HR: 1340h
AN: T23B-0547    [Abstracts]
TI: Time-dependent frictional restrengthening: Implications for fractured silicate rocks in geothermal systems
AU: * Karner, S L
EM: Stephen.Karner@inl.gov
AF: Idaho National Laboratory, PO Box 1625, Mailstop 2107, Idaho Falls, ID 83415-2107 United States
AB: Geothermal systems involve mechanical deformation of fluid-saturated rocks at elevated thermal and stress conditions. The combination of temperature, time, and fluid-rock interactions can alter strength properties of fractures in geothermal systems. I report on data compiled from laboratory experiments on simulated faults. Repetitive stick-slip events observed during room-dry, room-temperature shear of granite blocks show that stress drop amplitude increases with time between instabilities or as the imposed loading rate slows. When stress drop is taken as a measure of strength then these data indicate natural shear zones strengthen as seismic recurrence time increases or as the stressing rate is decreased. These observations agree with those of stress relaxation (or slide-hold-slide) experiments often used to study frictional strength of simulated faults and fractures. For these tests, steady-state deformation of shear zones (with/without fault gouge) is interrupted by timed intervals (holds) initiated by setting applied loading rate to zero. During holds, stress decays due to continued creep of the shear zone. Upon reloading after holds, shear stress increases to a peak level and subsequently resumes a steady-state sliding level. The difference between pre-hold stress and the peak shear stress on reloading is a measure of static strength for the shear zone. Room-temperature and room-humidity stress relaxation tests on simulated fault gouge (quartz, granitic) show that strength increases with hold time (hence, positive restrengthening rates), in agreement with stick-slip data. Stress relaxation tests at hydrothermal conditions show systematic variations in restrengthening rates as a function of temperature. Positive rates are observed for temperatures less than ~450C that systematically transition to negative rates at greater temperatures. These data can be interpreted in terms of the interplay between stress, enhanced reaction rates at temperature, and processes that change the character of frictional contacts (e.g. aging, solution-transfer, cementation, and mineral alteration). As these processes alter the mechanical properties of fractures and faults at laboratory timescales, they should significantly influence the temporal evolution of fracture/fault properties within the lifespan of exploitable geothermal systems.
DE: 5104 Fracture and flow
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8010 Fractures and faults
SC: Tectonophysics [T]
MN: Fall Meeting 2005