Levy, Stanford News Service (650) 725-1944; firstname.lastname@example.org
Zoback, Stanford Department of Geophysics (650) 725-9295; email@example.com
The Parkfield pilot hole drill site is on private
land and off-limits to visitors. USGS will offer a
scheduled media tour of the site later in the summer.
Betacam video of on-site drilling can be obtained
from Susan Garcia or Wendy Shindle at USGS. Graphics
Scientists launch major earthquake drilling project along the San Andreas Fault
An international research team announced Monday it has begun drilling a 1.4-mile-deep hole along the San Andreas Fault near the Central California town of Parkfield -- site of one of the longest ongoing earthquake experiments in the world.
When drilling is completed this summer, the research team -- spearheaded by the U.S. Geological Survey (USGS) and Stanford University -- will make field and laboratory measurements and install a variety of underground instruments that will help scientists better predict the timing and severity of earthquake activity along the 800-mile-long fault.
A major objective of the project is to provide geological data for an even more ambitious drilling project called the San Andreas Fault Observatory at Depth (SAFOD) -- a parallel borehole designed to cross the fault some 2.4 miles below the surface. If approved by Congress, SAFOD would be the first underground earthquake observatory to penetrate a seismically active fault zone -- giving scientists a unique opportunity to continuously monitor a section of the fault where earthquakes actually happen. The current project will serve as a pilot hole for SAFOD by providing critical engineering data needed to drill through the San Andreas Fault itself.
"The pilot hole is really a warm-up exercise for SAFOD," said Stanford geophysicist Mark Zoback. "It was conceived about a year ago as a way to begin studying the upper crust adjacent to the fault zone, while at the same time helping us identify earthquake targets for SAFOD."
Zoback, along with geophysicists Stephen Hickman and William Ellsworth of the USGS Earthquake Hazards Team in Menlo Park, Calif., are longtime proponents of the Parkfield drilling effort.
When drilling is completed in the next few weeks, researchers will lower instruments into the hole to measure stress, fluid pressure, heat flow and other properties to characterize the geologic environment of the San Andreas Fault Zone and to determine the amount of stress required to make the fault slip. They will then install an extensive array of seismometers and other instruments in the hole to help study and precisely locate earthquakes within the fault zone that will be targets for later SAFOD drilling.
"The earthquakes that occur here are quite remarkable," Ellsworth said. "Many of them recur time and time again with near clocklike regularity. The pilot hole instruments will give us a powerful new tool for understanding what makes them tick."
Hickman added: "We'll also be analyzing in the laboratory rock, water and gas samples collected during drilling to determine how changes in fluid circulation and chemistry might be related to the earthquake cycle."
Relation to Parkfield Experiment
Located on the San Andreas Fault halfway between San Francisco and Los Angeles, Parkfield is considered an ideal place to study the physical processes associated with recurring earthquakes. Geologists have been monitoring the picturesque rural town for more than 20 years.
Throughout most of California, the San Andreas Fault is locked, causing stress to build up underground and trigger occasional large temblors of magnitude 7.0 or greater -- such as the devastating magnitude 7.8 San Francisco quake of 1906.
But in Central California, the San Andreas Fault is slipping all the time, through a combination of steady (aseismic) creep and small-magnitude earthquakes. As a result, Parkfield, which is located at the southern transition between the creeping and locked sections of the fault, experiences only small-to-moderate-sized earthquakes -- including a remarkable series of six magnitude 6.0 events that have occurred, on average, every 22 years since 1857.
In 1985, the USGS and the state of California launched the Parkfield Earthquake Experiment, which has been collecting real-time data from an array of instruments on or near the surface of the fault. The pilot hole is the first deep underground probe to be included in the experiment.
"One important goal of the Parkfield Experiment is to try to catch a moderate earthquake," Zoback explained. "The last 6.0 quake struck Parkfield in 1966, so right now we're overdue. Hopefully, both the pilot hole and SAFOD will be completed in time to capture the next magnitude 6 Parkfield earthquake."
The pilot hole is being drilled on private land about a mile southwest of the fault and is closed to visitors. Several companies, including ThermaSource Inc., an engineering firm in Santa Rosa, Calif., have been subcontracted to perform the drilling, thanks to a $1.5 million grant from the International Continental Drilling Program, a multinational geosciences project based in Potsdam, Germany. The National Science Foundation (NSF) is providing an additional $400,000 in research grants to scientists from U.S. universities that is being matched by a comparable contribution of scientific staff and funding from the USGS.
The SAFOD project -- which will be located just east of the pilot site -- will require drilling a larger, deeper and more directionally complex hole than the pilot hole being drilled this summer. SAFOD drilling would start on the Pacific Plate, which forms the western boundary of the San Andreas Fault Zone, and continue in a vertical direction about 1.4 miles below the surface. Then, using technology developed by the oil industry, engineers would alter the course of the hole -- angling it eastward until it crossed the fault zone and ending up in the North American Plate.
Scientists from more than 20 universities and government laboratories spent a year examining the entire San Andreas Fault before selecting the SAFOD site, Ellsworth said. "Everyone agreed that this was a most promising place to address some of the most fundamental questions about earthquakes that have long eluded surface-bound observers."
According to Hickman, SAFOD will drill into, or very close to, a repeating microearthquake source -- a region along the fault where small magnitude 2.0 quakes repeatedly rupture the same area at regular time intervals. Thus, for the first time, SAFOD will allow around-the-clock monitoring of subtle changes in deformation, fluid pressure and temperature during earthquake initiation and rupture directly within the fault zone at depths where earthquakes originate.
"These magnitude 2.0 clusters are rarely felt, but they can end up rupturing a football-sized piece of the fault," Hickman said.
SAFOD is one of four large-scale experiments proposed under the umbrella of EarthScope, an interdisciplinary geophysical and geological program dedicated to answering fundamental questions about the geology of North America: Why do earthquakes and volcanic eruptions occur? How do continents form and evolve?
The EarthScope program involves NSF, USGS, NASA and more than 100 universities. The proposed budget for the SAFOD project alone is about $30 million over six years -- $16 million to drill through the San Andreas Fault, and $14 million to make downhole geophysical measurements, conduct laboratory studies of fault rocks and fluids recovered from the hole, and install a long-term earthquake observatory within the fault zone that will operate for 20 years. Whether SAFOD goes forward depends on whether the U.S. Congress decides to fund EarthScope. A decision could be made by September.
By Mark Shwartz