On Monday residents of northeastern Japan were rattled by a massive magnitude 7.7 earthquake off the coast and warned of possible tsunamis, as well as a slim chance of a magnitude 8 or higher “megaquake” in the coming days. A new study, published Thursday 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 each other. Once this accumulated stress overcomes the friction holding the rocks together at a specific point called the hypocenter, 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 that makes it stop instantly, like a speeding train crashing into a concrete wall.
That process of a rupture hitting a barrier creates a signature called a stopping phase—a seismic shudder traveling the opposite direction to the main rupture. “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 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 explains.
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But observational data showing that signature has been lacking, so Kearse and his colleague Yoshihiro Kaneko, a geophysicist at Kyoto University, hunted for it in the seismic and geodetic data registered by sensors placed close to 12 large earthquakes across the globe.
Five of the earthquakes the researchers studied had enough sensors along the fault that they could isolate the stopping phase for these quakes. The team also found that certain near-surface features, such as softer rock layers above where the stopping phase happens, can further enhance it, 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 scientists know how to identify a stopping phase signature, the researchers say, they can pinpoint these phases in past earthquakes’ data to map out underground barriers and assess how much energy they can absorb, plus whether there are any 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.
But 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 one another, because there are simply more data for them. Monday’s 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.”
