A study for the prediction of precursory slips based on the contact mechanics of asperities

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This study consists of the following three projects.1. Indentation experiment. We performed a series of indentation scratch tests mainly on metal samples, such as aluminum, copper, and brass. We found that the indenter penetrated deeper into the sample with the onset of horizontal movement of the indenter, and that it floated up with the progress of the horizontal movement. A simulation was performed with a model, which well agreed with the observation. We focused on the fact that the initial friction force was always slightly higher than the friction at steady state. This may correlate the fact that static friction is larger than the dynamic friction.2. An experimental trial to detect precursory slips by transmission waves across a fault. Using a biaxial sandwich type apparatus at ERI, the University of Tokyo, we performed a series of stick-slip experiments in which P- and S-waves were transmitted across the fault. We found that the transmitted waves are sensitive to the contact states of the fault. With the increase in shear stress, the amplitude of the transmitted waves significantly increased. However, the increasing rate was slightly decreased with the onset of the precursory slips. This is interpreted as the effects of the replacement of asperity contacts.3. An experiment to investigate the effects of fault geometry (asperity height, asperity size, spacing etc.) on the transmission waves. Elastic waves were transmitted across an artificial fault. Parallel grooves with various widths and depths were cut on the fault surfaces. We found that the transmissivity of the waves is solely determined by the ratio of the groove depth/width to wavelength. We have also made a mathematical model based on the stiffness of the fault. The model well explains the observation when the wavelength is 4 times longer than the size of contacting asperities.