How Faults Heal

Time heals all wounds, even cracks in Earth. According to a Report in the current issue of Science, the network of tiny cracks left by a quake slowly heal in the following years. Understanding how much these breaks have sealed up could help geologists predict the timing and magnitude of future quakes.

Earthquakes create tiny cracks that penetrate and surround the fault. In turn, these are thought to affect the behavior of the fault and the size of the next quake, because strong rock with few cracks will unleash more energy when it fractures in an earthquake. Yong-Gang Li, a geologist at the University of Southern California in Los Angeles, and his colleagues thought they might get a better handle on this process by measuring how quickly the cracks seal up after a major quake.

Li and his colleagues tested how sound waves travel along the Johnson Valley fault in the Mojave desert, which ripped apart during the magnitude 7.5 Landers earthquake of 1992. In 1994 and 1996, the team set off explosions in a 30-meter borehole, then timed shock waves in both directions along the fault, a few kilometers deep. The team found that the waves traveled about 1% faster in 1996 than they did in 1994. This indicates that there were fewer cracks in the rock, which they speculate would worsen any subsequent quake.

The finding "puts a number on what we thought was going on," say John Vidale, a geologist at the University of California, Los Angeles, and a co-author of the paper. Vidale says that by measuring the strength of the rock and perhaps other parameters such as water content as well, geologists might be able to predict the strength of a future earthquake. Bill Ellsworth, a geologist at the U.S. Geological Survey in Menlo Park, California, says this work is the first to show how ruptures heal over time, and similar experiments could help uncover how faults change. "We'd like to understand the full cycle of earthquakes," he says.