EXPERIMENTAL EVIDENCE FOR FAULT LUBRICATION BY MELTING DURING SIMULATED COSEISMIC SLIP

A series of experiments have been performed on machined Westerly granite core using friction welding apparatus. The experiments involved bringing into contact the faces of steel-sleeved, 2.5 cm diameter rock cylinders at mean velocities of up to 7 m s-1. The friction welder was computer controlled and included a digital velocity encoder and a high-resolution optical pyrometer, both of which took readings every millisecond. This means that the velocity and interface temperature were measured with a high degree of precision. The pyrometer was deployed in two modes: (1) aimed obliquely at the periphery of one of the rotating faces via a wedge cut in the stationary sample, and (2) via a porthole machined into the steel sleeve where the contact interface was shrouded by an overlapping sleeve. The first mode of pyrometry measured temperatures directly from the interface, the second deployment measured temperatures at right angles to the interface. Experiment durations were typically between 2-5 seconds. The experiments were designed to assess the rate of deceleration of the drive motor to stalling under a contact force of 0.5 kN (equivalent to 0.5 MPa).

Results indicate that there is a direct correlation between interface velocity and temperature. Once a certain temperature is realized, the rate of deceleration decreases and stalling is delayed. This initiating temperature was found to be ~750 C, with maximum negative deceleration effects being realized at ~1400 C. These temperatures equate with the breakdown and melting of micas at ~750 C, following by the breakdown and melting of orthoclase and plagioclase at temperatures between 1100-1400 C. Quartz was not melted to any significant degree and remained as clasts in the friction melt. The breakdown temperatures also correlate with mineral shear yield strengths and fracture toughnesses. The comminution process is dependent on the strength of the minerals, with greater energy being expended on the mechanically weaker phases, such that their surface areas increase dramatically and they melt first.

This work proves a direct relationship between interface temperature and slip velocity and provides evidence of interface melting and lubrication during high-speed slip.