After an earthquake, fault zones may need no more than a couple of years to regain their strength, research suggests. But the recovery is not without setbacks: Large, distant quakes can redamage fragile faults and prolong healing.
A fault weakened by a rupture won’t begin to build up stress again until it’s strong enough. So the new findings, published in the June 28 Science, should help researchers understand the timing of earthquake cycles in a fault zone.
After a magnitude 7.9 earthquake devastated southern China in 2008, scientists drilled a borehole 1,201 meters into the Longmen Shan fault zone to monitor the healing process. Researchers have previously analyzed fault strengthening in the laboratory or with surface measurements. But this is the first time anyone has peered directly into a fault to observe recovery. “We’re very hard up for evidence about what’s happening down there,” says seismologist John Vidale of the University of Washington in Seattle.
Researchers indirectly measured the Chinese fault’s strength by looking at how easily water flowed through the rock. When a fault fails during a quake, rocks break. Many processes reseal the cracks, such as the crystallization of minerals dissolved in groundwater. The more cracks a fault has, the weaker it should be, says coauthor Emily Brodsky, a geophysicist at the University of California, Santa Cruz.
From January 2010 to August 2011, Brodsky, Lian Xue, also of UC Santa Cruz, and colleagues observed changes in the rock’s permeability by measuring shifting groundwater levels in the borehole. Permeability plummeted, and the team calculated that recovery after an earthquake in the Longmen Shan fault zone should have taken anywhere from seven months to 2½ years. Some previous studies indicated that faults should take decades or more to heal.
In the case of Longmen Shan, recovery was interrupted by periodic spikes in permeability that coincided with big, faraway quakes. Seismic waves unleashed by those events probably fractured the still-delicate fault, Brodsky says.
One open question, says Elizabeth Cochran of the U.S. Geological Survey in Pasadena, Calif., is whether seismic waves from distant temblors can also damage strong, fully repaired faults.