Constitutive properties of clayey fault gouge from the Hanaore fault zone, southwest Japan

摘要

Velocity step tests at a range of slip rates (0.0154-155.54 pm s-1) are performedusing natural fault gouge containing smectite, mica, and quartz collected from an outcropof the Hanaore Fault, southwest Japan. Field and microscopic observations reveal thatthe shear deformation is localized to a few centimeters or thinner layer of black clayey,fault gouge. This layer is formed by multiple stages, and determining the width ofthe shear zone due to a single event is difficult to determine. The experimental data on theabrupt jumps in the load point velocity are fitted by a rate- and state-dependent frictionallaw, coupled with the spring-slider model, the stiffness of which is treated as a fittingparameter. This treatment is shown to be essential to determine the constitutive parametersand their errors. The velocity steps are successfully fit with typically two state variables:larger b1 with shorter del and smaller b2 with longer dc2. At slip rates higher than1 pm s-1, negative b2 is required to fit the data in most of the cases. Thin gouge layers(-200 pm) in the experiment enables us to simulate large averaged shear strain which isimportant to recognize the evolution of the state variable associated with negative b2and long dc2. Observation of microscopic structure after experiments shows poordevelopment of Y planes. This may be consistent with the mechanical behavior observed:weak occurrence of initial peak strength at yielding and displacement hardeningthroughout the experiments.