An Empirical Wear Law for Rocks during High-velocity Fault Motion

An empirical wear law of rocks is examined by using a rotary-shear high-speed frictional testing apparatus, in order to reveal the gouge-generation processes during coseismic sliding of faults. Hollow cylindrical specimens of gabbro, granite and sandstone with inner and outer diameters of 15 and 25 mm, respectively, were slid at velocities of 0.02 to 0.3 m/s and normal stresses of 0.3 to 4.2 MPa under unconfined and dry conditions. Powdered rock (gouge) was continuously produced by abrasive wear of initially bare fault surfaces during sliding. Because the fault was not confined in our experiments, the gouge was extruded from the sliding surfaces, resulting in shortening of axial length of specimen. In this study the wear rate was defined that an axial shortening rate of the specimen was divided by velocity, although it is commonly described as the ratio of thickness of gouge zone to total slip. The experimental results on gabbro and granite indicate that the wear rate increases drastically with velocity and normal stress. The relationship between the wear rate and normal stress can be fit well with an exponential equation. The increase in wear rate must be caused by a subtle thermal cracking owing to the frictional heating on the sliding fault surfaces. When the velocity is of more than 0.08 m/s, the wear rate of sandstone decreases with increasing in normal stress. It might be associated with the formation of a consolidated or sintered layer on sliding surface. The wear rate of gabbro and granite is an order of 10-5 at normal stress of 2 MPa and velocity of 0.2 m/s. If this wear rate is extrapolated to high normal-stress conditions in nature (i.e., 50 MPa), the estimated value is higher by several orders of magnitude than the wear rates of 10-3 to 100 reported from natural fault (e.g., Scholz, 1987). This large gap between laboratory and nature might be associated with the extrusion of gouge from sliding surface in the experiment. We will try to prevent the leak of gouge from sliding surfaces and modify the wear law of rocks during coseismic sliding.