I've always been skeptical about earthquake predictions, but this new Israeli study, which focuses on friction movement, says it could improve these predictions. The researchers looked at the waves (or fronts) of detachment between two surfaces. And they found that even if the two traditional fronts, which propagate at sonic and supersonic velocities, are present at the time of rupture, a recently discovered much slower wave is the dominant force leading to the rupture. These slow waves are not felt before or during an earthquake, but can be measured and used to prevent future ones. However, this implies that their method of microscale measurements in the lab can successfully be adapted at the macroscale of earth subsurface. So even if this study is interesting, I doubt it will be used for accurate earthquake prediction before a long time. Read more...
Here is the introduction of this news release of the Hebrew University of Jerusalem.
A new study on "waves (or fronts) of detachment" involved in the process of friction offers a new perspective on an old scientific puzzle and could provide a key to improving predictions of future earthquakes, say scientists [of the Racah Institute of Physics] at the Hebrew University of Jerusalem.
Using near-field optics and recent technological advances in rapid imaging, the Hebrew University researchers have observed for the first time how three different types of waves govern the onset of friction. These waves, which function within the micron-thick interface between sliding surfaces, move at widely different velocities, from sonic and supersonic and down to slow speeds. The researchers showed that detachment -- the actual separation of the points of microcontact between one surface and another that occurs during frictional movement -- is governed mainly by the newly discovered, slow-wave phase.
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"The top illustration shows two surfaces, greatly enlarged, with the microcontacts connecting them. In the middle illustration, the surfaces are starting to move against each other, with the microcontacts being broken. In the bottom drawing, sliding takes place as a slow-motion wave (white area) moves between the surfaces." (Credit: The Racah Institute of Physics). |
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And this is a schematic diagram of the experimental apparatus. On the top part, a normal force, F N, is applied to the base and slider blocks of plexiglas. A shear force, F S, applied at the trailing edge of the slider, is increased until motion ensues in the x direction. In the bottom part, a laser sheet, incident at an angle beyond the critical angle for total internal reflection, is solely transmitted at the net contact points along the rough interface (inset) between the base and slider. Thus, the light intensity transmitted across the interface at any location is proportional to the net contact area at that location. The transmitted light is imaged by a fast camera (Credit: The Racah Institute of Physics). |
But how this discovery can be applied to earthquake prediction?
These findings, says Prof. Fineberg, have relevance for the issue of earthquake measurement and predictions, as well as for other future scientific and industrial applications. (Over 5 percent of losses due to both wear and energy dissipation in industry are due to friction, resulting in the loss of hundreds of billions of dollars each year worldwide, says Prof. Fineberg.)
An earthquake is felt (and is measured seismically) as a sudden, rapid movement, or sliding, of tectonic plates in a frictional action, says Fineberg. However -- based on the Hebrew University researchers’ findings -- it would seem that it is actually the slow, "unfelt" or "silent" waves in the earth’s crust to which we should be paying closer attention and that are apparently the precursors of the frictional movement that we call earthquakes, says Fineberg.
He might be right, but how these experiments at the microscale level can be adapted to earth's movement? He doesn't say.
The research work has been published in a letter to the journal Nature on August 26, 2004 under the title "Detachment fronts and the onset of dynamic friction." Here are two links to the abstract and to the full paper (PDF format, 5 pages, 417 KB). The diagram of the experimental apparatus above and its legend come from this document.
A final word: thanks to reader Philip Wasson who sent me a pointer about this story.
Sources: Hebrew University of Jerusalem news release, September 26, 2004; and various other websites
2:35:36 PM
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