• Rempe, M.
• Smith, S. A.
• Di Toro, G.
• Mitchell, Thomas

Abstract

Laboratory studies designed to understand fault and earthquake processes often use rotary-shear apparatus to investigate the frictional properties of rocks. Rotary-shear apparatus can apply a wide range of slip velocities and impose high displacements on cohesive and non-cohesive rocks and are thus ideal to study deformation processes taking place throughout the seismic cycle. However, in experiments performed on gouges, the mechanical results and the developing microstructures are often hard to interpret, because little is known about the distribution of strain (i.e., the ratio of displacement to the thickness of the deforming zones) within the gouge layer. A better understanding of the strain distribution may help to interpret microstructures associated with varying degrees of strain localization and infer the underlying physical-chemical processes. We report preliminary results from a series of experiments carried out with strain markers using the rotary-shear apparatus SHIVA at INGV in Rome. Within an annular steel sample holder (with outer and inner radii of 55 and 35 mm, respectively), a 3 mm thick calcite gouge layer (starting grain size <250 μm) was prepared with a 2 mm wide dolomite gouge strain marker cutting vertically through the calcite layer. The marker was orientated perpendicular to the imposed shear direction. Dolomite was selected for the strain marker because it has similar mechanical properties to calcite. The strain marker experiments were conducted at varying slip rates (10-3 m/s and 1 m/s), displacements (8 cm to 1.5 m), normal stresses (3 to 20 MPa) and ambient conditions (room-humid and wet) to test the dependence of the strain distribution. Mechanical data (shear stress, shortening and dilation, etc.) were recorded during the experiments. Afterwards, the preserved samples were cut vertically through the dolomite strain markers (approximately parallel to the slip direction) and microstructural analysis was conducted with the scanning electron microscope. Preliminary results show that at a slip velocity of 0.1 m/s and normal stress of 3 MPa, localization to a relatively high-strain slipping zone occurs rapidly. Progressive strain localization at the onset of shearing is associated with gouge layer dilation and strain hardening. The onset of dynamic weakening (i.e. at the end of the strain hardening phase) broadly correlates with the establishment of a discrete slip surface that develops within the high strain slipping zone. Interestingly, the value of strain (γ ~2) in the low-strain zone does not change significantly with increasing displacement, suggesting that, once formed, the high strain slipping zone and slip surface accommodate most of the ongoing displacement. More quantitative information on the dependence of strain distribution on displacement, slip rate, and normal stress will be gained from further microstructural analysis....

Topics

• impedance spectroscopy
• surface
• grain
• grain size
• phase
• experiment
• steel