SHEAR HEATING AND PARTIAL MELTING OF WET FAULT GOUGE DURING EARTHQUAKE SLIP

We consider water infiltrated faults like in the shallow and middle crust and assume that earthquake slip is accommodated by a narrow fault core.. For very low permeability k, pore pressure P will rise by shear heating towards the fault-normal stress Sn so that frictional resistance drops. That reduces strength and the rate of continuing temperature rise so that melting is precluded for small enough slip.

However, the fragmented zones bordering an active fault core will have just experienced the high stress fluctuations associated with passage of the rupture front and will be freshly cracked with renewed, much higher k. Because of the resulting incomplete confinement of pore fluid, and because of continued shear heating from the residual viscous shear strength (even when P ~ Sn) of a thin fluidized core, onset of melting is expected to occur with large enough displacements. That is so at least if the friction coefficient relating strength to Sn - P does not drop significantly due to other mechanisms.

We model that heating process, and the resulting fault shear resistance, using properties inferred from fault zone studies. Slip duration is very short (average seismic slip rates of order 1 m/s), so melting is modeled as a non-equilibrium, taking place over several hundred K, as supported by T estimates from pseudotachylyte field exposures. Results show that even if drainage is effective so that P is much less than Sn at onset of melting, P within the water/melt pore fluid will generally rise to P ~ Sn over a very small amount of further shear. That is because the fluid phase is too viscous to permeate out of the core, and the fluid volume fraction increases in continued melting so that still solid grains no longer maintain effective contacts. Further shear heating then occurs due to the viscous resistance, not yet well characterized, of that fluidized (P ~ Sn) core. Results are compatible with seismic esimates of slip-weakening and its scaling with total slip.