Deep Hole Dug on San Andreas Fault for Quake Study

Stanford University and the U.S. Geological Survey announced Monday that they have begun drilling a 1.4-mile-deep hole along a particularly active portion of the San Andreas fault near Parkfield as an experiment to better predict earthquakes.

When the project is completed later this summer, an international team of scientists will implant instruments underground that they hope will enable them to monitor the earthquake process in an area where a magnitude-6 quake has been expected since 1985.

Mark Zoback, a Stanford geophysicist, said that a major objective of the experiment is to facilitate a more ambitious project, for which Congress is being asked to appropriate $30 million over six years. The project calls for a 2.4-mile-deep hole to be drilled through the fault itself, not just alongside it.

The research is to focus on an area in Central California where there have been magnitude-6 quakes at more or less regular intervals since the mid-19th century. The last one occurred in 1966.

In 1985, in what was named the Parkfield prediction experiment, scientists said they considered it 95% likely that another such quake would occur by the end of 1993.

But none has, and some scientists have speculated that the 6.5 Coalinga quake of May 2, 1983, may have reduced stress in the area and delayed it.

In any event, numerous small quakes have continued to occur in the area, and instruments in the new hole will prove useful in studying those.

The entire project, including the second drilling, would be the first underground earthquake observatory to penetrate a seismically active fault zone at such an extreme depth.

“The pilot hole is really a warmup exercise for SAFOD, the San Andreas Fault Observatory at Depth,” Zoback said. “It was conceived about a year ago as a way to begin studying the upper crust adjacent to the fault zone, while at the same time helping us identify earthquake targets for SAFOD.”

When the first hole is drilled, researchers will lower instruments into it to measure stress, fluid pressure, heat flow and other characteristics of the San Andreas geologic environment. The idea is to better delineate the amount of stress necessary to make the fault slip.

Bill Ellsworth, chief scientist for the Geological Survey’s earthquake work in the Western states, said the quakes that occur near Parkfield “are quite remarkable. Many of them recur time and time again with near clocklike regularity. The ... instruments will give us a powerful new tool for understanding what makes them tick.”

Instrumental readings will be analyzed in laboratories at participating institutions to determine how changes in fluid circulation and chemistry relate to the earthquake cycle.

One of the factors inhibiting understanding of earthquakes is that they have occurred deep under the surface, where scientists were unable to directly examine them.

Parkfield is about halfway between Los Angeles and San Francisco, in a region where the San Andreas fault is slipping all the time through a combination of steady creeping and small-magnitude quakes. This contrasts with other regions where the slippage is much more infrequent.

The pilot hole is being drilled on private land about a mile southwest of the fault and is closed to visitors.