Fault friction

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Fault friction describes the relation of friction to fault mechanics. Rock failure and associated earthquakes are very much a fractal operation (see Characteristic earthquakes). The process remains scale-invariant down to the smallest crystal. Thus, the behaviour of massive earthquakes is dependent on the properties of single molecular irregularities or asperities.[1]

Two crystal asperities approaching
Two crystal asperities approaching

If we take two clean nano-asperities and bring them together in a vacuum, we will get a cold weld. That is, the crystal tips will fuse as if one (cohesion). In nature these tips are actually covered with a thin film of foreign material. By far, the most important component of this film, is water.

Crystal asperities with a thin film
Crystal asperities with a thin film

If this water is removed, by extreme drying, the rock minerals do not behave at all as expected[2]: they exhibit no fault healing or dynamic friction. The entire behaviour of earthquakes (as we know it) depends on very thin films.

After a major earthquake, there starts a process known as fault healing [3]. This is a well-demonstrated phenomenon involving a slow increase in the static coefficient of friction. With our nano-model, it is a matter of slowly pushing away the junk for a good cohesive bond. With typical minerals and water, there is another mechanism, whereby the water causes stress corrosion and weakening of the main asperity body (smoothing the irregularities), allowing more plastic deformation, and more contact.

The most important aspect is that this bond-strengthening is time-dependent. For a fault being stressed to the point of an earthquake, these bonds begin to stretch and break. They don’t have time to heal again. Once the critical distance has been achieved , there is a significant strength loss, and the fault begins to slide.

Earthquakes only exist because there is a very large loss in frictional strength. It could be that the earthquake "skids" are greased by silica gel[4], the water acts as a standard bearing lubricant, or that there is a "lift and separate" mechanism at work.

[edit] References

  1. ^ Visual Glossary - asperity. USGS.gov. Retrieved on 2008-05-10. “An asperity is an area on a fault that is stuck. The earthquake rupture usually begins at an asperity.”
  2. ^ Kevin M. Frye; Chris Marone (2002-11-20). Effect of humidity on granular friction at room temperature (PDF). Journal Of Geophysical Research. Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge. DOI:10.1029/2001JB000654. Retrieved on 2008-05-10.
  3. ^ Chris Marone (1997-05-29). The effect of loading rate on static friction and the rate of fault healing during the earthquake cycle (PDF). Nature. Macmillan Magazines Ltd.. Retrieved on 2008-05-10.
  4. ^ Chris Marone (2004-01-29). Faults greased at high speed (PDF). Nature. Retrieved on 2008-05-10.