Earthquakes occur when rocks suddenly break on a fault – a boundary between two blocks or slabs. During large earthquakes, rock breaking can spread the fault line. Now geoscientists have recorded an earthquake ‘boomerang’ in the equatorial Atlantic Ocean, where the eruption initially spreads from initial break, but then rotates and runs the other way back at higher velocities.
The 7.1 magnitude earthquake in 2016 at the Romanche Fracture Zone in the equatorial Atlantic Ocean. The location of the map is given by the red rectangle on the input world. Image Credit: Hicks and others, doi: 10.1038 / s41561-020-0619-9.
While large (magnitude 7 or higher) earthquakes occur on land and are measured by nearby networks of seismometers, these earthquakes often trigger movement along complex networks of faults, such as a series of dominoes.
This makes it difficult to follow the underlying mechanisms of how this ‘seismic slip’ occurs.
Under the ocean, many types of fault have simple shapes, giving the possibility under the hood of the earthquake engine. However, they are far from large networks of seismometers on land.
Dr Stephen Hicks of Imperial College London and the University of Southampton used a new network of underwater seismometers to check the Romanche Fracture Zone, a fault line that stretches 900 km (559 miles) below the Atlantic Ocean near the equator.
In 2016, investigators recorded a magnitude 7.1 earthquake along the fracture zone and followed the fault along the fault.
This showed that the eruption initially traveled in one direction before half-reversing the earthquake and breaking the ‘seismic sound barrier’, and became an ultra-rapid earthquake.
Only a handful of such earthquakes have been recorded worldwide.
The authors believe that the first phase of the fracture was crucial for causing the second, rapid abrasive phase.
Reconstructed image of the Romanche Fracture Zone. Image Credit: Hicks and others, doi: 10.1038 / s41561-020-0619-9.
“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 guilt,” said Drs. Hicks.
“Even though the error structure seems simple, the way the earthquake did not grow, and this was in stark contrast to how we expected the earthquake to look before we started analyzing the data.”
“However, if similar types of inversions such as boomerang earthquakes could occur on land, a seismic eruption that rotates halfway through an earthquake could dramatically affect the amount of soil caused,” the scientists said.
“Given the lack of observational evidence for now, this mechanism has not been responsible in modeling earthquakes and assessing the dangers of such earthquakes.”
“The detailed tracking of the boomerang earthquake could allow us to find similar patterns in other earthquakes and add new scenarios to their modeling and improved earthquake forecasts.”
The findings were published in the journal Nature Geoscience.
_____
SP Hicks and others. Backward Propagating Supershear Break in 2016 Mw 7.1 Romanche Transform Error Earthquake. Nat. Geosci, published online August 10, 2020; doi: 10.1038 / s41561-020-0619-9
