Understanding the frictional behavior of fault gouge materials has important implications for interpretation of mechanisms involved in fault movements that result in earthquakes. Fault behavior and stability are significantly influenced by properties of the gouge material, especially in old faults. In this study, the main objective is to investigate the relation between changes in ultrasonic wave properties and micro-scale processes that occur during shear failure of gouge materials. We present results from novel ultrasonic wave propagation measurements in which the gouge materials are shearing under constant different values of normal stress in a single direct shear apparatus. Ultrasonic measurements are conducted by a fast data acquisition system and four pairs of shear wave transduces are embedded at the sides of the specimen. The experimental results provide measures of variations in rate and state friction law parameters with changes in the sliding velocity and normal stress as well as changes in ultrasonic wave properties such as the transmitted amplitude and wave velocity. The variations in ultrasonic waves were monitored and compared during both velocity weakening and velocity strengthening phases. Our results indicate that changes in transmitted amplitude are related to changes in contact area between the particles, while on the other hand, the velocity variations are more related to changes in contact stiffness between the particles. Moreover, the transmitted amplitude and wave velocity variations were observed to be negligible during steady state sliding. With this, during stick slip cycles, the trends were clearly observed to follow the stick-slip behavior.

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