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Getting to the bottom of the San Andreas.

Getting to the bottom of the San Andreas

Last month, scientists began drilling a5-kilometer-deep hole in California, which they expect will get to the bottom of a long-standing paradox about the stresses and heat generated in the nearby San Andreas fault.

The hole, located within the fault zone,3.5 kilometers from the fault itself, is in the Cajon Pass, northeast of Los Angeles. It will be the deepest hole ever drilled in the United States solely for research, according to Mark Zoback, a geophysicist at Stanford University and chief scientist on the project. By using the hole to study fundamental properties of the earth's crust as well as to monitor the San Andreas over the next few decades, scientists hope to improve their earthquake prediction abilities considerably.

Earthquakes are generated along theSan Andreas as the Pacific plate to the west of the fault grinds northward past the North American plate. In the Cajon Pass area, the Pacific plate moves at a rate of a few centimeters per year. This motion is resisted by friction in the fault. Scientists, using laboratory studies of the strength of rocks, calculate that the frictional stress shearing the fault should be high, about 1,000 bars. Indeed, measurements at shallow levels reveal a strong stress gradient, suggesting that high stresses are present at lower depths.

But with such high frictional stress,scientists have also expected that the San Andreas would be generating much heat as the rocks grind past one another. And there is the rub, because essentially none of the more than 100 shallow heat-flow measurements made during the last 20 years has detected any anomalous heat coming from the San Andreas; the heat levels near the fault are no greater than those 10 kilometers away. These heat-flow measurements are consistent with shear stresses in the fault that are 5 to 10 times lower than the 1,000 bars predicted by the laboratory and theoretical studies.

Because the heat and stress studieshave been made near the surface, where all kinds of factors can complicate the measurements, scientists are cautious about extrapolating their results to deeper levels of the fault. That's why Zoback and others have wanted to drill a deep hole near the San Andreas. "It's been our inability to address the [stress-heat] question with anything but direct observations that has been the primary motivation for this project," says Zoback.

The National Science Foundationagrees. It is providing most of the $8 million required for the Cajon Pass project. In addition to studies relating to the stress-heat paradox, 30 researchers from a dozen institutions plan to do a wide variety of experiments to investigate everything from regional geologic questions to geophysical properties of the crust. According to Zoback, the hole should be completed by March 1988.

While drilling has just begun, projectscientists concerned about the stress-heat paradox already have given much thought to what they may find. One scenario is that at depths greater than about 2 kilometers, they may measure both high stresses and the "missing" anomalous heat. If so, then it's possible that the flow of groundwater, or some other mechanism, is carrying the heat away at shallow levels.

Alternatively, they may find that theshear stresses and heat levels are low at greater depths. This would mean that the fault is much weaker than previously supposed, says Zoback. "The implications of this would be quite dramatic," he says, since scientists have assumed that frictional stress on the fault is an important force controlling the movement of the earth's tectonic plates.

In support of a weak fault, Zoback andhis wife, Mary Lou Zoback of the U.S. Geological Survey in Menlo Park, Calif., have recently found that compressional stresses in California's crust point about 70[deg.] away from the trend of the fault. This orientation, they say, would tend to move the San Andreas, rather than other faults in the area, only if the San Andreas were weak enough to be susceptible to such a stress.

If the Cajon Pass study also suggeststhat the fault is weak, then scientists will probably want to drill other holes in the fault zone to find out why. One possibility is that fluids trapped in the pores of fault rocks and clays are exerting a strong pressure, jacking the rocks apart and nearly nullifying the frictional stresses.

The Cajon Pass hole probably won'tanswer all the questions scientists have about fault mechanics, but Zoback says it may help them to move beyond the realm of describing plate motion, and toward an understanding of the physics behind plate tectonics.
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Title Annotation:5-kilometer-deep hole being dug to study San Andreas Fault
Author:Weisburd, Stefi
Publication:Science News
Date:Jan 31, 1987
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