Melt Patches Formation during Dynamic Fault Motion; its Braking Effect and Implications for Modeling Incipient Frictional Melting

Following the first slip-weakening associated with flash heating at asperity contacts [Rice, 1999], frictional melting occurs in three stages; (1) incipient frictional melting, accompanied by marked strengthening of a fault, where melt patches form and develop into a continuous molten layer, (2) growth of molten layer resulting in the second slip-weakening, and (3) steady state with a nearly constant shear resistance where melt production is nearly balanced with melt loss. Growth of molten layer (stage 2) can be modeled as a Stefan problem with viscous shearing of molten layer as a heat source and with melting surfaces as moving boundaries [Hirose & Shimamoto, 2002]. Satomi & Shirono [2003, 2004] and Matsuzawa & Takeo [2004] solved frictional melting problem as a 1D Stefan problem and obtained results in reasonable agreement with laboratory data. In particular, the latter authors analyzed rupture propagation along a fault incorporating frictional melting. The incipient frictional melting and melt loss have to be included for complete modeling in the future. Modeling the incipient frictional melting is difficult because formation of melt patches is a 3D problem, so that the above authors started modeling from a continuous molten layer. This paper focuses on how melt patches grow to form a molten layer during the incipient frictional melting, based on frictional melting experiments on gabbro. This process causes marked strengthening of a fault and is significant to evaluate a breaking effect of the incipient frictional melting on fault motion. Hollow-cylindrical specimens of gabbro were sheared dry to 3.6-76.5 m in displacements at a slip rate of 0.85 m/s and at a normal stress of 1.3 MPa using a rotary-shear high-velocity friction apparatus in Kyoto. Frictional coefficient μ is ∼0.9 at the first peak-friction and it drops to transient steady-state friction (μ of ∼0.35) after the first weakening with the slip-weakening distance of ∼0.5 m. Melt patches of 7-10 μ m in thickness and of 110-160 μ m in width form sporadically on fault surfaces just after the first slip-weakening. Melt patches first increase in number without much change in their geometry, and then begin to coalesce, thickens to 16-20 μ m and widens to 2-4 mm. This process eventually leads to the formation of a continuous molten layer of about 20 to 25 μ m in thickness near the second peak-friction. Fault area occupied by melt patches increases from about 10 % at the onset of melt patches formation to 70-80 % close to the second peak-friction. Viscous shear resistance of melt patches no doubt causes marked strengthening of a fault, but measured shear resistance alone cannot separate contributions from solid friction and from melt patches. Infinitely-thin molten layer poses infinite shear resistance for a given slip rate. Nature avoids such difficulty by forming melt patches of finite thickness. What determines the geometry of melt patches and how they deform under extreme shear would be a clue to model the incipient frictional melting.