Never before have scientists had available for study rock samples from deep inside one of the actively moving tectonic plate-bounding faults responsible for the world's most damaging earthquakes. Now, with this newly recovered material, scientists hope to answer long-standing questions about the fault's composition and properties.
Altogether, the geologists retrieved 41 metres of 4-inch core (four inches are just over 10 centimetres). The last of the cores was brought to the surface in the predawn hours of September 7.
Zoback is one of three co-principal investigators of the San Andreas Fault Observatory at Depth (SAFOD) project, which is establishing the world's first underground earthquake observatory. William Ellsworth and Steve Hickman, geophysicists with the U.S. Geological Survey (USGS) in Menlo Park, California, are the other co-principal investigators. SAFOD first broke ground in 2004. It is a major research component of EarthScope, a National Science Foundation-funded program being carried out in collaboration with USGS and NASA to investigate the forces that shape the North American continent and the physical processes controlling earthquakes and volcanic eruptions.
"This is tremendously exciting. Obtaining cores from the actively slipping San Andreas Fault is truly unprecedented and will allow truly transformative research and discoveries," said Kaye Shedlock, EarthScope program director at the National Science Foundation.
In the next phase of the experiment, the science team will install an array of seismic instruments in the four-kilometre long borehole that runs from the Pacific plate on the west side of the fault into the North American plate on the east. By placing sensors next to a zone that has been the source of many small earthquakes, scientists will be able to observe the generation process with unprecedented acuity. They hope to keep the observatory operating for the next 10 to 20 years.
As Zoback told reporters at a press conference yesterday, "The really big earthquakes occur on plate boundaries like the San Andreas Fault." The SAFOD site, located about 37 kilometres northeast of Paso Robles near the tiny town of Parkfield, sits on a particularly active section of the fault that moves regularly. But it does not produce large earthquakes. Instead, it moves in modest increments by a process called creep, in which the two sides of the fault slide slowly past one another, accompanied by occasional small quakes, most of which are not even felt at the surface.
One of the big questions the researchers seek to answer is how, when most of the fault moves in violent, episodic upheavals, can there be a section where the same massive tectonic plates seem, by comparison, to gently tiptoe past each other with the delicate tread of little cat feet"
"There have been many theories about why the San Andreas Fault slides along so easily, none of which could be tested directly until now," Hickman said. Some posit the presence of especially slippery clays, called smectites. Others suggest there may be high water pressure along the fault plane lubricating the surface. Still others note the presence of serpentine, exposed in several places along the surface trace of the fault, which - if it existed at depth - could weaken the fault and cause it to creep.
Zoback said the correlation between the occurrence of serpentine, a metamorphosed remnant of old oceanic crust, and the slippery nature of the fault motion in the area, have been the subject of speculation for more than 40 years. However, it has never been demonstrated that serpentine actually occurs along the active San Andreas at depth, and the mechanism by which serpentine might lubricate the fault was unknown.
Then, in 2005, when the SAFOD drill pierced the zone of active faulting using rotary drilling (which grinds up the rock into tiny fragments), mineralogist Diane Moore of the USGS detected talc in the rock cuttings brought up to the surface. This finding was published in the 16 August 2007, issue of Nature. "Talc is one of the slipperiest, weakest minerals ever studied" Hickman says.
Might the same mineral that helps keep a babies' bottoms smooth also be smoothing the backsides of tectonic plates? Chemically, it's possible, because when serpentine is subjected to high temperatures in the presence of siliceous waters it forms talc. Serpentine might also control how faults behave in other ways. "Serpentine can dissolve in ground water as fault particles grind past each other and then crystallise in nearby open pore spaces, allowing the fault to creep even under very little pressure" Hickman says.
The SAFOD borehole cored into two active traces of the fault this summer, both contained within a broad fault "zone" about 700 feet wide. The deeper of the two active fault zones, designated 10830 for its distance in feet from the surface as measured along the curving borehole, yielded an 8-foot-long section of very fine-grained powder called fault gouge. Such gouge is common in fault zones and is produced by the grinding of rock against rock. "What is remarkable about this gouge is that it contains abundant fragments of serpentine that appear to have been swept up into the gouge from the adjacent solid rock," Hickman said. "The serpentine is floating around in the fault gouge like raisins in raisin pudding."
The only way to know what role serpentine, talc or other exotic minerals play in controlling the behaviour of the San Andreas Fault is to study the SAFOD core samples in the laboratory. "To an earthquake scientist, these cores are like the Apollo moon rocks," Hickman says. "Scientists from around the world are anxious to get their hands on them in the hope that they can help solve the mystery of how this major, active plate boundary works."
In early December, a "sample party" will be held at the USGS office in Menlo Park, where the cores will be displayed and scientists will offer competing research proposals in a bid to be allowed to analyse parts of the core.
Zoback said most of the initial testing will be non-destructive. "But then, some of the material will be made available for testing that simulates earthquakes and fault slip in the lab" he says. When not being examined, the core samples will be refrigerated and kept moist to prevent the cores and the fluid in them from being disturbed.