Boomerang Earthquakes Don’t Just Move Forward — They Bounce Back
Earthquakes are generally considered one-way events. Stresses build up deep underground, a fault ruptures, and seismic ruptures propagate outward from their point of origin, shaking the ground as they go. But in some cases, researchers have observed something strange: earthquakes that seem to reverse their direction, shaking regions that had already passed through moments earlier.
These unusual events, called “boomerang” earthquakes, are typically associated with complex fault systems, that is, places where multiple fractures intersect and interact in unpredictable ways. Now, a new study published in AGU progress suggests that these seismic reversals may not be so rare after all.
According to the researchers, even simple, straight faults – such as segments of the San Andreas Fault in California – may be capable of producing earthquakes that reverse on themselves under the right conditions.
Learn more: Seismic faults can heal themselves in just a few hours, adding power to disastrous earthquakes
What is a Boomerang earthquake?
A boomerang earthquake is exactly what it sounds like: a seismic rupture that propagates along a fault, then partially reverses its trajectory. Scientists call them “back-propagating fronts,” meaning that the rupture doesn’t just move forward but splits, with one part continuing forward while another returns to where it started.
A handful of real earthquakes have exhibited this behavior in recent years. In 2016, an earthquake in the middle of the Atlantic Ocean appeared to move east before ricocheting west. Similar seismic signatures may have occurred in the massive 2011 earthquake off Japan and the devastating 7.8 magnitude quake in 2023 in Turkey and Syria. These events occurred both in complex fault zones and in regions dominated by simple, straight faults – an observation that raised new questions for seismologists.
“Our work suggests that these boomerang earthquakes may have gone undetected in a number of cases,” study author Yudong Sun said in a press release. “We think this behavior might be more common than we’ve seen in seismic data so far.”
How Researchers Recreated Boomerang Earthquakes
To explore how a boomerang earthquake might form, the research team turned to computer simulations. Using a physical model of a simple fault, they recreated earthquakes under a wide range of conditions, modifying the displacement of ruptures and the behavior of friction along the fault surface.
In these simulations, friction along the fault played a crucial role. When friction decreased, increased, and quickly fell again, sections of the fault that had already slipped could become stuck and then rupture a second time. Unilateral earthquakes, which rupture in only one direction rather than propagating in both directions, are the only type of earthquake to consistently produce boomerang behavior.
“When the earthquake propagates in one direction, it produces a ‘rupture’ effect that reduces slip speed, increases friction, and allows only a narrow section of the fault to slip at a time. The region behind the earthquake, which stops slipping, may then rupture again, because it has accumulated more stress to slip again,” explained co-author Camilla Cattania.
What can we learn from Boomerang earthquakes?
Understanding boomerang earthquakes is not only an exercise in curiosity for scientists, but could also help them better predict and prevent earthquakes and their devastating damage in the future.
“We know that shaking is amplified in the direction of rupture and that buildings would shake more in response. So there is a real effect in terms of resulting damage. This is why it is important to understand where these boomerang events could occur,” Cattania concluded in the press release.
Overall, the new study suggests that older, simpler faults – long thought to be more predictable – may still harbor hidden seismic surprises.
Learn more: Mini-earthquakes reveal hidden tectonic features buried beneath the Earth’s surface
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