang earthquakes can occur on the ground, a seismic rupture that turns in the middle of the road through an earthquake can dramatically affect the amount of earthquake caused,” the scientists said.
“Given the lack of previous observational evidence, this mechanism has not been calculated for earthquake scenario modeling and risk assessments from such earthquakes.”
“Detailed boomerang earthquake tracking can allow us to find similar patterns in other earthquakes and add new scenarios to their modeling and improve earthquake impact predictions.”
The findings were published in the journal Natural geosciences.
Hicks PS et al. The rear supershear-propagation rupture in the fault earthquake transformed the Romanche 2016 Mw 7.1. Nat. Geosci, published online August 10, 2020; doi: 10.1038 / s41561-020-0619-9