Stress-driven fault reactivation and failure

Twenty-nine bare interface fault experiments were undertaken in nominally dry conditions (Appendix 4) with existing fault angles ranging from θr = 25°-70°. The

experiments were conducted at two confining pressures, Pc= 50MPa / 100MPa, thus

allowing the effect of varying confining pressure on the behaviour and microstructural development of the fault zone to be explored. The experimental data displays good reproducibility and the faults show largely comparable mechanical behaviour between the two confining pressures (for comparison loading curves see Appendix 5). As the existing fault orientation increases from θr = 25° to θr = 55°, mechanical behaviour is

characterised by increasing fault strength at the yield point. Following yield, the fault experiences slip hardening (Fig. 4A), and the rate of slip hardening also positively correlates with the increasing angle of misorientation.

As the slip distance increases, the sliding behaviour transitions into a stick-slip regime, where fault behaviour alternates between elastic loading and rapid sliding accompanied by co-seismic stress relief. The load that the fault supports before initiating rapid slip also rises with the increasing θr, resulting in an increase in stored elastic strain within

the apparatus and the development of large stress drops and slip displacements during stick-slip events.

Samples where θr = 60° show little evidence of slip on the pre-existing fault prior to the

onset of stick-slip behaviour. These samples do not display an apparent yield point, although there is a slight inflexion on the stress-strain curve prior to the first slip event, possibly indicating the onset of microfracturing, minor frictional slip or pore collapse. After the occurrence of the first major stick-slip event on θr = 55°-60° faults, the

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samples experience nearly pure elastic re-loading until a second stress peak, which is followed by the macroscopic shear failure of a new, approximately optimally-oriented fault. During the second loading period, some samples, especially those where θr = 60º,

show a distinct yield point and roll-over of the loading curing, immediately prior to the onset of failure of the new fault. Experiments MIS017 & MIS018 (with θr = 60º) were

undertaken to observe the microstructural development of the new favourably oriented fault. MIS017 was halted immediately after the first slip event and MIS018 was stopped just after the second yield point, prior to the onset of the failure of the new fault.

The mechanical behaviour of samples MIS028 and MIS029, in which θr = 65º and 70º

respectively, are characterised by loading curves very similar to that observed during intact rock failure, with elastic loading followed by the macroscopic failure along a new fault. A rapid stress drop and axial shortening of between 600-800µm accompany failure. However, in contrast to the loading curve of the intact rock failure experiments, these samples have a markedly better defined yield point which occurs approximately 10 seconds prior to rupture and possibly indicating the onset of fracture development. The retrieved samples show that where a new fault has formed and θr, ≤ 60º, the new

fault has a ‘strike’ similar to that of the existing fault, but dips at a more favourable angle relative to the sample shortening direction. In the samples where θr≥ 65º, the fault

trace of the new fracture is not consistent with the location and orientation of the pre- ground surface, supporting the idea of complete frictional-lock up of the original fault. Two stress-driven slip experiments were undertaken on the Fontainebleau sandstone at controlled pore fluid pressures (see Appendix 5 for loading curves). These experiments involved activating slip on the pre-existing fault by increasing differential stress at a nominally constant axial shortening rate, while maintaining a constant pore fluid pressure. The first experiment (MIS022) was undertaken at an effective confining pressure of 50MPa (Pc = 80MPa, Pf = 30MPa) on a sample with a fault oriented at θr =

60°. The results show behaviour very similar to that of the nominally dry experiment undertaken at equivalent conditions (MIS012). A second experiment (MIS023) was undertaken where θr = 55°, and at an effective confining pressure of 25MPa (Pc =

80MPa, Pf = 55MPa). During this experiment, the wet stress-driven reactivation

experiment resulted in a much lower yield strength (211MPa as opposed to 391MPa), consistent with the effective confining pressure being approximately half that

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Figure 4: Representative loading curves for the misoriented fault experiments.

(A) Composite plot of the loading curves for a suite of misoriented fault experiments undertaken at 100MPa confining pressure. The different experiments show the effect of changing the angle of the fault relative to the maximum shortening direction from 25º to 60º. (B) Shows the temporal evolution of differential stress, pore fluid pressure and fault displacement for a fluid-driven fault reactivation

experiment. Each stress drop represents an incremental slip event on the θr=50º fault. Note how small the

stress drops and displacements are in comparison with the stress-driven failures on a similarly oriented fault, reflecting the lower σn.

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of the dry experiment. The behaviour of the fault after the yield point in both experiments is characterised by slip hardening followed by the ultimate failure along a new optimally-oriented fault.