The San Francisco Bay Area is a little less prone to major earthquakes than scientists thought. New satellite radar images of the northern Hayward fault--one of the major faults that threaten the area--show that deep underground, its two sides are steadily slipping past each other at about 7 millimeters per year. That's enough to prevent the buildup of destructive tension along the fault line.
The grinding of tectonic plates has splintered Earth's crust near the earthquake-prone San Francisco Bay Area, leaving it cracked like a shattered window. Geological faults can release the pressure in three ways: a steady slippage called creep, a stuttering series of nearly imperceptible microquakes, or a sudden catastrophic earthquake. Most scientists blamed the northern Hayward fault for a massive 1868 earthquake, because measurements showed the fault ruptured during the quake. Disrupted layers in excavated trenches implicated the northern Hayward in four other suspected major quakes during the last 2200 years. There was a problem, however: The surface of the northern Hayward fault also creeps, suggesting that the fault might be slipping underground and might not be a major shaker after all.
Because his office is in the danger zone, Roland Bürgmann of the University of California, Berkeley, decided to find out. Bürgmann's team searched for the warping on radar maps of the ground along the fault made by the InSAR satellite from 1992 to 1997. This kind of deformation would indicate that no deep creep was relieving the strain. They also analyzed microquakes that had ruptured 10 kilometers beneath the surface. In the 18 August issue of Science, the team concludes that the fault is creeping at 5 to 7 millimeters per year--not just at the surface, but also down to a depth of at least 10 kilometers. This should defuse the strain that energizes major earthquakes. The new results dramatically reduce estimates of the probability of an earthquake along the northern Hayward and imply that the 1868 quake actually started in a nearby fault, like the southern Hayward or Rodgers Creek.
But San Francisco residents' worries are far from over. "Even if we are right about the likelihood of an earthquake nucleation on this segment being low, there is no reason at all to relax," Bürgmann says. The southern Hayward fault is still very hazardous, notes seismologist Bob Simpson of the U.S. Geological Survey in Menlo Park, California. "Don't stop paying your earthquake insurance," he warns.