The San Andreas Fault Observatory at Depth (SAFOD) reached asignificant goal on Aug. 2 when scientists drilled into a seismicallyactive section of the fault approximately two miles below the surfaceof the Earth.
"This is a milestone for SAFOD," says Mark Zoback, a professor ofgeophysics at Stanford University. "For the first time, scientists havedrilled directly into the San Andreas Fault Zone at a depth that willallow us to observe earthquakes up close for decades to come."
Zoback is co-principal investigator of the SAFOD project, alongwith geophysicists Steve Hickman and Bill Ellsworth of the U.S.Geological Survey (USGS) in Menlo Park, Calif.
"It's the first time we've been inside the earthquake machine,"Ellsworth says. "We've looked at the fossil earthquakes, we've madecomputer models, and we've made laboratory earthquakes. We've studiedthem from afar, but we've never been inside the machine where theaction is."
When completed in 2007, SAFOD will be the only earthquakeobservatory with instruments installed directly within an active faultwhere earthquakes form or "nucleate." Scientists also will be able tobring up actual rock and mineral samples from the earthquake zone."With SAFOD, we'll be able to recreate the earthquake process in thelaboratory using real materials and under real conditions that exist inthe San Andreas Fault Zone at depth," Hickman says. "That's unique."
Launched in 2003, SAFOD is one of three major components ofEarthScope, a National Science Foundation-funded initiative beingcarried out in collaboration with USGS. EarthScope is designed toinvestigate the powerful geological forces that shape the NorthAmerican continent. The other EarthScope projects, USArray and thePlate Boundary Observatory, are large-scale research efforts focusingon deformation and properties of the Earth's crust in North America.
EarthScope is combining data from the SAFOD borehole withthousands of seismic, strainmeter and GPS measurements from across thecontinent. "We now have the first opportunity to measure directly theconditions under which earthquakes initiate and grow," says HermanZimmerman, director of the NSF Division of Earth Sciences. "This is anunprecedented step forward in understanding these dangerous phenomena."
SAFOD is being built on private ranchland near the rural town ofParkfield in central California, about halfway between San Franciscoand Los Angeles. The ranch straddles the San Andreas Fault, an800-mile-long rift that marks the boundary between the Pacific andNorth American tectonic plates. These two enormous landmassesconstantly grind against each other in opposite directions, triggeringearthquakes of various magnitudes up and down the fault.
"Almost everything we know about earthquakes has been gatheredeither at or very close to the Earth's surface, where all we see is theelastic part of the process, the part that carries seismic waves togreat distance," Ellsworth says. "SAFOD gets into the inelastic partwhere things are actually breaking. That's the part we can only see bygetting into the fault zone."
Drilling of the observatory borehole began in June 2004 andcontinued until mid-October, the end of the dry season in California.Drilling resumed on June 10, 2005, and on Aug. 2 drill operatorsfinally entered the San Andreas Fault Zone, reaching a maximum depthabout 2 miles below the surface of the Earth.
The borehole begins on the Pacific plate just west of thefault, passes through the active earthquake zone and winds up in theNorth American plate east of the fault--a distance of 3 miles. Seismicinstruments will be installed along both plates in a section of thefault where small temblors of magnitude 2.0 are frequent. While thesemicroearthquakes usually aren't felt at the surface, they can offerimportant clues about the origin of bigger, more destructive quakes."Microearthquakes provide scientists an exciting opportunity to studyevents that occur about every two years in roughly the same place,"Zoback explains. "It's a live, active system, and we're building anobservatory directly within it."
SAFOD instrumentation will provide around-the-clockobservations of temperature, fluid pressure, strain accumulation andother processes before, during and after microearthquakes occur."That's really at the heart of determining whether earthquakeprediction is possible, and if it is, how you might go about doing it,"Hickman notes. "You cannot do those kinds of in-depth observations inparts of the fault that only produce big earthquakes, because thoseusually occur at intervals of 100 to 150 years or so."
In addition to monitoring theearthquake nucleation process, SAFOD researchers plan to address anumber of fundamental scientific questions. For example, in what waysare plate boundaries such as the San Andreas unique? Why are they sonarrow? Why do they persist for millions of years? What makes them soweak relative to that crust that's adjacent to them?
"We have numerous theories about how earthquakes work that havebeen developed over the last 20 years based on remote geophysicalobservations of active faults or geologic examination of faults exhumedby erosion that are no longer active," Hickman says. "For the dozens ofscientists involved in SAFOD, this is really their first opportunity totest these ideas and see which ones are right."
When drilling is completed in August, the entire borehole willbe encased in steel and cement so that sensitive instruments--such asseismometers, strainmeters, and fluid and temperature gauges--can beinstalled underground. Meanwhile, scientists will begin to collectrock, gas and mineral samples from the fault zone for laboratoryanalysis.
Over the next two years, geophysicists also will try toidentify precise areas in the fault zone where microearthquakesregularly occur. In 2007, project engineers will begin drilling intothose active areas and installing the instruments. The observatory isexpected to operate for 20 years and give researchers a unique windowinto the process of stress buildup and release in the fault zone duringnumerous microearthquakes.
"It's a whole new type of experiment," Zoback concludes. "It'sopening doors to research we haven't been able to consider beforebecause we've never been able to do experiments within an active fault.It's a very exciting time for earthquake science."
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