The time-dependent volumetric constitutive relation for a San Andreas fault gouge and a consolidated kaolinite are experimentally determined at confining pressures to 200 Mpa, under creep condition and at ultrasonic frequency. At any given pressure, the bulk modulus determined at 1Mhz frequency is identical to the "elastic" modulus determined at a stepwise change of pressure. After the pressure change, there occur a time-dependent change of volume and thus the effective bulk modulus. The constitutive relations may be modeled by a standard linear viscoelastic medium with pressure-dependent moduli and viscosity. For the San Andreas fault gouge, viscosity increases from 2x10¹³ poise, at 17 Mpa, and 10¹4 poise, at 100 Mpa. For kaolinite sample, the corresponding values are 3x10¹³ poise and 10¹³ poise. These results imply that both stress and pore pressure in fault zones may show time-dependent variations in response to a change of state of stress, such as that occurring after an earthquake.
Studies of the mechanical properties of fault gouge may be important for gaining an understanding of the mechanics of faulting. Factors which influence the fault gouge behavior include the confining pressure (e.g., Summers and Byerlee, 1977; Wang and Mao, 1979; Wang et al., 1980; Shimamoto and Logan, 1981; Morrow et al., 1982), strain rate (Logan, 1978; Solberg and Byerlee, 1981), hydration and dehydration (Logan et al., 1981), water saturation (Wang and Mao, 1979; Morrow et al., 1982), grain size and tomposition (Chu et al., 1981; Morrow et al., 1982).In the present study the time-dependent volumetric constitutive relations for a San Andreas fault gouge and an air-dired, consolidated kaolinite are experimentally determined. The volumetric strain of fault gouge is important not only because it is a fundamental variable in the constitutive relation but also because it affects pore pressure in the media. The time-dependent volumetric constitutive relation of fault gouge, therefore, bears on the question of redistribution of pore pressure and effective stress in a fault zone after an earthquake and other types of change in the in situ state of stress.
Core samples of fault gouge were obtained through a USGS deep drilling program, from depths down to 400 m in the San Andreas fault zone in Dry Lake Valley near Hollister, California. The sample used for this study is #725 obtained from a depth of 725 feet. It was in the shape of a cylinder of 2.4 cm in diameter and about 4 cm long, with the ends of the cylinder cut into flat surfaces perpendicular to the axis of the cylinder, but otherwise left in the cored state with as little mechanical disturbance as possible. The sample was consolidated at a confining pressure of 200 Mpa for a period of 7 days, and was left in air for one and a half years before experimentation. Modal analysis of the sample using standard point counting technique showed that it contains 12.5% quartz, 5.1% feldspar, 6.4% calcite and 76% clay on a volume basis.