San Andreas earthquake observatory achieves milestone as drillers penetrate the active fault zone

The San Andreas Fault Observatory at Depth (SAFOD) reached a significant goal on Aug. 2 when scientists drilled into a seismically active section of the fault approximately two miles below the surface of the Earth.

“This is a milestone for SAFOD,” says Mark Zoback, a professor of geophysics at Stanford University. “For the first time, scientists have drilled directly into the San Andreas Fault Zone at a depth that will allow us to observe earthquakes up close for decades to come.”

Zoback is co-principal investigator of the SAFOD project, along with 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 made computer models, and we’ve made laboratory earthquakes. We’ve studied them from afar, but we’ve never been inside the machine where the action is.”

When completed in 2007, SAFOD will be the only earthquake observatory with instruments installed directly within an active fault where earthquakes form or “nucleate.” Scientists also will be able to bring up actual rock and mineral samples from the earthquake zone. “With SAFOD, we’ll be able to recreate the earthquake process in the laboratory using real materials and under real conditions that exist in the San Andreas Fault Zone at depth,” Hickman says. “That’s unique.”

Launched in 2003, SAFOD is one of three major components of EarthScope, a National Science Foundation-funded initiative being carried out in collaboration with USGS. EarthScope is designed to investigate the powerful geological forces that shape the North American continent. The other EarthScope projects, USArray and the Plate Boundary Observatory, are large-scale research efforts focusing on deformation and properties of the Earth’s crust in North America.

EarthScope is combining data from the SAFOD borehole with thousands of seismic, strainmeter and GPS measurements from across the continent. “We now have the first opportunity to measure directly the conditions under which earthquakes initiate and grow,” says Herman Zimmerman, director of the NSF Division of Earth Sciences. “This is an unprecedented step forward in understanding these dangerous phenomena.”

Plate tectonics

SAFOD is being built on private ranchland near the rural town of Parkfield in central California, about halfway between San Francisco and Los Angeles. The ranch straddles the San Andreas Fault, an 800-mile-long rift that marks the boundary between the Pacific and North American tectonic plates. These two enormous landmasses constantly grind against each other in opposite directions, triggering earthquakes of various magnitudes up and down the fault.

“Almost everything we know about earthquakes has been gathered either at or very close to the Earth’s surface, where all we see is the elastic part of the process, the part that carries seismic waves to great distance,” Ellsworth says. “SAFOD gets into the inelastic part where things are actually breaking. That’s the part we can only see by getting into the fault zone.”

Drilling of the observatory borehole began in June 2004 and continued until mid-October, the end of the dry season in California. Drilling resumed on June 10, 2005, and on Aug. 2 drill operators finally entered the San Andreas Fault Zone, reaching a maximum depth about 2 miles below the surface of the Earth.

The borehole begins on the Pacific plate just west of the fault, passes through the active earthquake zone and winds up in the North American plate east of the fault–a distance of 3 miles. Seismic instruments will be installed along both plates in a section of the fault where small temblors of magnitude 2.0 are frequent. While these microearthquakes usually aren’t felt at the surface, they can offer important clues about the origin of bigger, more destructive quakes. “Microearthquakes provide scientists an exciting opportunity to study events that occur about every two years in roughly the same place,” Zoback explains. “It’s a live, active system, and we’re building an observatory directly within it.”

SAFOD instrumentation will provide around-the-clock observations of temperature, fluid pressure, strain accumulation and other processes before, during and after microearthquakes occur. “That’s really at the heart of determining whether earthquake prediction is possible, and if it is, how you might go about doing it,” Hickman notes. “You cannot do those kinds of in-depth observations in parts of the fault that only produce big earthquakes, because those usually occur at intervals of 100 to 150 years or so.”

Fundamental theories

In addition to monitoring the earthquake nucleation process, SAFOD researchers plan to address a number of fundamental scientific questions. For example, in what ways are plate boundaries such as the San Andreas unique? Why are they so narrow? Why do they persist for millions of years? What makes them so weak relative to that crust that’s adjacent to them?

“We have numerous theories about how earthquakes work that have been developed over the last 20 years based on remote geophysical observations of active faults or geologic examination of faults exhumed by erosion that are no longer active,” Hickman says. “For the dozens of scientists involved in SAFOD, this is really their first opportunity to test these ideas and see which ones are right.”

When drilling is completed in August, the entire borehole will be encased in steel and cement so that sensitive instruments–such as seismometers, strainmeters, and fluid and temperature gauges–can be installed underground. Meanwhile, scientists will begin to collect rock, gas and mineral samples from the fault zone for laboratory analysis.

Over the next two years, geophysicists also will try to identify precise areas in the fault zone where microearthquakes regularly occur. In 2007, project engineers will begin drilling into those active areas and installing the instruments. The observatory is expected to operate for 20 years and give researchers a unique window into the process of stress buildup and release in the fault zone during numerous microearthquakes.

“It’s a whole new type of experiment,” Zoback concludes. “It’s opening doors to research we haven’t been able to consider before because we’ve never been able to do experiments within an active fault. It’s a very exciting time for earthquake science.”