Chapter 3: Experimental insights into the mechanics and microstructures
2. Experimental and analytical methods
4.2 Interpretation of microstructural processes
4.2.3 Development of new faults
Cohesive strengthening through melt welding
Fault healing and strength recovery is commonly thought of as a time-dependent process resulting from compaction and cementation of the fault core, over timescales of decades to centuries [Fredrich and Evans, 1992; Tenthorey et al., 2003; Tenthorey and Cox, 2006]. However, the current suite of experiments demonstrates that cohesive fault strengthening can also occur due to melt welding, almost instantaneously after seismic rupture in a stick-slip regime. The most dramatic examples of the effects of fault strengthening occurs on unfavourably-oriented faults where θr ≥ 55°. During fault
rupture significant areas of the fault interface frictionally heats and melts, forming a
Figure 30: Mohr diagram showing effect of melt welding on the strength of fault surfaces.
Mohr diagram illustrating the experimental data from MIS004, where θr=60º. The geometry of the failure/
reactivation envelopes reflect the friction coefficient and cohesive strength defined in section 4.1. Ci
refers to the cohesive strength of intact rock and Cr is an indicative estimation of the cohesive strength of the existing fault following melt welding. The first stick-slip event (see inset) occurs on the pre-existing fault surface as shown by the intersection of the first Mohr circle (grey circle) with the incohesive fault reactivation envelope. During this rupture melting occurs over a considerable area of the fault interface, effectively welding the surfaces and increasing the cohesive strength of the fault. Subsequent re-loading results in the second (black) Mohr circle intersecting the intact rock failure envelope prior to conditions being met that would allow the continued reactivation (intersection with melt-welded reactivation envelope).
semi-continuous zone of melt-welding. Subsequent re-loading impedes reactivation of the existing misoriented slip surface, and drives the formation of a new optimally- oriented fault. The evolving stress states in sample MIS004 (θr = 60º) are depicted using
a Mohr circle in Figure 30.
During this experiment the first slip event occurs on the pre-existing fault, where θr =
60º (fault ruptures are indicated on the schematic loading curve shown in Fig. 30 - see inset). Slip occurs when the first Mohr circle (indicated by the grey line) intersects the incohesive fault reactivation envelope, marked with a red star. During this rupture frictional melting occurs on the existing fault interface, welding the surfaces at the end of the slip event. It is assumed that the main change induced by the generation of frictional melt is an increase in the cohesive strength of the existing fault surface and that any change in the coefficient of static friction is negligible. Although, the increase in the cohesive strength of the fault has not been quantified experimentally, a minimum increase can be estimated based on sample behaviour during subsequent re-loading. During the second loading the existing misoriented fault de-activates when a new optimally-oriented fault nucleates. For this to occur, conditions necessary for the failure of intact rock must be achieved prior to the conditions necessary to reactivate the melt- welded existing fault. On the Mohr diagram this is shown by the intersection of the second Mohr circle (black circle indicating the stress states immediately prior to the second failure) with the intact rock failure envelope, as indicated with the second red star.
Differences in fault development processes between intact-rock failure and new faults formed after frictional lock-up of misoriented faults
The mechanical and microstructural data from the current suite of experiments indicate differences in the process of fault development between fault zones formed during intact rock failure and new optimally oriented faults formed following lock-up of unfavourably-oriented faults. Faults formed during intact rock failure are observed to propagate approximately co-seismically, as shown by the significant co-seismic sample dilation and associated large pore fluid pressure drops in experiments where pore fluid pressures were monitored. In contrast, new favourably-oriented faults, generated as a consequence of lock-up of a pre-existing misoriented fault, propagate ‘slowly’ without fast rupture and accompanying stress drop. For ease of discussion, faults that develop by this latter method are referred to developing aseismically (or without sudden,
Reactivation of Misoriented Faults
macroscopic stress drop), although on a grain scale this is unlikely to be true. Previous experiments investigating fault growth have reported acoustic emissions associated with microfracture formation and localisation prior to macroscopic failure [Lockner et al., 1992]. Experiments halted at peak stress and prior to sudden stress drop capture the new fault in the process of development, and indicate nucleation from the pre-existing unfavourably-oriented fault surface in regions of melt welding. Precursory brittle creep on the newly-forming fault is associated with up to ~200µm of displacement.
The volume of frictional melt produced within the new fault zone is also significantly different between the two styles of fault propagation. During intact rock failure, melt is localised, forming mainly at restraining bends created by the uneven fault surface, or at locally impinging grain contacts. In contrast, the new fault developed during reactivation experiments contains much larger volumes of melt, with some melt layers being up to 1.2mm long and 20µm wide. This indicates that there is a significant difference in the way energy is expended during rupture in the two types of experiments.
During rupture propagation and co-seismic slip, elastic energy that is stored in the wall rock, sample assembly and machine is released. The release of this energy produces mechanical work that is commonly referred to as an ‘energy budget’ that is defined as:
𝑊𝑓 =𝑄+𝐸𝑠 +𝑈𝑠 5
where 𝑊𝑓 is total mechanical work, 𝑄 is heat produced during frictional sliding, 𝐸𝑠 is energy radiated as seismic waves and 𝑈𝑠 is surface energy for gouge and fracture formation [Scholz, 2002].
Previous research suggests that the majority of energy is expended in heat production with relatively minor contribution expended as seismic energy [McGarr, 1999] and an almost negligible contribution from the formation of the rupture surface [e.g. Chester et al., 2005]. However, the latter of these assumptions is based on Griffith’s fracture model where fractures are assumed to propagate under equilibrium conditions [Scholz, 2002]. During seismic rupture this contribution may be significantly higher as a result of complex fracture surface development and gouge production [Reches and Dewers, 2005]. If a greater dissipation of energy occurs through fracture development, abrasive
wear and dynamic pulverization, the amount of energy that is converted to heat may be reduced, delaying the onset of melting.
In light of this discussion it is suggested that the ‘slow’ growth of new, favourably- oriented faults results in fault zones that are well-developed prior to the first sudden slip event. The existence of a fault prior to rupture minimises the proportion of energy that is consumed by damage processes and maximises frictional heating and melt production. On the other hand, during sudden intact rock failure, fault growth and slip are more contemporaneous with fault rupture and associated sudden stress drop. Consequently a greater portion of the energy budget is consumed by the formation of damage, which results if lower volumes of melt.