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

Abstract

Velocity step tests at a range of slip rates (0.0154–155.54 μm s−1) are performed using natural fault gouge containing smectite, mica, and quartz collected from an outcrop of the Hanaore Fault, southwest Japan. Field and microscopic observations reveal that the 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 of the shear zone due to a single event is difficult to determine. The experimental data on the abrupt jumps in the load point velocity are fitted by a rate‐ and state‐dependent frictional law, coupled with the spring‐slider model, the stiffness of which is treated as a fitting parameter. This treatment is shown to be essential to determine the constitutive parameters and their errors. The velocity steps are successfully fit with typically two state variables: larger b1 with shorter dc1 and smaller b2 with longer dc2. At slip rates higher than 1 μm s−1, negative b2 is required to fit the data in most of the cases. Thin gouge layers (∼200 μm) in the experiment enables us to simulate large averaged shear strain which is important to recognize the evolution of the state variable associated with negative b2 and long dc2. Observation of microscopic structure after experiments shows poor development of Y planes. This may be consistent with the mechanical behavior observed: weak occurrence of initial peak strength at yielding and displacement hardening throughout the experiments.