ABSTRACT
We analyzed the strain field along the developing faults in Westerly granite samples during triaxial compression tests. Samples with experimentally developing faults were prepared by recovering samples after the peak stress level had been attained, but before the samples were bisected during the triaxial compression tests. After attaching strain gauges around the fault trace on the sample surface, the samples were again compressed in a triaxial cell. First, we conducted triaxial compression tests under a confining pressure of 100 MPa at ambient temperature, after we attached strain gauges to the sample surface parallel to the fault trace (tangential strain). Tangential strain deviated in the process zone (around fault tips). The polarity of strain deviation was in accord with the calculated static strain deviation due to slip on a fault in an elastic medium, although dynamic slip had not occurred on the fault in our sample. Deviation near the fault nucleation region preceded peak stress. Tangential strain never deviated behind the fault tips, even though strain deviation had been observed during the previous loading, when the sample was again loaded without sufficient time to adhere. Then, another series of triaxial compression tests was performed under the same conditions, except that the samples were held under a hydrostatic condition of 100 MPa for three hours prior to loading. Three components of strain gauges, parallel and perpendicular to the loading axis and parallel to the fault trace on the sample surface, were attached in the process zone (around the fault tip) in this series of tests. Tangential strain deviation was still present in the process zone after three hours of healing. Due to strain deviation, the principal strain axes rotated in the same sense as the theoretical model prior to the peak stress. The angle between the local maximum compressive axis at the fault tip and the fault strike was about 45° at the peak stress level.
