Drilling yields San Andreas rock cores
PALO ALTO -- Rock samples extracted from two miles deep within the San Andreas fault -- the first samples of their kind -- are a breakthrough that could provide new understanding of how earthquakes function, scientists said Thursday.
Earthquake scientists from Stanford University and the United States Geological Survey said that the minerals taken from the core of the active fault could help scientists learn what triggers an earthquake and whether quakes can be predicted.
“Now we can hold the San Andreas fault in our hands,” said Mark Zoback, a Stanford professor of earth sciences and a leader of the project. “We know what it’s made of. We can study how it works.”
Until now, geologists studying the physical evidence of faults have focused on outcrops that were uplifted to the surface and weathered over millions of years.
The rock samples taken from the San Andreas fault near Parkfield in Central California are so fresh that they were 240 degrees Fahrenheit when they were extracted.
“This is very significant,” said Thomas Jordan, a USC professor of geophysics and director of the Southern California Earthquake Center who is not involved in the project. “For the first time we have samples from the core of the fault zone at the depths where these earthquake processes are actually happening.”
Jordan added: “These may turn out to be as precious as the rocks brought back from the moon.”
The core samples are part of an ambitious $197-million federal project called EarthScope to gather detailed data on the geologic structure and evolution of North America. In California, it includes the $24-million San Andreas Fault Observatory at Depth, which is establishing the world’s first underground earthquake observatory at Parkfield.
The small town near Paso Robles was chosen as the site for the study because it is an unusual spot on the San Andreas fault where small earthquakes occur frequently, many of them unnoticed at the surface.
Scientists have long wondered why the two huge tectonic plates that are separated by the San Andreas slide by each other at Parkfield with few major jolts, unlike other sections of the fault. They hope the core samples will provide the answer.
“The story of how it moves is contained in these rocks,” said William Ellsworth, a USGS geophysicist who is another leader of the project. “We are going to get some of the secrets of the San Andreas fault out of these rocks.”
The drilling at Parkfield began in 2004, and scientists recovered ground-up soil from the fault in 2005. In drilling the hole, they discovered two active strands of the fault about 350 feet apart.
This year, they drilled diagonally through the two strands at a depth of about two miles and extracted intact core samples from both. The final samples were brought up Sept. 7. On Thursday, project leaders displayed several sections of the core at a Stanford news conference.
The scientists are confident that they extracted minerals from the fault because the steel casing they inserted into the ground was deformed by the shifting of the tectonic plates during the project. The plates move about an inch a year along the fault line.
“That was the proof,” Ellsworth said.
Altogether, they extracted about 135 feet of core samples 4 inches in diameter and weighing about a ton. Most of the samples are solid rock, but in the area of the active fault -- which scientists call “the gouge” -- the soil was pulverized. One gouge sample is 3 feet long, the other 8 1/2 long, an indication of how wide each strand is.
Among the immediate findings was the discovery of serpentine in the samples, which could help explain why the plates there creep along relatively smoothly.
When exposed to water, heat and pressure, serpentine can dissolve and crystallize as talc. And talc, long used in baby powder, is well known for reducing friction. Talc was found in some of the soil extracted in 2005, but the new samples have not yet been analyzed for its presence.
“The serpentine is floating around in the fault gouge like raisins in raisin pudding,” said Stephen Hickman, a USGS geophysicist and the third project leader. “We will be looking very carefully to see if talc is in the core samples.”
The scientists will hold a “sample party” in December to show the cores to experts from around the world and invite them to propose research projects. Until then, the samples will spend most of their time refrigerated at a temperature just above freezing and wrapped in plastic so they remain moist.
At Parkfield, work to establish the observatory will continue with the insertion of sensing devices such as seismometers, tiltmeters and accelerometers into the two-mile hole to monitor the fault’s activity.
But for the scientific community, the extraction of the core samples has already made the project a success.
“This is an important milestone for all of earth sciences,” said Kaye Shedlock, EarthScope project director for the National Science Foundation.
