Geological survey probes inside San Andreas Fault Geological survey probes inside San Andreas Fault (August 31, 2005)

Two miles under the cow pastures of Parkfield, California, local scientists are probing inside the active zone of the San Andreas Fault to study the nitty-gritty of how and why earthquakes are triggered.

On August 2, the first effort to drill down into the part of the fault where earthquakes originate reached its target.

"It's the first time we've been within the earthquake machine," said Bill Ellsworsh of Menlo Park, chief scientist in the Earthquake Hazards Center of the U.S. Geological Survey in Menlo Park. "We've looked at fossil earthquakes, we've made computer models, and we've made laboratory earthquakes. We've studied them from afar, but we've never been inside the machine where the action is."

Dr. Ellsworth is one of three principal investigators of SAFOD, the San Andreas Fault Observatory at Depth. The others are Mark Zoback, professor of geophysics at Stanford, and Steve Hickman, a geophysicist at USGS in Menlo Park.

When completed in 2007, SAFOD will be the only earthquake observatory with instruments installed directly within an active fault where earthquakes form or "nucleate." Scientists will be able to bring up actual rock and mineral samples from within the earthquake zone, and install instruments to measure what's happening inside the fault.

Parkfield, located along the San Andreas Fault about half-way between San Francisco and Los Angeles, has been a favored place to study the fault, where two great plates of the earth's crust grind past each other. It is known for small and moderate earthquakes at fairly regular intervals, rather than the large quakes that occasionally devastate areas to the north and south.

"Almost everything we know about earthquakes has been gathered either at or very close to the earth's surface, where all we see is the elastic part of the process, the part that carries seismic waves to great distance," Dr. Ellsworth said. "SAFOD gets into the inelastic part where things are actually breaking. That's the part we can only see by getting into the fault zone."

Over the next two years, scientists will try to identify precise areas in the fault zone where micro-earthquakes regularly occur so that engineers can drill into these areas and install instruments. The observatory is expected to operate for 20 years. "

"It's a whole new type of experiment," said Dr. Zoback.

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