Boomerang Earthquakes: New MIT Study Reveals Ruptures Can Reverse Direction
Earthquakes are typically thought to propagate outward from their epicenter, sending seismic waves along fault lines in one or two directions. However, groundbreaking research from the Massachusetts Institute of Technology suggests that under specific conditions, a rupture can briefly turn back along its original path. This phenomenon, known as a "boomerang" earthquake, challenges long-held assumptions in seismology.
Findings Published in AGU Advances
The study, published in the journal AGU Advances, indicates that boomerang earthquakes are not confined to complex fault systems. Computer simulations demonstrate that even a single straight fault may experience a reversal if friction changes rapidly during the event and if the rupture travels sufficiently far in one direction. According to a press release from EurekAlert, researchers believe this behavior may have gone undetected in historical seismic records and could significantly influence how earthquake hazards are assessed.
Boomerang Earthquakes on Straight Faults
Boomerang earthquakes have been recorded only a few times in history. For instance, a 2016 earthquake in the Atlantic Ocean appeared to move eastward before turning back westward. Similar patterns have been suggested in the 2011 Tohoku earthquake in Japan and the 2023 Turkey-Syria earthquake. Previously, such events were often attributed to intricate networks of intersecting faults. The new study challenges this assumption, proposing that mature, straight faults—including sections of the San Andreas Fault—could also produce this type of rupture. The researchers focused on whether geological complexity is always necessary to explain the effect, with results suggesting it is not.
Friction Changes Trigger Reversal
The team developed a computer model representing a simple elastic crust with one straight fault. They tested how ruptures behaved under varying lengths, starting points, and travel directions. Only earthquakes that moved in one direction exhibited the reversal pattern. In these cases, friction along the fault did not simply drop and remain low. Instead, it fell, then rose, and fell again. This fluctuation created conditions where part of the rupture could split and move back toward its origin.
The explanation is technical but centers on stress dynamics. When part of the fault stops sliding, stress can rebuild behind the moving rupture. This stored energy may then trigger a second slip in the opposite direction, leading to the boomerang effect.
Implications for Large Earthquakes
The simulations suggest that distance is a critical factor. A rupture must travel far enough before reversal becomes possible, implying that larger earthquakes may exhibit behaviors not observed in smaller events. From the surface, people would not easily notice the change in direction, as ground shaking is influenced by numerous factors. However, 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 suspect that current detection methods may overlook these back-propagating fronts. While the idea remains under study, it adds a new layer to our understanding of earthquake physics, particularly on faults once thought to behave in simpler ways.
