Earthquakes are generally understood to rupture outward from their starting point beneath the ground, sending seismic waves along a fault line in one or two directions. New research from the Massachusetts Institute of Technology suggests that, in certain conditions, a rupture can briefly turn back along the same path. The findings, published in AGU Advances, indicate that so-called boomerang earthquakes are not limited to complex fault systems. Computer simulations show that even a single straight fault may experience a reversal if friction changes quickly during the event and if the rupture travels far enough in one direction. According to a press release at EurekAlertresearchers say the behavior may have gone undetected in past seismic records and could influence how hazards are assessed.
“Boomerang” earthquakes can reverse direction on a single straight fault
Boomerang earthquakes have been recorded only a few times. In 2016, a quake in the Atlantic Ocean appeared to move east before turning back west. Similar patterns have been suggested in the 2011 Tohoku earthquake in Japan and the 2023 Turkey-Syria earthquake.Such events were often linked to complicated networks of intersecting faults. The new study challenges that assumption. It proposes that a mature, straight fault, including sections of the San Andreas Fault, could also produce this kind of rupture.The researchers focused on whether complexity in the Earth is always required to explain the effect. Their results suggest it is not.
Friction changes along a fault can trigger reversal.
The team built a computer model representing a simple elastic crust with one straight fault. They tested how ruptures behaved under different lengths, starting points and travel directions.Only earthquakes that moved in one direction showed the reversal pattern. In those cases, friction along the fault did not simply drop and stay low. Instead, it fell, then rose, then fell again. This shift created conditions where part of the rupture could split and move back towards its origin.The explanation is technical but centers on stress. When part of the fault stops sliding, stress can build again behind the moving rupture. That stored energy may then trigger a second slip in the opposite direction.
Large earthquakes may behave differently from small ones.
The simulations suggest that distance matters. A rupture must travel far enough before reversal becomes possible. This implies that larger earthquakes may show behavior not seen in smaller events.From the surface, people would not easily notice the change in direction. Ground shaking is influenced by many factors. Still, shaking tends to be stronger in the direction a rupture travels. If a rupture reverses, some areas could experience intensified motion twice within seconds.Researchers believe that current detection methods may overlook these back-propagating fronts. The idea remains under study. For now, it adds another layer to how earthquake physics is understood, particularly on faults once thought to behave in simpler ways.
