On May 12, 2008, a powerful 7.9-magnitude earthquake struck the Wenchuan county of Sichuan province in south-central China. It was the most powerful quake to hit China in at least 50 years, killing more than 69,000 people, creating thousands of orphans and leaving millions of people temporarily homeless.
Given the depth of the suffering, it verges on thoughtlessness to use the words silver lining, but a study in the latest Science has relied on the Wenchuan quake to add a small but possibly important clue to the mystery of why and how earthquakes happen. “Everyone who studies earthquakes would love to be able to predict them,” says Emily Brodsky, a seismologist at the University of California, Santa Cruz, and a co-author of the new study. “This is one piece of that.”
In this case, the piece wasn’t the quake itself, but the geological healing process that took place afterward. Like the great quakes that periodically rattle California’s San Andreas Fault, this one was triggered by a sudden, sideways slip of one huge slab of underground rock past another. Above ground, the energy released by this geological spasm was enough to shake the landscape for hundreds of miles in all directions and send buildings tumbling.
But down below, the quake also shattered rock on both sides of the fault itself, out to a distance of 304 m or more, opening up a network of cracks. Over time, geologists have learned, those cracks gradually repair themselves. “We think it’s some combination of the rocks fusing back together,” says Brodsky, “along with water flowing through the cracks bearing minerals that crystallize. But we don’t really know.”
To begin trying to figure it out, Brodsky, lead author Lian Xue, a Santa Cruz graduate student, and several colleagues worked with geologists in China, who drilled a series of boreholes close to the fault very soon after the quake happened. For the following 18 months or so, they monitored the levels of groundwater in one of the holes, and saw the water rise and fall twice every day.
What they were seeing were tides in the rock itself. It isn’t just the ocean that experiences tidal forces, the solid earth does as well, as gravity from the moon pulls harder on one side of our planet at any given time than it does on the other. The tidal bulges are easy to see in the ocean, where they can reach several feet, but the change in dry land is only on the order of centimeters.
That’s enough, however, to squeeze groundwater in and out of the newly formed underground cracks, making the water in the borehole rise and fall with the twice-daily rhythm. But that rhythm changes: it takes time for the water to squeeze into the borehole as the cracks tighten during low tide — and as the cracks slowly repair themselves and water flows less easily, the investigators observed the delay getting longer and longer.
“That’s our secret sauce,” says Brodsky. “The only way this could happen is if the permeability of the rock is changing.”
The healing was not without setbacks. As with a fractured bone that’s slowly knitting back together can be reinjured if you move it the wrong way, the lag time in the boreholes occasionally shortened, suggesting some kind of geological trauma. “We interpret this as a redamaging of the fault zone,” says Brodsky, “possibly due to seismic waves from distant earthquakes, as the delicate, fragile fault is trying to heal.”
In theory, says Brodsky, the speed at which the rock recovers could have a bearing on how soon the next major quake on a given fault might happen. But scientists are a long way from answering a question like that with certainty. They don’t even know whether fractured rock ever heals fully or whether, like a broken bone too, it’s more prone to breakage in the future. “Maybe,” she says, “but that’s well beyond the scope of this study.”
At best, therefore, this is just a small bit of progress in the ongoing attempt to solve the extraordinarily complex problem of how to predict major earthquakes. Given the terrible damage they do, however, both direct and indirect, even a small bit of progress is a big deal.