POSTED 3 NOVEMBER 2005
Getting down to business
We've
talked about where earthquakes start, but not
how they start. What exactly happens when friction is overcome
and a fault starts to slip? One ambitious effort to answer this question
is taking place at the San Andreas Fault, where the National Science Foundation,
the U.S. Geological Survey, and a raft of cooperating institutions have
drilled a hole to the heart of darkness, directly through (they hope)
the San Andreas Fault.
This rig drilled 3.2 kilometers into the San Andreas Fault at Parkfield, Calif. Photo: EarthScope
The San Andreas Fault Observatory at Depth (SAFOD) was drilled in Parkfield, Calif., which has endured, on average every 22 years, earthquakes measuring 6 on the Richter scale.
The hole, which has now reached a depth of more than 3 kilometers, is part of the ambitious EarthScope project, which also funds a movable array of 400 seismographs, intended to make an earth-shaking map of the entire United States, and global positioning stations to measure crustal movement in unprecedented scope and detail.
Boring, boring
SAFOD is the first borehole to penetrate a seismic fault at a depth where earthquakes occur, and it will be used to retrieve rock samples, and to house various instruments for measuring conditions in the basement. "We put in a lot of instruments, are trying to record anything that happens before the event, change in water levels, background seismicity, the velocity of waves through the rock, the strain, the gas content," says University of California seismologist Thorne Lay, who is on the EarthScope facilities executive committee.
One key question concerns any fluids in the rock: Many seismologists have speculated that pressurized fluid may help lubricate and activate a fault.
These rock cores were removed from a drill hole
that reached inside the active region of the San Andreas Fault. Do they
hold clues to better earthquake prediction? Photo:
EarthScope
SAFOD has yet to produce much in the way of results, but seismologists are salivating at the chance to peek inside a fault. "We don't really know what to expect," says Cliff Thurber, a professor of geology and geophysics at the University of Wisconsin-Madison, who has been leading an effort to aim the borehole directly at the active fault. "This has never been done before. We can observe old eroded faults at the surface, and we can interpret them, but we don't know how much their character has been altered by the process of being uplifted and exposed."
So what does he expect the inside of the San Andreas to look like? "We expect to see a localized zone of damaged rock, highly fractured, within a scale of centimeters," says Thurber. "We don't know what the character of the rocks will be, or the presence or absence of fluids. This will show us what the fault is really like."
Early EarthScope evidence will be enunciated at the December, 2005 meeting of the American Geophysical Union.
Plate tectonics: The history of a scientific heresy.
