Evolution of Fault Friction Following Large Velocity Jumps
Abstract
Since the early 1980's, when the modern equations of rate-and-state friction were formalized, two empirical laws for the evolution of the state variable have been commonly used. Neither accurately describes the full range of fault behaviors observed in the laboratory. One law (Slip) predicts no evolution of the fault surface at zero slip velocity; the other (Aging) predicts that the state variable increases linearly with time during stationary contact. Near steady-state the two laws are asymptotically identical, but far from steady-state they diverge. In particular, the slip law exhibits symmetric stress changes with slip distance following step velocity increases and decreases, while the aging law exhibits asymmetric changes. In addition, the effective slip- weakening distance of the surface following step velocity increases is constant under the slip law, but is proportional to the logarithm of the velocity jump under the aging law. Recently, Rubin and Ampuero (2005) showed that the size and character of nucleation zones on elastic faults are very sensitive to the behavior of the fault surface in response to large, abrupt velocity increases, and are entirely different under the two laws. Because (1) most velocity-stepping experiments have been conducted using jumps of a single order of magnitude, (2) the difference between the two laws becomes much more pronounced as the jump magnitude increases, and (3) velocity jumps of many orders of magnitude are apparently relevant to earthquake nucleation, we set out to impose large velocity steps experimentally. We used a double-direct shear geometry to shear 3-mm thick layers of granular quartz gouge between rough surfaces held at 25 MPa normal stress. In one set of experiments we servo-controlled off a displacement transducer mounted directly on the sample, straddling the sliding surface. Velocity jumps of 1 and 2 orders of magnitude were symmetric for step increases and decreases, and d[stress]/d[slip] for the 2 order of magnitude increases was twice that for the single order increases. This behavior is entirely consistent with the slip law and inconsistent with the aging law. Velocity increases of 3 orders of magnitude could not be reliably stabilized in this fashion. Because the fault displacement transducer was noisier than that fixed to the loading ram, we designed a velocity history for the loading ram that was intended to produce a true velocity step on the fault surface, and servo-controlled off the loading ram transducer. As this approach requires knowledge of the fault rate-and-state parameters, in addition to the machine stiffness, a range of parameters was explored. We obtained one well-characterized 3-order-of magnitude velocity increase; this again is consistent with the slip law.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.S31A0180B
- Keywords:
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- 8034 Rheology and friction of fault zones (8163);
- 8163 Rheology and friction of fault zones (8034)