Experimental observations of slow aseismic failure due to intra-crystalline plasticity in Carrara marble

Two triaxial compression experiments were performed on Carrara marble under wet and dry conditions respectively. The rock samples, first deformed at high confining pressure in the cataclastic regime, were brought back into the brittle field at constant differential stress by increasing the pore pressure and/or reducing the confining pressure. When returning to the brittle field, both samples exhibited exponential increases in axial strain, which eventually led to tertiary creep and failure nucleation. Very little energy was released in the acoustic frequency range (100KHz-1MHz) during rupture. Slip propagation was slow (60 and 500 seconds in the dry and wet case respectively), although failure was accompanied by stress drops of the order of 150 MPa, and millimetric slips. Even in dry conditions, a continuous acoustic recording over the course of rupture shows that the slip initiated aseismically. In this test, the continous recording shows that fast frictional sliding then induced some acoustic activity, probably due to asperity shearing, thus illustrating the transition from aseismic to seismic slip. Elastic wave velocity recordings demonstrated aseismic damage accumulation prior to failure and elastic wave velocity inversion showed that rupture occurred at crack densities close to one, both in the wet and dry experiments. Microstructural analysis highlighted strong interactions between plastically accommodated shear deformation at the intragranular scale and brittle deformation at the macroscopic level. Microscopically, shear deformation was accommodated by twinning and dislocation glide. Howevere, differential plastic strains due to random crystallographic orientations were necessarily accommodated by the growth of ductile cracks at the macroscopic level, in part due to dislocation pileups. Plastic relaxation and dislocation loops at propagating crack tips enabled ductile aseismic growth and slow aseismic failure was triggered in a way analogous to silent earthquakes observed in the field. To our knowledge, these experiments provide the first clear case of silent, strain dependent failure in rocks as a result from intragranular plasticity.