During earthquakes, mineral gel may reduce rock friction to zero


ARLINGTON, Va.-Researchers have discovered a mineral gel created when rocks abrade each other under earthquake-like conditions. If present in faults during a quake, the gel may reduce friction to nearly zero in some situations, resulting in larger energy releases that could cause more damage.

Terry Tullis and David Goldsby of Brown University in Providence, R.I., and Giulio Di Toro of the University of Padova in Italy announce their findings in the Jan. 29 issue of the journal Nature.

The researchers sheared quartz-rich rocks against each other under controlled conditions, simulating several aspects of a geologic fault environment. Future experiments will take advantage of a salvaged 100-horsepower BMW motorcycle engine, which will allow the apparatus to reach seismic slip speeds of one meter per second.

As the shearing progressed, resistance between the rocks approached zero at the highest shearing speeds. Scanning electron microscope images suggest that mineral powder, in this case comprised of silica, generated during the abrasion combines with water from the atmosphere to form a gel that lubricates the rock surfaces.

If confirmed with field observations, researchers could apply these findings to computer earthquake models. The simulations may help scientists and emergency personnel better predict the magnitude of strong ground motions that damage man-made structures.

All three researchers were supported by NSF awards 0003543 and 0352548, and grants from the United States Geological Survey. Di Toro was also supported by an Italian MURST grant.

What the scientists said:

Importance of this research:

"We don't know how much resistance faults experience during rapid earthquake slip. If it is much lower than resistance during periods between earthquakes then much more energy could be released and the damage caused by the earthquakes could be greater." - Terry Tullis

"Many different studies are needed, but one important approach is to do experiments at earthquake slip speeds. Under these conditions, we may be able to discover if the earthquake greatly reduces resistance to slip and what processes may cause the weakening. If these processes also occur on earthquake faults, then we can enhance earthquake simulations with mathematical models of fault weakening from experimental observations and determine the consequences for earthquake hazards." - Terry Tullis

"While the full significance of our work for earthquake rupture mechanics remains to be seen, the mechanism we discovered provides a simple, straightforward means to cause very low friction on faults. The mechanism does not require more complicated processes involving fluid pressure, loss of contact due to elastic waves, or fluidization of water-rich rock powder. The low values of friction predicted by our study would explain the lower-than-expected amount of heat generated along some major faults like the San Andreas Fault during earthquakes". - David Goldsby

Importance of lower friction during an earthquake:

"Rocks rub against each other at about one meter per second during earthquakes. If it takes only a tenth of a second to get up to speed, then the acceleration is greater than the acceleration due to gravity that holds us onto the Earth. In that case, objects such as people and cars can be thrown in the air." - Terry Tullis

"It is much more expensive to construct a building, for example a nuclear reactor, that is designed to withstand higher earthquake accelerations. Experiments and simulations to understand what we might expect are an important way to anticipate earthquake accelerations and thus permit us to build to reduce damage without expensive over-design." - Terry Tullis

The experimentation:

"One of the important processes that can occur during earthquakes is melting due to frictional heating, an extreme version of what happens when you rub your hands together to get them warm. Rocks called pseudotachylites are products of frictional melting and in some cases are found on faults. We believe faults with pseudotachylites slipped rapidly during earthquakes, since otherwise they couldn't get hot enough to melt. We need to learn more about how low fault resistance can fall during frictional melting." - Terry Tullis

"We have purchased a used 100 HP BMW motorcycle and plan to use its engine to rotate our samples faster. If we rotate at 400 RPM, we can attain a truly seismic slip speed of one meter per second, allowing us to study how weak a fault may become during an earthquake in cases where rock melts along the fault plane." - Terry Tullis

Why researchers studied quartz:

"Quartz has been studied for many years in a wide variety of ways, so much is known about its properties and thus understanding its behavior should be easier." - Terry Tullis

"Studying one mineral rather than a rock with many minerals makes it easier to understand what is happening." - Terry Tullis

"Quartz is a very common mineral in the Earth's crust; it is not only a constituent of many rocks such as granite, but it is also frequently found along fault zones. Even for faults that cut rocks containing no quartz, one frequently finds abundant quartz where it is carried in solution by hot water. Whether the gel weakening we find in our experiments is important along faults during earthquakes is still unknown, but it is certainly possible." - Terry Tullis

Additional comments:

"This work is a nice example of how fascinating scientific research can be and how a team of workers at all levels of experience can learn new things by working together." - Terry Tullis

"David Goldsby, as a post-graduate Research Associate working with me as a Professor, initially discovered the extraordinary reduction in frictional resistance for quartz rocks in experiments at higher stresses and lower speeds than in this work, but we could not figure out what caused it. While he was a graduate student, Giulio Di Toro' visited from Italy to do experiments to help understand fault-zone rocks in the Alps. His presence resulted in our expanding the scope of experiments in the atmospheric pressure tension-torsion Instron machine, which is part of an NSF-funded Materials Research Lab at Brown University. This led to the findings of our Nature paper, namely that the friction got progressively lower as the slip speed increased." - Terry Tullis

"Despite our findings regarding lowered friction, we were still mystified as to the process causing the weakening. Finally, two undergraduate students, Brian Titone and Katharine Sayre Hinkle, working one summer as NSF REU (Research Experience for Undergraduates) students, discovered the important role water plays in causing the weakening. This led to the discovery that the weakening is apparently due to the production of a lubricating layer of silica gel. None of us would have done this without the contributions of the others. We all learned a lot from each other and had a great time doing it." - Terry Tullis

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