Dynamic friction: Speeds and modes of frictional ruptures

Kaiwen Xia, Brown University

Friction is one of the oldest yet still poorly understood problems, and dynamic friction has remained rarely explored [1] until recently largely due to experimental challenges. Motivated to understand the physics of friction in general and earthquake physics in particular, researchers have conducted laboratory experiments to understand the nature of friction between rock-like brittle materials.

Utilizing dynamic photoelasticity combined with high-speed photography as diagnostics for high temporal and spatial resolutions, we designed laboratory fault models for investigating dynamic friction under a highly controlled environment [2]. Rupture was triggered and propagated along the contact zone (an artificial fault between two materials) with varying contact zone geometry and initial loading conditions. Different sliding propagation (rupture) speed regimes along the contact zone between two identical materials, and the transition of the speed from sub-shear (speeds slower than the shear wave speed of the material) to supershear (speed faster than the shear wave speed of the material) were identified for the first time [3]. Very rich physical phenomena including the directionality of rupture propagation due to the broken symmetry were also observed along the contact zone between two materials with slightly different elastic properties [4]. In the case where a lower velocity material was introduced inside the contact zone between two identical materials, stable self-healing pulse-like propagation front and unstable propagation front were observed. These pulse-like ruptures are different from traditional crack-like ruptures in that the duration of the slip is much shorter than the crack-like ruptures where sliding continues till the end of the event [1].

These experimental observations can be used to construct proper dynamic friction laws. Among the common frictional laws, namely Coulomb's friction law, the slip weakening law, the slip rate weakening law, and the rate- and state-dependent friction law, the rate- and state-dependent law resulted in a prediction closest to our experiments for the frictional rupture along the contact between different materials. Our experiments can also be used directly to explain earthquake observations. For example, the rupture speed transition can be used to explain the 2001 Kunlunshan earthquake rupture speed history [3], and the directionality observed in our experiments can be used to understand rupture scenarios in both the 1999 Izmit earthquake and the 2004 Parkfield earthquake [4]. The self-healing pulse rupture mode, clearly demonstrated in our experiments, might be the only way to reconcile the low heat flux observed in San Andreas Fault [1].

Continuum-scale and molecular dynamics simulations, and improvement of current theories are certainly desirable for an ultimate understanding of the physics of dynamic friction. The well-controlled boundary and initial conditions in our experiments are advantageous for a salient comparison with the predictions of theoretical models and simulations.

References:

[1] J.R. Rice, "New perspectives in crack and fault dynamics," in Mechanics for a New Millennium (Proceedings of the 20th International Congress of Theoretical and Applied Mechanics, 27 Aug–2 Sept 2000, Chicago), eds. H. Aref and J.W. Phillips, Kluwer Academic Publishers, pp. 1–23, 2001.
[2] Xia K.W., Rosakis, A.J., and Kanamori, H. "Supershear and subRayleigh-intersonic transition observed in laboratory earthquake experiments," Experimental Techniques, to appear 2005.
[3] Xia, K.W., Rosakis, A.J. and Kanamori, H. "Laboratory Earthquakes: The Sub-Rayleigh-to-Supershear Rupture Transition," Science, 303, 859–1861, 2004.
[4] Xia, K.W., Rosakis, A.J., Kanamori, H., and Rice, J.R. "Laboratory Earthquakes along Inhomogeneous Faults: Directionality and Supershear", Science, to appear 2005.

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