The dynamics of asperities and the resulting real contact area play an important role in the evolving frictional strength of fault zones, particularly during the interseismic period. The aperture of fractures controls the ability of fluids to traverse meaningful distances in geologically and industrially relevant timescales. Here, we show the evolution of the real contact area and aperture at increasing normal load in a triaxial compression experiment with X-ray tomography on a Westerly granite rock core with a preexisting core-spanning fracture oriented perpendicular to the maximum compressive principal stress direction, at a confining pressure of 15 MPa and fluid pressure of water of 10 MPa. At the onset of normal loading, the real contact area increases linearly with the differential stress, at a rate of about 0.2 %/MPa until the real contact area is 20 %, mostly though the accumulation of greater numbers of individual contacts, consistent with elastic deformation. Then the real contact area increases more rapidly with load, with increasing areas of individual contacts, consistent with plastic deformation. The real contact area reaches maximum values of 35 %, and then declines with increasing loading as secondary axial fractures propagate from the large sub-horizontal fracture. The average mechanical aperture first declines rapidly with loading, reaches a minimum value of 13 μm, and then increases before failure as secondary axial fractures develop. The rough sub-horizontal fracture and secondary axial fractures produce a long-lived zone of elevated porosity, which is 4 % above the background porosity (1–2 %) just before failure. Thus, a closing rough fracture can provide a conduit for fluid flow due to non-zero aperture and the production of secondary damage.
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