 |
|
Geologists recover rocks yielding unprecedented insights into San Andreas Fault
October 05, 2007For the first time, geologists have extracted intact rock samples from 2 miles beneath the surface of the San Andreas Fault, the infamous rupture that runs 800 miles along the length of California.
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 135 feet of 4-inch diameter rock cores weighing roughly 1 ton. They were brought to the surface through a research borehole drilled more than 2.5 miles into the Earth. The last of the cores was brought to the surface in the predawn hours of Sept. 7.
Scientists seeking to understand how the great faults bounding Earth's vast tectonic plates evolve and generate earthquakes have always had to infer the processes through indirect means. Up until now, they could only work with samples of ancient faults exposed at the Earth's surface after millions of years of erosion and uplift, together with computer simulations and laboratory experiments approximating what they think might be happening at the depths at which earthquakes occur.
"Now we can hold the San Andreas Fault in our hands," said Mark Zoback, the Benjamin M. Page Professor in Earth Sciences at Stanford. "We know what it's made of. We can study how it works."
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, Calif., are the other co-principal investigators.
SAFOD, which first broke ground in 2004, is a major research component of EarthScope, a National Science Foundation-funded program being carried out in collaboration with the 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 2.5-mile-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 temblors, scientists will be able to observe the earthquake generation process with unprecedented acuity. They hope to keep the observatory operating for the next 10 to 20 years.
Studying the San Andreas Fault is important because, as Zoback noted, "The really big earthquakes occur on plate boundaries like the San Andreas Fault." The SAFOD site, located about 23 miles 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 a mineral called serpentine exposed in several places along the surface trace of the fault, which-if it existed at depth-could both 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 has 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 limber up 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 Aug. 16, 2007, issue of Nature.
"Talc is one of the slipperiest, weakest minerals ever studied," Hickman said.
Might the same mineral that helps keep a baby's bottom smooth also be smoothing the way for the huge tectonic plates" Chemically, it's possible, for when serpentine is subjected to high temperatures in the presence of water containing silica, 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 crystallize in nearby open pore spaces, allowing the fault to creep even under very little pressure," Hickman said.
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 behavior 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 said. "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."
Will these new samples allow scientists to predict earthquakes" The short answer is no. But research on these samples could provide clues to answer the question of whether earthquakes are predictable. The observatory will allow scientists to begin to address whether there are precursory phenomena occurring within the fault zone.
The other fault zone, called 10480, contains 3 feet of fault gouge. It also produces small earthquakes at a location about 300 feet below the borehole. "Remarkably, we observe the same earthquake rupturing at the same spot on the fault year after year," Ellsworth said. This repeating earthquake, always about a magnitude 2, will be the focus of the observatory to be installed inside the fault in 2008.
Sensitive seismometers and tiltmeters to be installed in the SAFOD borehole directly above the spot that ruptures will observe for the first time the birthing process of an earthquake from the zone where the earthquake energy accumulates. Preliminary observations made in 2006 already have revealed the tiniest earthquakes ever observed-so small they have negative magnitudes.
In early December, a "sample party" will be held at the USGS office in Menlo Park, where the cores will be on display and scientists will offer their proposals to do research projects in a bid to be allowed to analyze part of the core.
Zoback said most of the initial testing will be nondestructive in order to preserve the samples for as long as possible. "But then, some of the material will be made available for testing that simulates earthquakes and fault slip in the lab," he said.
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.
Stanford University
Scientists seeking to understand how the great faults bounding Earth's vast tectonic plates evolve and generate earthquakes have always had to infer the processes through indirect means. Up until now, they could only work with samples of ancient faults exposed at the Earth's surface after millions of years of erosion and uplift, together with computer simulations and laboratory experiments approximating what they think might be happening at the depths at which earthquakes occur.
"Now we can hold the San Andreas Fault in our hands," said Mark Zoback, the Benjamin M. Page Professor in Earth Sciences at Stanford. "We know what it's made of. We can study how it works."
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, Calif., are the other co-principal investigators.
SAFOD, which first broke ground in 2004, is a major research component of EarthScope, a National Science Foundation-funded program being carried out in collaboration with the 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 2.5-mile-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 temblors, scientists will be able to observe the earthquake generation process with unprecedented acuity. They hope to keep the observatory operating for the next 10 to 20 years.
Studying the San Andreas Fault is important because, as Zoback noted, "The really big earthquakes occur on plate boundaries like the San Andreas Fault." The SAFOD site, located about 23 miles 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 a mineral called serpentine exposed in several places along the surface trace of the fault, which-if it existed at depth-could both 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 has 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 limber up 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 Aug. 16, 2007, issue of Nature.
"Talc is one of the slipperiest, weakest minerals ever studied," Hickman said.
Might the same mineral that helps keep a baby's bottom smooth also be smoothing the way for the huge tectonic plates" Chemically, it's possible, for when serpentine is subjected to high temperatures in the presence of water containing silica, 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 crystallize in nearby open pore spaces, allowing the fault to creep even under very little pressure," Hickman said.
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 behavior 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 said. "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."
Will these new samples allow scientists to predict earthquakes" The short answer is no. But research on these samples could provide clues to answer the question of whether earthquakes are predictable. The observatory will allow scientists to begin to address whether there are precursory phenomena occurring within the fault zone.
