HR: 08:45h
AN: MR31B-04    [Abstracts]
TI: Process Zone Evolution and Fault Propagation in Granite
AU: * Stanchits, S
EM: stanch@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg D423, Potsdam, 14473 Germany
AU: Dresen, G
EM: dre@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg D423, Potsdam, 14473 Germany
AB: To investigate micromechanics of fault nucleation and propagation, we performed nine triaxial compression tests on two granite rocks (Aue Granite, Tono Granite) in a servo-hydraulic loading frame at confining pressures between 5-40 MPa. Samples were 50 mm in diameter and 100-125 mm in length. Twelve P-wave sensors, eight S-wave sensors and four strain gages were glued directly to the sample surface monitoring acoustic emissions (AE), seismic velocities and volumetric strain. We analyzed AE hypocenter distributions, AE source types, b-values and the correlation coefficients (C-values) versus time and axial strain. We performed tests with displacement-controlled and AE rate-controlled loading resulting in process zone propagation rates between 2 cm/s and 20 μm/s. Specimens were loaded with or without a stress-concentrating indenter at the sample top surface (asymmetric and symmetric tests). In general, AE source type distributions show double-couple events and non-double couple tensile and collapse events irrespective of loading conditions for both rocks types. However, indenter tests consistently show higher contribution of collapse events. During all experiments we observed a drastic decrease in relative contribution of tensile events from >50% to <10% between yield stress and failure stress. Horizontal P-wave velocities start to decrease significantly below the yield stress and prior to onset of appreciable AE activity. At all test conditions, b-values typically start decreasing prior to peak stress. C-values indicate non-correlated randomly distributed cracking during initial loading stages. In tests performed with a linear indenter faults nucleated at the stress concentration. In the remaining experiments, nucleation occurred mostly near the sample periphery. Fracture nucleation and propagation was not affected by loading rate. Source type discrimination and mapping of b-values significantly improve identification of fault nucleation patches in hypocenter distributions.
DE: 5102 Acoustic properties
DE: 5104 Fracture and flow
DE: 5112 Microstructure
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005