Spontaneous Earthquake generation Experiments by Controlling Shear Stress in a Rotating Shear Apparatus

Earthquake faults are stationary in a critical state where they either slip or not.

The fluid pressure fluctuation in the fault zone at this critical state is thought to be one of the factors that cause a variety of earthquakes that progress from slow to fast. This process of earthquake generation has been studied using the conventional "slip displacement - rate control" method, however, this does not reproduce the process of initiation and acceleration of slip as natural earthquake generation. In this study, we conducted experiments to induce spontaneous fault slip using a rotating friction apparatus that can control torque and fluid pressure rather than slip rate.

First, as a simple experiment, a sample (standard SiO₂) whose frictional behavior was measured at low and constant velocity was used to continuously and gradually increase torque (0.3 N-m/s) under normal stress of 5 MPa. Slip started when the torque reached 200 N-m (corresponding to a shear stress of 4 MPa and a coefficient of friction of 0.8) and gradually accelerated to 170 microns/sec, at an acceleration of 20 micron/s/s. We also conducted an experiment in which the fluid pressure was increased in steps after the axial pressure and torque were applied at fixed values in advance, and similar acceleration behavior was obtained here as well. For both experiments, the termination velocity was 170 microns/sec and did not accelerate to the cm/sec order, which was set as the limit. These could have reproduced the onset of the slow earthquake, but it is also possible that the result may have been due to experimental problems that the torque was not controlled and decreased with the onset of the slip. The presentation will include this and other experimental results, as well as the interpretation of the torque-controlled experimental results and initiation of slow earthquake.