The other fault zone, called 10480, contains 3 feet of fault gouge. It also produces small earthquakes at a location about 300 feet below the borehole. "Remarkably, we observe the same earthquake rupturing at the same spot on the fault year after year," Ellsworth said. This repeating earthquake, always about a magnitude 2, will be the focus of the observatory to be installed inside the fault in 2008.
Sensitive seismometers and tiltmeters to be installed in the SAFOD borehole directly above the spot that ruptures will observe for the first time the birthing process of an earthquake from the zone where the earthquake energy accumulates. Preliminary observations made in 2006 already have revealed the tiniest earthquakes ever observed-so small they have negative magnitudes.
In early December, a "sample party" will be held at the USGS office in Menlo Park, where the cores will be on display and scientists will offer their proposals to do research projects in a bid to be allowed to analyze part of the core.
Zoback said most of the initial testing will be nondestructive in order to preserve the samples for as long as possible. "But then, some of the material will be made available for testing that simulates earthquakes and fault slip in the lab," he said.
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.
Stanford University
San Andreas Fault Current Events and San Andreas Fault News RSSA stress meter for fault zones
For the first time, scientists from Rice University, the Carnegie Institution of Washington, and the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have measured - in the field rather than in the laboratory - how changes in stress in rocks affect changes in the speed of seismic waves at depths where earthquakes begin.
Why do earthquakes stop?
The underlying structure of a fault determines whether an earthquake rupture will jump from one fault to another, magnifying its size and potential devastation.
Newly discovered active fault building new Dalmatian Islands off Croatian coast
A newly identified fault that runs under the Adriatic Sea is actively building more of the famously beautiful Dalmatian Islands and Dinaride Mountains of Croatia, according to a new research report.
Los Angeles enjoying 1,000 year seismic lull
The Los Angeles basin appears to be in a seismic "lull" characterized by relatively smaller and infrequent earthquakes, according to a study in the September issue of Geology.
SMART-1 diagnoses wrinkles and excess weight on the Moon
Owing to SMART-1's high resolution and favourable illumination conditions during the satellite's scientific operations, data from Europe's lunar orbiter is helping put together a story linking geological and volcanic activity on the Moon.
Scientists explain source of mysterious tremors emanating from fault zones
Tiny tremors and temblors recently discovered in fault zones from California to Japan are generated by slow-moving earthquakes that may foreshadow catastrophic seismic events, according to scientists at Stanford University and the University of Tokyo.
New data shakes accepted models of collisions of the Earth's crust
New research findings may help refine the accepted models used by earth scientists over the past 30 years to describe the ways in which continents clash to form the Earth's landscape.
Discovery sheds new light on cause of earthquakes
Research at the University of Liverpool into a large fault zone in the Atacama Desert in northern Chile has produced new insight into how fluid pressure can cause earthquakes.
New evidence shows New Madrid Seismic Zone may be cold and dying
New results about the temperatures of rock deep below the New Madrid Seismic Zone in the central United States shed light on the puzzling questions of why large earthquakes happened there in 1811 and 1812 and when they may happen again.
Tiny tremors and earthquakes provide intriguing clues about seismic activity, study finds
The elusive science of earthquake prediction has been reinvigorated in recent years with the discovery of "non-volcanic tremors"—faint vibrations that originate deep inside active fault zones.
More San Andreas Fault Current Events and San Andreas Fault News Articles
 | Life on the San Andreas Fault: A History of Portola Valley by Nancy Lund, Pamela Gullard |
| San Andreas Fault in Southern California (Guide to San Andreas fault from Mexico to Carrizo Plain, Special Report 118) This timely guide has been compiled and published as a joint effort between the Calif. Division of Mines and Geology and the Cordilleran Section of the Geological Society of America in an effort to provide a greater understanding of the San Andreas fault and the regional tectonics of Southern California at a time when earthquakes and seismic hazards are of keen... | |
| San Andreas Fault in Southern California. A guide to San Andreas Fault from Mexico to Carrizo Plain. by John [Ed] Crowell | |
| Guidebook to the Geology of Salinas Valley and the San Andreas Fault, 1963, Annual Spring Field Trip, Pacific Section AAPG-SEPM | |
| The San Andreas Fault System, California by Robert E Wallace | |
| Well, It's Not My Fault!: About the San Andreas Fault and Other Things by J. M. A. Lenihan | |
 | Magnitude 8: Earthquakes and Life along the San Andreas Fault by Philip L. Fradkin Environmental historian Philip L. Fradkin offers a vivid history of earthquakes and an eloquent guide to the San Andreas Fault, the seismic scar that bisects the Golden State's spectacular scenery. The author includes dramatic stories of legendary earthquakes elsewhere: in New York, New England, the central Mississippi River Valley, Europe, and the Far East. Combining human and natural dramas, he... |
| Fault Stress States, Pore Pressure Distributions, and the Weakness of the San Andreas Fault by James R Rice | |
| Selected Papers Presented to San Joaquin Geological Society: San Andreas Fault, A Sleeper in the Old Midway-Sunset Oil Field, Meganos Gorge, and Blue Sky Laws and the Geologist by G. B. et al. Oakeshott | |
| The San Andreas Fault by Sandra S Schulz |
| © 2008 BrightSurf.com |