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STANFORD - Geophysicists from Stanford and the U.S. Geological Survey may have solved one of the most intriguing mysteries about major earthquake-prone faults: How does a fault rip furiously when there doesn't seem to be much stress on it?

The answer may be a mechanism involving constantly cycling ground water, according to Norman Sleep, Stanford professor of geophysics and geology, and Michael L. Blanpied of the geological service, who first proposed their theory, based on lab experiments and field data, in a recent issue of Nature.

The ground water acts almost as a lubricant, "reducing the force at the contact points, something like the way the rain can make a car hydroplane or a block slide down a ramp when the ramp is wet," Sleep said.

In the long term, knowing that the mechanism is true could help with earthquake prediction. For now, Sleep said, it just adds another variable to all the equations, complicating things, Sleep said.

"But if you know that there is another variable, then you can hope to understand it," he said.

For instance, Stanford geophysicist Mark Zoback, studying the San Andreas Fault in a deep drilling experiment, found that there is almost no measurable heat from friction that might be caused by the two sides of the fault rubbing against each other, and that the stress on the faults seems oriented the wrong way for the fault to slip at all. The plane of the San Andreas is almost perpendicular to the principal axis of compression.

Several other theories have been propounded, Sleep said, but all have major inconsistencies with nature.

Sleep and Blanpied suggested that ground water -- found in most earthquake zones - shows a gradual increase of pressure before a quake and a sudden decrease of pressure after one.

Ductile creep (in which pressure distorts the shape of the rock) along the faults gradually and almost immeasurably increases the water pressure until it's almost great enough to force the two sides of the fault apart. What stress is present is released, causing a quake.

When the quake occurs, cracks and fissures form in the fault zone, permitting the water to flow into them and quickly decreasing the pressure. Gradually, the pressure builds back to a hydrostatic level (equal to the surface pressure), then continues to increase enough to cause the fault to rip again.

"The water can circulate in and out of the fault zone and downward," he said, "but there is no need for a net flow from the outside. The whole process would be self-adjusting."

The action would occur several kilometers below the surface. It probably would not be measurable in well water levels, an old theory of earthquake prediction. Indeed, the change in water pressure in the fault is almost impossible to measure, Sleep said, perhaps a few millimeters in a fault zone that is typically a meter wide over a long period. Although this motion has never been measured, he said there are places where rocks contain evidence that something like it happened. Pigeon Point, southwest of San Francisco, shows ductile deformation in calcite that once was at great depths, suggesting a similar process.

The Sleep-Blanpied theory has a long and colorful lineage. In 1968, scientists working in Rangely, Colo., near the Denver airport, discovered that they could trigger quakes by pumping water into fault zones. Once they even considered disarming the faults by triggering small quakes that would relieve pressure along the fault. The potential for instead instigating a damaging earthquake deterred them.



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