Two hours before a major earthquake, the ground appears to move — infinitely small and without causing any tremors — a new study shows. This motion, known as an aseismic slip, can cause a possible path to predicting damaging earthquakes before they happen, the researchers say.
Many seismologists have long argued that earthquakes are impossible to predict because the Earth’s crust provides no discernible warning before it bursts. The new study, published Thursday in Science, suggests that there are indeed warning signs. They’re just very, very subtle. “It’s solid evidence that something happens before major earthquakes,” said study co-author Quentin Bletery, an earth sciences researcher at France’s National Research Institute for Sustainable Development (IRD) at Cote d’Azur University. “It doesn’t mean we know how to predict them, but it means it’s physically possible.”
But just because there’s a pattern of events before an earthquake doesn’t mean the new finding itself amounts to a prediction method, warns Lucy Jones, a seismologist and founder of the Dr. Lucy Jones Center for Science and Society. The patterns seen in the aseismic slip are similar in many ways to the patterns seen in foreshocks — smaller earthquakes that happen before major quakes. Seismologists once hoped they could use foreshocks to predict large earthquakes, says Jones, who was not involved in the current study. But that didn’t happen. It turns out that after an earthquake there is a 5 percent chance that a bigger one will follow. It is not clear that a earthquake is a foreshock until a larger earthquake occurs. The same problems are likely to plague the use of aseismic motion for predictive purposes, Jones says. “Their analysis requires knowing the biggest shock,” she says. “To be useful in any kind of predictive thing, you have to figure out how you do it [without] knowing that the biggest shock is coming.”
An aseismic slip is ground motion that occurs without producing seismic waves. These events are sometimes called “slow earthquakes” because they can move the ground the same amount as an earthquake, but the slip motion happens so gradually that there are no jolts, jolts, or tremors. Still, the movement is significant enough for researchers to detect using GPS sensors.
To find out if faults show changes in pre-earthquake slip behavior, Bletery and his co-author Jean-Mathieu Nocquet, a fellow IRD researcher, combined data from 3,026 GPS stations near the centers of 90 earthquakes with magnitudes of 7.0 or greater. Each station automatically recorded its exact location every five minutes, and the researchers analyzed the 48 hours leading up to each earthquake. They looked at the amount and direction of displacement for each station and tried to spot any patterns in the signals.
For the first 46 hours of the 48-hour period before each earthquake, they saw no patterns. But in the past two hours, they saw an exponential acceleration in the horizontal movement of the sensors. “This last one [data] point is twice the maximum of the first 46 hours,” says Bletery, “so it’s very unlikely to happen by chance.”
To confirm that this signal was earthquake-specific, the researchers used the same method on 100,000 random 48-hour periods that did not precede an earthquake. They saw last-hour increases in just 0.03 percent of the samples, giving a rough estimate of the likelihood of this pattern occurring randomly.
The average amount of energy released from slow slip was comparable to that produced by a magnitude 6.3 earthquake, even though there was no shaking. (The exact distance of ground motion that occurs in a magnitude 6.3 earthquake can vary, and the researchers couldn’t calculate these measurements — just the energy released by the motion.) It seems that before a fault breaks and triggers an earthquake, Bletery says, there’s a slow shift of crust against crust that acts as a precursor.
But the slide pattern is subtle, and the current analysis required more than 3,000 widely distributed sensors to detect it. Detecting these silent changes in a single location on a single fault would require sensors at least 100 times more sensitive than what is available today, Bletery says.
Even then, such a feat is probably impossible, Jones says. The researchers discovered the slow slip by adding up all the data from individual stations and using the timing of the main shock as a benchmark. The exponential growth pattern seen at all stations makes sense, she says, because if you start with the moment of a major earthquake, chances are something interesting will happen in the few minutes before or after — just as foreshocks and aftershocks are most likely to be very close to the quake. This probability decreases exponentially as you move further in time from the event, whether you move forward or backward.
This pattern holds when analyzing many earthquakes at once, because you’ve chosen to start looking for each earthquake when things are most interesting — but that doesn’t mean that a single earthquake in a given location will be heralded by an exponential increase in silent slip, says Jones.
“It doesn’t give you foresight, predictive ability, because it’s an explanation of how you summarized it rather than what’s going on in a particular earthquake,” she adds. “‘I see this before big shocks’ and ‘I see this and know what’s coming’ are completely different questions.”