On April 20 residents of northeastern Japan were rattled by a massive magnitude 7.7 earthquake off the coast. They were warned of possible tsunamis, as well as a slim chance of a magnitude 8 or higher “megaquake” in the coming days. A study published just a few days later in Science investigates how such megaquakes evolve, what can eventually stop them and how we can predict their destructive power.
An earthquake starts deep underground when huge tectonic forces cause stress to build up along a fault line: a massive fracture in Earth’s crust where blocks of rock have shifted and moved past one another. If this accumulated stress overcomes the friction holding the rocks together, the fault slips, and a rupture rapidly spreads along it, generating powerful seismic waves that cause the ground to shake. This process continues until the spreading rupture reaches an area of low stress and slowly loses momentum or until it hits a physical barrier underground, such as a change in the ground’s makeup or trapped high-pressure fluid, that makes it stop instantly like a speeding train crashing into a concrete wall.
Not all underground barriers will stop a rupture. But if one does, the impact creates a signature traveling in the opposite direction called a stopping phase. “When the rupture is going fast and encounters some barrier that suddenly makes it stop, it sends out a shock wave,” says study co-author Jesse Kearse, an Earth scientist at Victoria University of Wellington in New Zealand. A human standing above such a barrier would first feel the ground move in the same direction as the rupture and then sharply jump back in the opposite direction. “It’s like you’re in a car and the brakes suddenly engage, and you snap back in your car seat,” Kearse says.
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But observational data showing that signature have been lacking. Kearse and his colleague Yoshihiro Kaneko, a geophysicist at Kyoto University, hunted for it in the seismic and geodetic data registered by sensors in the paths of 12 large earthquakes across the globe. Five of the earthquakes the researchers studied were equipped with enough sensors along the fault that the team could isolate a stopping phase. They also found that certain near-surface features, such as softer rock layers above the stopping phase, can further enhance the signal, leading to more severe shaking of the ground at the surface.
Every barrier a rupture hits on its way works as a checkpoint. If the barrier holds, it stops the earthquake, which can end up as a minor, localized event. But if the advancing rupture has enough energy to shatter through the checkpoint, it spills over into the next fault segment, potentially cascading into a megaquake monster. “This demonstrates the extremely valuable role of near-field observations in understanding why earthquakes grow big or remain small,” says Yihe Huang, a geophysicist at the University of Michigan, who was not involved in the study.
Now that they know how to identify a stopping phase signature, the researchers can pinpoint these phases in past earthquakes’ data to map out underground barriers and assess how much energy they can absorb. They can also identify amplifying near-surface features nearby. “This new insight can potentially transform earthquake hazard analysis,” Huang adds, by showing where an earthquake of a particular strength might be stopped and where it might be enhanced.
There’s still a lot of research to do before the new findings help to build more accurate earthquake models. Kearse and Kaneko limited their study to strike-slip earthquakes, in which two blocks of rock slide horizontally past each other, because there are simply more data for them. The April event in Japan was a thrust earthquake that made the ground move up and down—a motion that is much more likely to cause a tsunami. “The obvious continuation of this work is to make it more general,” Kearse says. “But we expect this stopping mechanism is a common feature of the earthquake process that does apply to thrust events, too. We just cannot confirm that yet.”