  We advance the novel hypothesis that water in the presence of finite
localized strain within fault gouges may lead to the phase transformation of
stable minerals into metastable polymorphs of higher free energy density. Under
increasing strain, the transformed minerals eventually become unstable, as
shown from an application of Landau theory of structural phase transitions. We
propose that this instability leads to an explosive transformation, creating a
slightly supersonic shock wave propagating along the altered fault core leaving
a wake of shaking fragments. As long as the resulting high-frequency acoustic
waves remain of sufficient amplitude to lead to a fluidization of the fault
core, the fault is unlocked and free to slip under the effect of the tectonic
stress, thus releasing the elastic part of the stored energy. We briefly
discuss observations that could be understood within this theory.
