Generation of Billow-like Wavy Folds by Thermal Pressurization in a Seismic Slip Plane of Nojima Fault Gouge

Abstract:

Nojima fault gouge has recorded physical processes during seismic slip, and one can find billow-like wavy folds along slip planes in the gouge. The fold patterns are similar to the ones of Kelvin Helmholtz (KH)–instability which occurs in fluid. Therefore, the presence of such folds suggests the fluidization of gouge materials. This instability occurs at the interface between two fluids of different densities shearing at different velocities (Thorpe, 2005), and in low viscous fluid (Woods, 1969). If a temperature range for the generation of such billow-like folds could be determined, we can constrain the weakening mechanism of frictional strength of faults. Here we show rock magnetic studies to prove the temperature rise in the generation of the billow-like folds, using a scanning magneto-impedance magnetic microscope. Its results showed the folds and the slip zones have been magnetized by the production of magnetite through thermal decomposition of siderite in the gouge. The thermal decomposition of siderite generally occurs at over about 350℃. This heating implies that thermal pressurization or melting were the driving mechanism of faulting. In order to clarify which mechanism mainly drove in the fault gouge, we constrained the temperature and viscosity condition of each melting model and thermal pressurization model from the perspective of the existence of the KH-instability. The melting model generates no such instability because of high viscosity (10 Pa·s), even if we postulate relatively high temperature melting (1300℃). On the other hands, we found that the thermal pressurization model can forced the fault slip zone very low viscosity (1.0×10^-3 Pa·s) enough to generate KH-instability, which requires a shear stress of at most 0.25 Pa during faulting. It is supposed that the existence of the low viscosity fluid and the frictional heating decreased the frictional strength of the Nojima fault at an ancient large seismic activity, accelerating the fault motion.