[S-SS08] Fault Rheology and Earthquake Physics

[SSS08-09] Mechanical amorphization of synthetic fault gouges during rotary-shear friction experiments

*Shunya Kaneki¹, Kiyokazu Oohashi², Tetsuro Hirono³, Hiroyuki Noda¹ (1.Disaster Prevention Research Institute, Kyoto University, 2.Graduate School of Sciences and Technology for Innovation, Yamaguchi University, 3.Graduate School of Science, Osaka University)

Keywords:Friction experiments, Fault gouge, Mechanical amorphization

Although mechanical amorphization of fault rocks and its possible effects on fault motion have been reported in both natural faults and laboratory experiments, the relationship between slip processes and generation rate of amorphous materials remains unclear. In this study, we performed rotary-shear friction experiments on quartz and kaolinite gouges under normal stresses of 1 or 3 MPa, at slip velocities of 0.001 or 1 m s^–1, with displacements of 1–100 m under room temperature and N₂ gas condition. Post-experiment XRD measurements revealed the occurrence of mechanical amorphization of both materials in some products after experiments. Microstructural observations demonstrated grain-size reduction, particle rounding, and generation of ultrafine particles (several tens to several hundreds of nanometers in diameter) within the localized slip zone with thickness of several tens to several hundreds of microns. The amount of amorphous materials generated is strongly dependent on total frictional work and mineralogy of fault gouge regardless of slip velocity, whereas the nominal dependence of amorphous content on seismic fracture energy and frictional power has no physical meaning. Estimated high amorphous content of up to 89 wt% indicates that mechanical amorphization may need to be considered for understanding dynamic mechanisms that affect strength of natural faults, such as thermo-chemical pressurization, fault lubrication by amorphous silica, and fault healing by pressure solution.