HR: 0800h
AN: S41B-0998    [Abstracts]
TI: Experimental observations of slow aseismic failure due to intra-crystalline plasticity in Carrara marble
AU: Schubnel, A
EM: alexandre.schubnel@utoronto.ca
AF: Lassonde Institute University of Toronto, 170 college street, Toronto, On M5S3E3 Canada
AU: Walker, E
EM: walker.e@noos.fr
AF: Ecole et Observatoire des Sciences de la Terre de Strasbourg (EOST), 5 rue René Descartes, Strasbourg, 67084 France
AU: * Thompson, B D
EM: B.Thompson@liverpool.ac.uk
AF: Lassonde Institute University of Toronto, 170 college street, Toronto, On M5S3E3 Canada
AU: * Thompson, B D
EM: B.Thompson@liverpool.ac.uk
AF: Department of Earth Sciences University of Liverpool, 4 Brownlow Street, Liverpool, L693GP United Kingdom
AU: Fortin, J
EM: fortin@geologie.ens.fr
AF: Laboratoire de Géologie Ecole Normale Supérieure de Paris, 24 rue Lhomond, Paris, 75005 France
AU: Gueguen, Y
EM: gueguen@geologie.ens.fr
AF: Laboratoire de Géologie Ecole Normale Supérieure de Paris, 24 rue Lhomond, Paris, 75005 France
AU: Young, R P
EM: paul.young@utoronto.ca
AF: Lassonde Institute University of Toronto, 170 college street, Toronto, On M5S3E3 Canada
AB: 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.
DE: 4425 Critical phenomena
DE: 5104 Fracture and flow
DE: 5112 Microstructure
DE: 5120 Plasticity, diffusion, and creep
DE: 8034 Rheology and friction of fault zones (8163)
SC: Seismology [S]
MN: Fall Meeting 2005