title: Acoustic emission investigation of fracture and fault mechanics in the laboratory
Creators: name: David. Thompson, Benjamin
Publisher: name: University of Liverpool Repository
Year: 2023

description: This thesis is an experimental study of brittle rock fracture and frictional slip on preexisting faults, both of which are important in improving understanding of earthquake nucleation. The experiments featured Westerly granite samples under triaxial compression in which, full waveform Acoustic Emission (AE) data was continuously recorded. AE locations and source mechanism studies provide a non-invasive tool to resolve the temporal and spatial evolution of micro-fracturing, and understand the micro-scale processes that are operating within a sample. New observations of brittle fracture are presented, in which the transition from stable nucleation to unstable, dynamic fracture is demonstrated to be a three stage process consisting of. (1) the development of a fracture nucleus; (2) the sudden fracture propagation at a speed of 10's of mm/s; and (3) acceleration of the fracture to cause rupture. A consideration of fault length and time-to-rupture demonstrates that the fracture velocity must accelerate to an average rate of a few m/s in the 20-100 ms prior to rupture. Furthermore, by directly measuring the duration of the dynamic, high speed fracture propagation phase, a fracture speed of 175 m/s is inferred for the 0.6 ms prior to rupture. Source mechanism studies, b-values and velocity data are presented to further demonstrate the evolution of fracture. This research demonstrates similarities between fracture nucleation in intact rock, and the previous observations of frictional slip nucleation that were used to model earthquake nucleation. Laboratory stick slip events are thought to share the same mechanism as earthquakes, and so experiments were conducted to investigate the nucleation of stick slip events, and their premonitory AE. Stick slip characteristics are compared for a saw-cut (artificial) and a rough (naturally fractured) fault. As expected, the frictional coefficient for slip events on the smooth fault was lower (between 0.48 and 0.59) than the natural fault (0.72) and the number of premonitory AE were fewer (&lt;120 compared to &gt;3000). These AE were located, and source mechanisms and b-values calculated. For both fault surfaces, the first motion of each stick slip was recorded as a large-amplitude AE and was source located onto the fault surface. These represent the nucleation sites of the stick-slip events. The nucleation sites varied and were probably controlled by heterogeneity of stress or surface conditions on the fault. There is a close similarity between the seismic signatures of slip for both tests, appearing as impulsive events with no AE in the microseconds prior to slip. This contrasts with the gradual increase in amplitude observed for the fracture initiation tests, explained by the development of a process zone. The ability to identify stick-slip nucleation sites, and define the evolution of unstable fracture for intact rock fracture, has been demonstrated for the first time in this thesis, and has been used to improve the understanding of nucleation processes.