Multi-Scale Flash Heating and Frictional Weakening at Seismic Slip-Rates in Rock

Frederick M. Chester, Omid Saber, & J. L. Alvarado

Published August 15, 2016, SCEC Contribution #7041, 2016 SCEC Annual Meeting Poster #031

Sliding rock-samples at rates > 1 mm.s^-1 can lead to a significant reduction of friction, often referred to as dynamic weakening. Thermal effects are understood to be the cause of dynamic weakening, and the Flash Weakening (FW) mechanism that can be activated at the initial stages of seismic slip by Flash Heating (FH) at micrometer-scale contacts. FW can play a key role in transient friction weakening leading to nucleation and propagation of earthquake ruptures. We document FW in granite (Westerly) by conducting sliding experiments at 1-1000 mm/s, normal stress of 1-25 MPa, and displacement up to 40 mm using a double-direct shear configuration with a 75 cm^2 sliding-surface area. We document the characteristics of FH by direct measurement of the flash-temperature distribution on sliding surfaces using a high-speed Infra-Red (IR) camera. Images with a resolution of 75 µm are captured at 300 frame/sec (every ~2.5 mm slip during high speed sliding) to document FH during sliding and the subsequent cooling upon cessation of frictional slip. Stepping velocity from quasi-static slip-rates to sustained high velocities with high accelerations (up to 100g) over short slip displacements (as low as 1 mm) allows us to observe the initial stages of evolution of the transient friction. The IR images of the sliding surface document a non-uniform distribution of temperature generated by FH at the millimetric scale, reflecting localization of normal and shear stresses to small portions of the contact surface. Temperatures up to 550 C are observed on mm-sized contact areas after ~10 mm of slip at ~800 mm/s sliding-rate and moderate normal stress, while the majority of the surface area displays significantly lower temperature. The observations of FH are used to constrain models of flash weakening that include the effects of macroscopic normal stress, slip history and presence of heated contacts at multiple scales (micrometer and millimeter). The models can explain the evolution of transient friction with slip in the acceleration and deceleration phases (e.g. the enhancement of FH with slip, and hysteresis loops in the friction-velocity curve during acceleration and deceleration) that is essential for modeling rupture propagation and arrest.

Key Words
Friction, Flash heating, Dynamic weakening, Experiments,

Citation
Chester, F. M., Saber, O., & Alvarado, J. (2016, 08). Multi-Scale Flash Heating and Frictional Weakening at Seismic Slip-Rates in Rock. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)