The relatively smooth movement, called creep, happens because the fault creates its own lubricants---slippery clays that form ultra-thin coatings on rock fragments, geologist Ben van der Pluijm and colleagues at the University of Michigan and Germany's Ernst-Moritz-Arndt Universität Institut für Geographie und Geologie report in the July issue of Geology.
The question of why some fault zones creep slowly and steadily while others lock for a time and then shift suddenly and violently, spawning earthquakes, has long puzzled scientists. Some have speculated that fluids facilitate slippage, while others have focused on serpentine---a greenish material that can alter to slippery talc.
But when van der Pluijm and colleagues analyzed samples of rock from an actively creeping segment that was brought up from a depth of two miles below the surface as part of the San Andreas Fault Observatory at Depth (SAFOD) project, they found very little talc. Instead, they found that fractured rock surfaces were coated with a thin layer of smectitic clay, less than 100 nanometers thick, that acts something like grease on ball bearings.
"For a long time, people thought you needed a lot of lubricant for creep to occur," said van der Pluijm, who is the Bruce R. Clark Collegiate Professor of Geology and Professor of the Environment. "What we can show is that you don't really need a lot; it just needs to be in the right place. It's a bit like real estate: location, location, location." The nanocoatings occur on the interfaces of broken-up bits of rock in exactly the places where they affect the fault's "weakness"---how easily it moves.
The technique of argon dating provided key evidence, when the researchers determined that these clays, found only in fault rock, formed relatively recently.
"The clays are growing in the fault zone, and the fault is coating its own pieces of fragmented rock," van der Pluijm said. "At some point there's enough coating that it begins to drive the behavior of the fault, and creeping kicks in."
If the fault is greasing itself, then why do earthquakes still occur?
"The problem is that the fault doesn't always move at strands where the coating sits," van der Pluijm said. The San Andreas fault is actually a network of faults, with new strands being added all the time. Because it takes some time for the slick nanocoatings to develop in a new strand, the unlubricated, new strand "gets stuck" for a time and then shifts in a violent spasm.
Although the samples obtained through SAFOD are from a depth of only about two miles, van der Pluijm and colleagues think it's likely the clay nanocoatings also are forming and driving fault behavior at greater depths. What's more, analyses of older, inactive strands suggest that the coatings have been facilitating creep for the millions of years of fault activity.
The SAFOD project, which is establishing the world's first underground earthquake observatory, is a major research component of EarthScope, an ambitious, $197-million federal program to investigate the forces that shaped the North American continent and the processes controlling earthquakes, volcanoes and other geological activity.
Van der Pluijm's coauthors on the paper are U-M assistant research scientist Anja Schleicher and Professor Laurence Warr of Ernst-Moritz-Arndt Universität Institut für Geographie und Geologie. The researchers received funding from the National Science Foundation and the Deutsche Forschungsgemeinschaft.
Ben van der Pluijm: http://www.ns.umich.edu/htdocs/public/experts/ExpDisplay.php?ExpID=781
San Andreas Fault Observatory at Depth: http://www.earthscope.org/observatories/safod
National Science Foundation: http://www.nsf.gov/
Deutsche Forschungsgemeinschaft: http://www.dfg.de/en/index.jsp
Nancy Ross-Flanigan | EurekAlert!
Large-Mouthed Fish Was Top Predator After Mass Extinction
26.07.2017 | Universität Zürich
Strength of tectonic plates may explain shape of the Tibetan Plateau, study finds
25.07.2017 | University of Illinois at Urbana-Champaign
Physicists working with researcher Oriol Romero-Isart devised a new simple scheme to theoretically generate arbitrarily short and focused electromagnetic fields. This new tool could be used for precise sensing and in microscopy.
Microwaves, heat radiation, light and X-radiation are examples for electromagnetic waves. Many applications require to focus the electromagnetic fields to...
Strong light-matter coupling in these semiconducting tubes may hold the key to electrically pumped lasers
Light-matter quasi-particles can be generated electrically in semiconducting carbon nanotubes. Material scientists and physicists from Heidelberg University...
Fraunhofer IPA has developed a proximity sensor made from silicone and carbon nanotubes (CNT) which detects objects and determines their position. The materials and printing process used mean that the sensor is extremely flexible, economical and can be used for large surfaces. Industry and research partners can use and further develop this innovation straight away.
At first glance, the proximity sensor appears to be nothing special: a thin, elastic layer of silicone onto which black square surfaces are printed, but these...
3-D shape acquisition using water displacement as the shape sensor for the reconstruction of complex objects
A global team of computer scientists and engineers have developed an innovative technique that more completely reconstructs challenging 3D objects. An ancient...
Physicists have developed a new technique that uses electrical voltages to control the electron spin on a chip. The newly-developed method provides protection from spin decay, meaning that the contained information can be maintained and transmitted over comparatively large distances, as has been demonstrated by a team from the University of Basel’s Department of Physics and the Swiss Nanoscience Institute. The results have been published in Physical Review X.
For several years, researchers have been trying to use the spin of an electron to store and transmit information. The spin of each electron is always coupled...
26.07.2017 | Event News
21.07.2017 | Event News
19.07.2017 | Event News
27.07.2017 | Life Sciences
27.07.2017 | Life Sciences
27.07.2017 | Health and Medicine