Project Abstract
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We continue to focus our attention on the widely discussed dynamic weakening mechanism termed thermal pore-fluid pressurization. There are many theoretical and numerical studies of thermal pressurization and it is increasingly used in dynamic rupture and earthquake nucleation models. However, experimental data suggesting the operation of this mechanism is limited, a shortcoming we are working to resolve. The only machine in existence capable of doing the required experiments is our unique high-pressure rotary shear friction machine that combines arbitrarily large slip displacement with independent control of both confining pressure and flow-through pore pressure capability. Ours is the first study in which the thermal pressurization mechanism has been isolated and is beginning to be characterized under controlled conditions on confined samples. This year we have focused some of our attention on comparing dry and water-saturated diabase in order to remove any other velocity dependent effects on friction that might be included in the results on the fluid saturated rocks. We have also begun to measure both the permeability and the storage capacity of the samples using the pressure oscillation technique in order to know the fluid diffusivity of the samples. We have often found that following the expected decline in shear resistance that it unexpectedly increases somewhat. Although we do not understand this behavior, we are investigating the possibility that it could result from dependence of some of the critical parameters on the spatially and temporally varying effective stress, something not accounted for in current models.
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