Dynamic fault weakening and strengthening by gouge compaction and dilatancy in a fluid‐saturated fault zone

Abstract

Fault gouge deformation likely plays a significant role in controlling the strength of mature, large‐displacement faults. Experiments show that intact gouge deforms in an overall ductile and stable manner, readily compacting, but dilates and experiences brittle failure under large strain rate. Inelastic gouge compaction and dilatancy are modeled here using a combined Mohr‐Coulomb and end‐cap yield criterion in a dynamic rupture model of a strike‐slip fault with strongly velocity‐weakening friction. We show that large shear stress concentration ahead of the rupture associated with the rupture front causes the gouge layer to compact (e.g., by structural collapse and comminution), leading to rapidly elevated pore pressure and significant weakening of the principal fault surface. Shortly after the rupture front passes, strong dilatancy during strength drop and rapid sliding reduces pore pressure and strengthens the fault, promoting slip pulses. Large strain localization in the gouge layer occurs as a result of rapid gouge dilatancy and strain softening. The combination of prerupture weakening from compaction and restrengthening from dilatancy hardening leads to a smaller‐strength drop, and limits the stress concentration outside the gouge layer. This leads to a reduction of inelastic shear strain in the damage zone, which is more consistent with geological observations and high‐speed frictional experiments. With the presence of well‐developed fault gouge, the strength of mature faults may be limited by end‐cap rather than Mohr‐Coulomb failure; thus, their frictional strengths are significantly smaller than Byerlee friction.

Seismological evidence of fault weakening due to erosion by fluids from observations of intraplate earthquake swarms

“…The fault weakening processes are recognized to be important also for standard seismic sequences and can span various time scales. The dynamic fault weakening is reported during a rupture process itself being caused by rapidly elevated pore pressure due to compaction of fluid saturated gouge layer [ Hirakawa and Ma , ] or by thermal pressurization of pore fluids during the seismic slip [ Sibson , ; Lachenbruch , ]. The long‐term fault weakening due to fluid interaction with faults comes from their hydrothermal alteration and involvement of fluids passing through fault‐fracture meshes over a broad range of crustal depths.…”

An analysis of the factors that control fault zone architecture and the importance of fault orientation relative to regional stress

“…The relocation and redistribution of shearing that leads to the formation of multiple highstrain zones in a single fault requires some form of strain hardening (e.g., Chester and Chester, 1998;Di Toro and Pennacchioni, 2005;Cowgill et al, 2004a;Faulkner et al, 2008). Regardless of protolith, strain hardening and the development of wide multicored faults may result from the healing of gouge with compaction and fluid induced cementation, which gives rise to dilatancy strengthening (Morrow et al, 1982;Marone et al, 1990;Marone, 1998aMarone, , 1998bHirakawa and Ma, 2016). This hypothesis predicts that abandoned fault cores should be more strongly compacted and/or mineralized than active fault cores, but such evidence has yet to be well documented.…”

Effects of Off‐Fault Inelasticity on Near‐Fault Directivity Pulses

“…A value of 0.1Δ t for the viscous damping parameter (Day et al, 2005) and Δ x/β for the characteristic timescale of the latter viscosity (Andrews, 2005; Duan & Day, 2008) have been found effective in reducing high‐frequency noise, and those values are adopted here (Table 1). Other regularization schemes (e.g., Dunham et al, 2011a; Hirakawa & Ma, 2016) have been similarly introduced to effectively stabilize solutions in elastoplastic models.…”

“…Plastic yielding during rupture redistributes stresses near the rupture front, in turn affecting the subsequent rupture history and associated ground motion. These effects have been modeled in the framework of continuum plasticity (e.g., 3‐D Drucker‐Prager [DP], 2‐D Mohr‐Coulomb, End‐cap and Masing type) in recent studies (e.g., Andrews, 2005; Duan & Day, 2008; Dunham et al, 2011a; Hirakawa & Ma, 2016; Roten et al, 2014; Roten, Olsen, Day, & Cui, 2017; Roten et al, 2019; Shi & Day, 2013; Wang et al, 2019; Wollherr et al, 2019, 2018). These models suggest that inelastic deformation can not only reduce peak ground motions (Dunham et al, 2011a; Roten et al, 2012; Roten, Olsen, Day, & Cui, 2017) but also partially filter out high‐frequency radiation (Duan & Day, 2008; Ma & Hirakawa, 2013).…”