Earthquakes are generally understood to rupture outward from their underground origins, sending seismic waves in one or two directions along faults. New research from the Massachusetts Institute of Technology suggests that under certain conditions, the disruption could temporarily return along the same path. The findings, published in AGU Advances, show that so-called boomerang earthquakes are not limited to complex fault systems. Computer simulations show that even a single straight fault can cause inversion if the friction changes rapidly during the event and the rupture extends far enough in one direction. According to the press release of Yurec AlertResearchers say this behavior may not have been detected in past earthquake records and could affect how hazards are assessed.

‘Boomerang’ earthquake could reverse direction on a single straight fault

Boomerang earthquakes have only been recorded a few times. In 2016, an earthquake in the Atlantic Ocean appeared to move east and then veer west. Similar patterns have been suggested for the 2011 Tohoku earthquake in Japan and the 2023 Turkey-Syria earthquake.Such events were often associated with complex networks of intersecting faults. A new study questions that assumption. It proposes that mature straight faults, including parts of the San Andreas Fault, can also cause this type of rupture.The researchers focused on whether Earth’s complexity is always needed to explain the effects. Their results suggest otherwise.

Changes in friction along faults can cause reversals

The research team built a computer model that represents a simple elastic crust with a single straight fault. They tested how the fracture behaved under different lengths, starting points, and directions of travel.Only earthquakes that moved in one direction showed a reversal pattern. In such cases, friction along the fault did not simply drop and remain low. Rather, it fell, then rose, and then fell again. This change created a situation where part of the rupture could split and return to its original orientation.The explanation is technical but focuses on stress. When a section of a fault stops slipping, stress can build up again behind the moving fault. That stored energy can cause a second slip in the opposite direction.

Large earthquakes may behave differently than small earthquakes

Simulations suggest that distance matters. The destruction must reach far enough before reversal is possible. This means that larger earthquakes can exhibit behavior not seen in smaller earthquakes.From the surface, people will hardly notice the change in direction. Ground shaking is influenced by various factors. Even so, the shaking tends to be larger in the direction in which the crack propagates. If the destruction were reversed, some areas could experience two violent movements within seconds.Researchers believe that current detection methods may miss these backpropagation fronts. This idea is still being researched. For now, it adds a new layer to the way we understand earthquake physics, especially for faults that were once thought to have simpler behavior.