Figure 1

Schematic illustration of the sample assembly and experimental setup. Schematic illustration of the sample assembly and experimental setup during the unstable slip tests on simulated fault zones. For each experimental run, two cylindrical gabbro samples were separated by a simulated fault gouge layer inclined 45° to the cylindrical axis. Four strain gauges (purple, red, green, and orange) were mounted on the lower gabbro to directly monitor local strains parallel and adjacent to the fault, while one strain gauge (white) was mounted on the upper gabbro to measure the mean axial strain of the sample. The large arrows indicate the sense of shear.