This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Thursday, October 20, 2011

"Flash heating" as a mechanism for fault weakening during earthquakes

The San Andreas fault
USGS photo
One problem that has mystified seismologists and geophysicists for a long time is the lack of high heat flow over active fault zones. When rocks are stressed in laboratory experiments, data indicate that faults should be very strong, and theories suggest that when they fail during an earthquake, frictional heating should create high temperatures. But, that is not observed on faults like the San Andreas in California.  A variety of laboratory experiments over the past decades have suggested that rocks are actually very weak under conditions of fast sliding that are occur in earthquakes.

In a paper*** in this weeks Science, Goldsby and Tullis point out that when two rock surfaces are brought together, they only touch at a few small contact points compared to the total surface area. These points have average sizes of tens of microns. Like the floor under a spiky woman's high heeled shoe, the stresses are concentrated on these points, typically having local stresses of 10 GPa for even modest average stresses. When these microscopic contacts are sheared in an earthquake, very high temperatures can result.  If the shearing rate is slow, heat can diffuse away from these points and the temperature remains low. However, if the shearing rate is high, there is no time for diffusion and the temperature at the points increases, sometimes to melting temperatures.

The results are based on laboratory experiments on a number of different rock types.  In these experiments rocks were sheared past each other at velocities up to 0.4 m/s over distances up to 45 mm. The results showed that the friction coefficient decreased dramatically when sliding velocity exceeded about 0.1 meters per second. Visual inspection of the samples after the sliding experiment showed that a thin layer of gouge (melted rock and crushed rock) had formed. The gouge layer was less than 30 microns thick.

The authors propose that flash heating is the dominant mechanism of weakening in small-slip, small-magnitude earthquakes, and that it is likely to be the dominant mechanism determining the strength of a fault in the early stages of larger earthquakes.  During continued slip during large earthquakes other fault-weakening mechanisms may combine with or dominate over flash heating, such as melt lubrication, gel formation, or pore-fluid pressurization.

***Goldsby, D.L., and Tullis, T.E., Flash heating leads to low frictional strength of crustal rocks at earthquake slip rates, Science, 334, 216-218, 2011.


marthainvienna said...

Hello, Dr. Kieffer,
Thank you for continually posting such interesting articles! I found your blog sometime around the Japan earthquake in March and find it to be so valuable that I have, for some time now, linked to you on my blogroll (www.marthasvienna.blogspot.com). Geology was one of my favorite courses in college and I appreciate the continuing education you are providing.

Best wishes,

marthainvienna said...

It's Martha again. I just saw this TED video and thought of you. It's about using weather data to create sculptural data that can be the basis of music. http://www.ted.com/talks/nathalie_miebach.html?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+TEDTalks_video+%28TEDTalks+Main+%28SD%29+-+Site%29