04/01/93
CONTACT: Stanford University News Service (650) 723-2558
STANFORD - Geophysicists from Stanford, the U.S. Geological Survey (USGS), and the Department of Energy are planning a 10-year experiment to drill holes as deep as 10 kilometers (6.2 miles) directly into the San Andreas Fault.
The experiment would not only help resolve mysteries about how the fault works and why it seems to be under much less stress than scientists thought it should be, but it would monitor what happens on the fault during and between earthquakes, said Mark Zoback, chairman of Stanford's geophysics department and one of the co-investigators of the proposed experiment.
Zoback, Stephen H. Hickman at the USGS, Leland W. Younker of the Lawrence Livermore National Laboratory and William Ellsworth of USGS have submitted a proposal to several federal agencies to proceed with site-selection activities for this project.
The proposal was prepared to address scientific issues raised during a workshop held in Pacific Grove, Calif., this past December. The workshop was attended by 113 scientists and engineers from seven different countries, representing the more than 200 scientists and engineers who are now involved in this wide-ranging international project.
According to the proposal, the geophysicists would drill vertical holes at one site along the fault using technology recently developed as part of ultra-deep scientific drilling efforts in Germany. The holes would go straight down at first, and then, at depths of three, six, and nine kilometers, be aimed diagonally through the fault itself. These depths were chosen because they are associated with changes in seismicity, mineralogy, deformation mechanisms and fault hydrology.
As the holes were drilled, the scientists would use the core samples to analyze the composition of the fault zones. Once inside the fault zone, casing would line the borehole and fluid samples would be taken through holes in the casing, the proposal said.
The main task of the experiment would be to find out why the fault zone is as weak as it is. Studies by Zoback and others indicated that the stress along the fault is about one-fifth what had been expected.
"Although theories abound, there is no direct evidence or generally accepted mechanical explanation for the weakness of the fault," the scientists wrote.
Much of Zoback's work was done in an experiment in which a borehole 3.5 kilometers deep (2.1 miles) was dug at the Cajon Pass in southern California four kilometers from the fault.
The geophysicists have seven possible sites for the new experiment along the 1,000 kilometer-long fault that runs through much of California. The first stage of the study would be to cut that number down to four.
To select the final site, the geophysicists would have to have a detailed understanding of what happens at each potential area.
"Just identifying the best locations would tell us a great deal about the fault," Zoback said. "The process of selecting one site will necessarily involve making choices about which of the fundamental questions we may be able to answer with this project."
The researchers would have to create complex models of each site, including such things as the geology, crustal structure, geophysical environment, hydrology, the state of stress at the site and the fault movement history. A one-kilometer deep hole would be drilled at each site to provide data for that analysis.
"Collectively, these studies will have a major impact on models of how this specific fault works, as well as for more general models of how earthquakes occur," the researchers said.
The short list probably will include at least one site each in northern, southern and central California. Parkfield, in central California, is one of the most studied and instrumented areas in the world. Some work done there would serve as a prototype for the final stages of site characterization.
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