  Rate- and state-dependent friction law for velocity-step tests is analyzed
from a thermodynamic point of view. A simple macroscopic non-equilibrium
thermodynamic model with a single internal variable reproduces instantaneous
jump and relaxation. Velocity weakening appears as a consequence of a
plasticity related nonlinear coefficient. Permanent part of displacement
corresponds to plastic strain, and relaxation effects are analogous to creep in
thermodynamic rheology.
