Abstract
A series of laboratory experiments was performed on the shear strength of simulated gouge layers. The gouge layers were composed of quartz sand or calcite sand (particle size 0.125-0.250 mm) with 2 or 3 mm thickness. A stick-slip was occurred at the end of each experiment without exception because the upper block of the fault was pushed with a low stiffness spring. At the final stage of the experiments, a precursory slip and a dilatancy were observed prior to the stick-slip. This result and an experimental result using a photo-elastic material strongly suggest that stress chains (columnar structures with strong stresses) are created inside the gouge layer. They are gradually inclined to sustain the external additional shear force and finally rotated, lifting up the upper block. This seems to be the mechanism of the dilatancy in our experiments. The shear strength of the gouge layers is stronger for thinner layers than thicker layers and for calcite sand than quartz sand. These facts also suggest that the strength of stress chains determines the shear strength of the gouge layers. It will be a future problem whether this mechanism is applicable to the actual faults partly because of the fractal size distribution of fault gouge.
