Jan. 13, 1998 Working on the hard granite of California's Mojave Desert, scientists for the first time have directly observed the "healing" of an earthquake fault.
The post-quake "healing" process restores the stiffness of the Earth's crust and renews the fault zone's resistance to rupture, according to findings reported in the Jan. 9 issue of the journal Science.
Geophysicists Yong-Gang Li, Ph.D., a research fellow in the University of Southern California department of earth sciences, and John Vidale, Ph.D., an associate professor in the UCLA department of earth and space sciences, led a team that has followed the changes in the Earth's crust along the Mojave Desert's Johnson Valley fault since that fault ruptured on June 28, 1992, in the magnitude 7.5 Landers earthquake.
Ultimately, studies such as these may lead to refined estimates of how often and how predictably a given fault is likely to rupture, the researchers believe.
In 1994 and again in 1996, the team conducted two identical seismic experiments. Both times, the scientists set out an elaborate network of seismic listening posts -- some 80 instruments spaced around the Johnson Valley fault over an area of some 20 square miles. The team then used the array to record the Earth's response to the shock of explosions set off in 30-meter-deep boreholes near the fault -- one on Nov. 2, 1994, a second on Aug. 6, 1996.
The shock waves traveled significantly faster in the 1996 test -- 0.5% to 1.5% faster, with the largest increases in velocity recorded at the stations closest to the fault.
This indicates that crust torn and shattered by the Landers quake is rapidly "healed," says Dr. Li, lead author of the Science article. Underground cracks opened by the quake are closing.
The Johnson Valley fault is relatively inactive. Quakes as strong as the Landers event are estimated to occur on it no more than once in a thousand years.
The probe of the fault and the knowledge gained about the changes taking place in the rock "help us to understand the earthquake cycle, the way in which faults gradually accumulate strain over decades or even centuries, and then release the strain in a few seconds," Li says.
Theorists have proposed many ideas for the sequence of events leading to an earthquake, from simple theories of dry rock surfaces rubbing against one another to more complicated scenarios in which water with dissolved minerals is present.
The "plumbing" in the fault may also control when earthquakes occur, Dr. Vidale explains. An increase in water pressure may push the sides of the fault apart, encouraging the fault slip that makes the Earth quake.
"By comparing observed velocities of acoustic and shear waves from the explosions with those predicted by theory for dry rock, we inferred that microcracks opened by the quake contain some liquid, but they are not completely filled with fluid. We are monitoring whether the amount of fluid increases after a quake," Li says.
By comparing observed velocities of acoustic and shear waves from the explosions with those predicted by theory for dry rock, the scientific team inferred that microcracks opened by the quake contain some liquid, but they are not completely filled with fluid, and the data indicate that the amount of fluid increased from 1994 to 1996.
Team member Keiiti Aki, Ph.D., holder of the W.M. Keck Foundation Chair in Geological Sciences in the USC College of Letters, Arts and Sciences, says that because most faults have a long history of rupture and healing, "studies of the fault structure and healing process will help scientists to identify the real causes of the starting and stopping of rupture." Such studies, he says, could help scientists predict where an earthquake is likely to occur on a fault and what that earthquake's magnitude is likely to be.
New technology made these observations possible. "Ten years ago, a handful of people would have been unable to deploy 80 instruments in a few days and record the ground motion for an entire week at a time," Vidale says. "The necessary precision, a few milliseconds in absolute time, is made possible by timing from satellites, which also became possible just a few years ago."
Li and Vidale plan to continue to monitor the Johnson fault, trying to determine if the rate of healing is constant over time, or accelerates or decelerates, to develop their understanding of the faulting and healing process. They are also probing other active quake areas -- including Parkfield and Anza in California, and the Nojima Fault near Kobe, Japan -- to develop their understanding of the faulting and healing process.
Other collaborators on the Johnson Valley work were graduate student Fei Xu of UCLA and geophysicist Thomas Burdette of the Menlo Park office of the U.S. Geological Survey. The National Science Foundation and the USGS funded this research.
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