Tiny earthquakes tamed in the laboratory.

Tiny earthquakes tamed in the laboratory

In the heart of an earthquake, action moves at a furious pace. Fractures spread through rock at more than 7,000 kilometers per hour, literally faster than a speeding bullet. That lightning motion has long hampered scientists who model earthquakes in the laboratory. But a team of U.S. and Soviet researchers has slowed the clock on the cracking process, providing new insight into the way rocks fracture.

David A. Lockner of the U.S. Geological Surveyin Menlo Park, Calif., and his colleagues developed a system that retards crack growth within a rock sample, prolonging for minutes or hours an event that normally takes less than a millisecond. In one experiment, says Lockner, "We went almost a day. I went home and went to bed and let it keep going."

The researchers studied cylinders of granite a little larger than beer bottles. Pressure exerted on the end of the cylinder by a metal ram causes the granite to fracture.

Lockner's group altered the standard experimental procedure by mounting six sensors on the cylinder to listen for the ultrasonic "sound" of microcracks that develop as the granite breaks. By feeding the acoustic information into the system that pushes on the granite, the researchers can control the rate of fracturing: If the rock starts cracking too fast, the apparatus quickly compensates by easing off the pressure compensates by easing off the pressure to slow down the breaking process.

The sensors also serve as a miniature version of the seismometer networks used to locate earthquakes and faults. A computer determines the position of the microcracks by analyzing the time it takes the ultrasonic waves to reach each sensor. This allows the researchers to monitor action inside the cylinder and provides an unprecedented image of the way fractures grow, Lockner says.

The first microcracks develop in an unorganized fashion throughout the granite sample as it absorbs stress from the advancing ram, he and his co-workers report in the March 7 NATURE. But at some point, the action concentrates in one spot as the microcracks coalesce to form a virtual fracture. That crack expands until it cleaves the cylinder.

Among the surprises to come out of the experiment was the finding that as the fracture advances through rock, it leaves a quiet zone in its wake. Scientists see the same activity in real earthquakes but had not observed it in lab experiments. Because rock samples in the lab are so small, researchers had presumed it impossible to catch this type of behavior in experiments, says Lockner.

While scientists in Japan have previously succeeded in slowing crack growth, Lockner's group is the first to both retard fracturing and use acoustic emissions to tracks the developing crack, says Tengfong Wong of the State University of New York at Stony Brook. This technique will improve mathematical descriptions of the way rocks fracture, aiding efforts to study and predict earthquakes, Wong says.