Geologists affiliated with the EarthScope Project have successfully drilled a hole 2 miles deep into the San Andreas Fault, an 800-mile-long rift. The entire borehole will be lined with steel and concrete so that sensitive instruments can be installed underground.
EarthScope is a National Science Foundation (NSF)-funded project carried out in collaboration with the U.S. Geological Survey. “We now have the first opportunity to measure directly the conditions under which earthquakes begin and grow,” says Herman Zimmerman, director of NSF's earth sciences division, which funds EarthScope. “This is an unprecedented step forward.”
The borehole begins in the Pacific Plate just west of the fault. It passes through the active earthquake zone and ends in the North American Plate east of the fault. These enormous land masses constantly grind against one another in opposite directions, triggering earthquakes of various magnitudes up and down the fault.
“With SAFOD, we no longer will be limited to interpreting earthquakes through layers of thick rock,” explains NSF EarthScope program director, Kaye Shedlock. “We now can directly measure the conditions under which earthquakes happen.”
And, according to Gregory van der Vink, EarthScope project director, “SAFOD is critical to creating a comprehensive picture of how continents form.”
SAFOD will give researchers a unique window into the process of strain build-up and release in the fault zone during numerous “microearthquakes.” Seismic instruments will be installed along both plates in a section of the fault where small earthquakes of magnitude 2.0 are frequent. While these microearthquakes usually aren't felt at the surface, they offer important clues about the origin of bigger, more destructive quakes. “Microearthquakes provide scientists with an exciting opportunity to study events that occur about every two years in roughly the same place,” says Zoback.
Steve Hickman and William Ellsworth, SAFOD co-principal investigators with the U.S. Geological Survey, say the San Andreas fault is creeping at a rate of about 1 inch per year near the drill site. Most earthquakes occur in a zone no more than 10 yards wide.
Scientists now will be able to recreate the earthquake process in the laboratory using real materials and under real conditions that exist deep within the San Andreas Fault. “You can't do these kinds of in-depth measurements in parts of the fault that only produce big earthquakes, because those usually occur at intervals of 100 to 150 years,” Hickman explains.
In addition to retrieving fault zone rocks and fluids for laboratory analyses, intensive down-hole geophysical measurements and long-term monitoring will take place within and adjacent to the active fault zone.