Borehole Belches Radon : Borehole reaches fault's active zone

MONTEREY COUNTY

- David Perlman, Chronicle Science Editor
Thursday, August 4, 2005


Scientists drilling into the San Andreas fault to catch earthquakes right in the act of starting reported Wednesday they have finally penetrated the fault's active zone, a region two miles deep where clusters of tiny quakes continually rattle the earth underground.

The successful borehole marks a major milestone in an effort to understand how quakes originate, how they grow into violent temblors that can rupture the surface for miles, and whether scientists might learn to predict them, said William Ellsworth, chief of the Earthquake Hazards Team at the U.S. Geological Survey in Menlo Park.

An exuberant Mark Zoback of Stanford University said the borehole has already crossed the fault and is penetrating its active zone, where sequences of "microquakes" with magnitudes up to 2 have been repeating every two years but have never reached the surface.

Ellsworth, Zoback and Steven Hickman of the geological survey -- all geophysicists -- head the project known as SAFOD, the San Andreas Fault Observatory at Depth, a National Science Foundation project. The borehole's site is nine miles northwest of quake-prone Parkfield in southern Monterey County.

Success in studying those tiny quakes right in the rocks where they strike, Zoback said, will enable scientists to simulate the same conditions in the laboratory and learn more about the forces that create them.

Drilling of the hole began in June 2004. A year later, after delays due to winter and technical mishaps, the drill rig was thrusting into the fault zone with difficulty, at only 6 to 9 feet an hour, Zoback said. But during the past few days, the drill's speed picked up to 41 feet an hour as the borehole emitted bursts of radon, carbon dioxide and hydrocarbon gases. "And the long time of water torture was over," he said.

The hole is located at the southernmost end of a segment of the San Andreas where the fault has been slowly creeping for many years -- a segment that runs north for more than 90 miles to San Juan Bautista.

"This is a major physics experiment," Ellsworth said, "and our goal is to get very, very close to earthquakes where they start small and then grow until they rupture the ground dangerously."

The new understanding that the quake observatory brings to seismic science, Ellsworth said, will have tremendous practical value, too, because it will yield information crucial to improved building codes and quake-resistant structures.

The project, which required a section of difficult slant drilling, includes a second vertical hole nearly a mile and a half deep, not far from the current drill site, where a string of 32 seismic monitoring instruments has been installed. The experience gained there will help scientists when they send instruments down into the center of seismic activity in the main SAFOD hole.

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