UC seismologists dig deep to profile ‘mini-quakes’
Fault creep’ studied in Parkfield to help predict location and likelihood of major quakes elsewhere on the San Andreas Fault

By Diane Ainsworth, Public Affairs

A pilot station at Parkfield, Calif., extending 1.4 miles underground, runs parallel to the location of the proposed San Andreas Fault Observatory at Depth, which will drop to depths of 2.5 miles when built and become the first earthquake observatory to penetrate a seismically active fault zone. Graphic courtesy Steve Hickman, USGS

23 October 2002 | In Parkfield, Calif., a team of Berkeley seismologists has begun studying earthquakes deep beneath the ground to understand the great tectonic compression that is carrying an 800-mile sliver of California northward at an average rate of 1.5 inches each year —and producing catastrophic earthquakes at greater, far more alarming intervals.

Building on 15 years of earthquake research at Parkfield, which straddles the San Andreas Fault about 200 miles south of San Francisco, Berkeley earthquake expert Robert Nadeau says a new experimental earthquake station will help seismologists understand the mechanics and frequency of small quakes, those measuring 2.0 or less on the Richter scale. That will give them a better snapshot of how fast the fault is slipping.

“A picture of the small quakes occurring along this part of the fault can tell us where and how much the fault is moving,” says Nadeau, an assistant researcher in Berkeley’s Seismology Laboratory. “Studying them at depth will give us a better understanding of how the fault behaves, what types of rock are down there, and how the release of groundwater and other fluids may be altering or weakening the fault.”

An ideal location
Parkfield couldn’t be a better place to study the behavior and physics of earthquakes, says Barbara Romanowicz, a professor of earth and planetary science and director of the Berkeley Seismology Laboratory. This part of the fault is quietly “creeping,” producing hundreds of tiny tremors every day, most of which are never felt. The temblors are caused by the grinding of the Pacific and North American plates as they scrape past each other in opposite directions.

But on the northern and southern ends of the fault, she says, the land is locked, causing stresses to build up underground — stresses that occasionally lead to earthquakes of 7.0 magnitude or more. The last time that happened on the San Andreas was in 1906, when a catastrophic earthquake struck San Francisco.

“We really need to understand what is happening on and near a fault during an earthquake,” Romanowicz says. “Most of what we know today is based on assumptions rather than actual direct measurements.”

Piercing the heart of the fault
The Parkfield Earthquake Experiment was launched in 1985 by the USGS and the state of California to collect real-time data from a variety of instruments on or near the surface of the San Andreas Fault. Berkeley’s 10-station High Resolution Seismic Network, a set of 10 shallow boreholes instrumented with seismometers, was installed two years later to record small quakes occurring below ground. The subsurface installation was designed to minimize any noise or movements on the ground that could interfere with the instruments.

Last year, that network was expanded to 13 stations to support a new underground earthquake observatory, the deepest ever planned. The San Andreas Fault Observatory at Depth (SAFOD), to be funded by the National Science Foundation, will pierce the heart of the San Andreas Fault, extending 2 kilometers (1.25 miles) straight down, then continuing at an angle across the entire fault zone until it reaches relatively undisturbed rock on the east side of the fault, at a depth of approximately 4 kilometers (2.5 miles) It will be the first borehole to cross two plates and record shifts in the layers of rock where the quakes originate.

Nadeau says a pilot station extending about half that depth — 2.2 kilometers (1.4 miles) below the surface — is a precursor to the deeper underground SAFOD station. That experimental station is up and running now, and beginning to record the many mini-earthquakes of 2.0 magnitude or less that occur repeatedly along the Parkfield segment of the San Andreas.

“Instruments are recording the velocity of seismic waves that are produced during a quake,” he says. “Because seismic waves travel at different speeds through different types of rock, scientists will be able to determine what type of rock is present at those depths.”

Changes in the ground
Other instruments operating as part of the pilot station may be able to determine additional variables, so that scientists can build better earthquake models in the future, Nadeau says. For instance, better measurements of ground deformation — changes in the shape of the fault zone — are sorely needed.

Instruments provided by other institutions will begin recording other pieces of the puzzle — such as subsurface ground temperatures and changes in groundwater levels that occur in the aftermath of a quake — so that seismologists can determine how much stress has been relieved along specific parts of the fault. That, in turn, will allow them to look for other parts of the fault that haven’t been shaken for a while — and to better predict whether those regions are likely to rupture anytime soon.

“The new SAFOD observatory will be our opportunity to punch through a seismically active fault zone and take direct measurements,” Nadeau says. “It will give us a chance to explore earthquake initiation, and what happens to the ground at great depths when energy is released. All of this will contribute to a fundamental understanding of earthquakes, and enhance our ability to identify hazardous parts of the fault in the future.”