FRACTAL DIMENSION OF MELTING SURFACE AS A POSSIBLE LINK BETWEEN HIGH-VELOCITY FRICTIONAL BEHAVIOUR OF FAULTS AND NATURAL PSEUDOTACHYLYTES

Frictional heating has drastic effects on mechanical properties of faults and is very significant in understanding earthquake initiation processes. Our high-velocity friction experiments on gabbro have consistently revealed two stages of slip weakening (a potential source of fault instability); one following the initial slip, and the other immediately after the second peak friction.  The first weakening is probably associated with flash heating of asperity junctions, and the second weakening is due to the formation and growth of a molten layer along simulated faults.  The occurrence of pseudotachylytes along natural faults suggests that the second weakening may cause at least some large earthquakes.  This paper shows that the second weakening processes can be traced by the fractal dimension of molten surfaces.  This opens up a way to compare experimental molten surfaces with natural pseudotachylytes quantitatively and to infer what stages of mechanical behaviour the pseudotachylytes have experienced.

Experiments were done with our rotary shear apparatus at a slip rate of 0.85 m/s, a normal stress of 1.25 MPa and displacement to 76.5 m.  A thin continuous molten layer forms near the second peak friction.  At this stage, the fault surfaces are very straight and flat with a fractal dimension of 1.0 (determined over fault lengths of 5 to 500 mm using the divider method).  Faults are separated by a molten layer after this stage, and the source of heat is viscous shear heating in the molten layer itself.  The growth of the molten layer primarily causes the weakening of a fault, during which melting occurs heterogeneously along fault surfaces forming irregular embayed surfaces.  Fractal dimension thus changes systematically from 1.0 to 1.1 towards the steady-state friction.  A pseudotachylyte from the Outer Hebrides in Scotland possibly from the slip surface has fractal dimension of 1.1 and that from the injection vein has 1.0.  Thus quantitative comparisons between natural and experimental melting surfaces may be possible through surface fractal dimensions.