DUCTILE CREEP, COMPACTION, AND RATE AND STATE-DEPENDENT FRICTION WITHIN MAJOR FAULT ZONES

The shear traction on major strike-slip faults during earthquakes is m uch lower than that expected on a frictionally sliding surface in equi librium with hydrostatic pressure. The low shear traction is explained if the fluid pressure at the time of the earthquake is much greater t han hydrostatic pressure. Ductile creep within mostly sealed fault zon es compacts the matrix and thus increases fluid pressure between earth quakes. Frictional dilatancy during earthquakes decreases fluid pressu re below hydrostatic, and over the earthquake cycle, the fault zone is in long-term equilibrium with the country rock. This ductile mechanis m is formally unified with rate and state theory for time-dependent fr iction when the difference between a critical porosity where the rock loses all strength and the actual porosity of cracks is used as a stat e variable. This choice is justified by percolation theory of mostly b roken lattices. Time-dependent behavior associated with changes in nor mal traction in the laboratory is explained by the formalism. Instabil ity (earthquakes) sometimes occurs in the numerical experiments. Howev er, fairly small amounts of frictional dilatancy during initial fricti onal creep decrease fluid pressure and preclude unstable sliding. Two coupled mechanisms for producing dilatancy on faults once an instabili ty is well underway are evident. (1) Expansion of pore fluids associat ed with frictional heating increases fluid pressure offsetting the eff ects of increased pore volume during earthquakes. There is some tenden cy for pore volume increase to balance fluid expansion so that fluid p ressure stays relatively constant. (2) Production of isolated voids th at do not immediately decrease fluid pressure throughout the fault zon e during earthquakes can occur to the extent that the fault zone is no t significantly strengthened. Although the extent of both processes is constrained by energy considerations, the variation of fluid pressure during earthquakes is not yet well enough understood to predict stres s drop from observable material properties.