Effects of normal stress variations on frictional stability of a fluid‐infiltrated fault

Abstract

Abstract. This paper extends the spring-slider model of Segall and Rice [ 1995] for dilating fluid-infiltrated faults to include geometry-induced normal stress variations. Because of the coupling between normal and shear stresses on a dipping plane the shear stress drop accompanying slip is associated with a decrease in normal stress for reverse faulting and with an increase for normal faulting. This coupling stabilizes frictional sliding on normal faults and destabilizes it on reverse faults. When a fluid phase is present, these normal stress variations during slip events also trigger, through poroelastic coupling, compensating pore pressure changes: stabilizing pressure decreases for reverse geometry and destabilizing increases for normal geometry. Fault stability is quantitatively studied using two parameters: the critical spring stiffness kcr separating the stable and unstable slip domains and, when numerical simulations exhibit stick-slip cycles, the stress drop during the slip phases At. As expected, geometric coupling is found to reduce kcr and Ar for normal faults (in comparison with Segal! and Rice's [1995] strike-slip geometry) and to increase these parameters for reverse faults. Poroelastic coupling shows opposite consequences. The combined effect of shear-induced dilatancy and poroelastic coupling is stabilizing for reverse geometry but can be either stabilizing or destabilizing for normal geometry, depending on the dilatancy magnitude. Nevertheless, the stability differences between the types of faults are primarily controlled by the geometric coupling between shear and normal stresses: For all the parameters we investigated, normal faults exhibit lower stress drops (and critical stiffnesses) than reverse faults. Finally, extrema appearing in the graphs featuring A r or kcr versus the diffusion rate demonstrate that the influence of the fluid effects is not maximum in the undrained case. Surprisingly, their influence is maximum at a point intermediate between the drained and undrained limits, when the diffusion time is roughly equal to the frictional relaxation time associated with stick phases. This phenomenon is presumably caused by subtle second-order mechanisms involving the instantaneous pore pressure changes induced by the poroelastic coupling in response to instantaneous friction changes and the interseismic pressure recovery process.

Fault slip controlled by stress path and fluid pressurization rate

“…The observation that with constant fluid pressure, slip events occur during the lateral relaxation paths and not the axial compression paths indicate decreasing normal stress and shear stress promote accelerated slip. This result can be explained by previous experimental observations and analyses which show that decreasing normal stress can cause unstable fault slip [ Linker and Dieterich , ; Dieterich and Linker , ; Chambon and Rudnicki , ; He and Wong , ]. The instability is shown to occur because a decrease in the effective normal stress causes an immediate decrease in the friction coefficient and, therefore, shear stress ( τ = μ ( σ n − P f )) that is analogous to the direct effect in velocity‐stepping tests followed by an increase in friction coefficient to steady state.…”

Shear heating of a fluid‐saturated slip‐weakening dilatant fault zone: 2. Quasi‐drained regime

“…Nevertheless, if the stability of the slip solution is assumed to evolve monotonically with the drainage parameter ϵ from the drained to the undrained limit, then it is likely that equation (22) is a sufficient condition for instability for arbitrary drainage conditions (any ϵ > 0). This conjecture that stability depends monotonically on drainage does, however, require investigation; Chambon and Rudnicki [2001] give an example for a rate‐ and state‐dependent friction relation in which stability does not depend monotonically on a drainage parameter.…”

Stability regimes of a dilatant, fluid‐infiltrated fault plane in a three‐dimensional elastic solid

“…Several numerical studies [ Lockner and Byerlee , 1995; Segall and Rice , 1995; Miller et al , 1996; Sleep , 1997; Taylor and Rice , 1998; Miller et al , 1999; Chambon and Rudnicki , 2001; Fitzenz and Miller , 2001] outlined principal mechanical implications of crustal pore pressure regimes in excess of hydrostatic. To pursue previous work, we focus on the role of dilatancy in active faulting [ Lockner and Byerlee , 1994; Rudnicki and Chen , 1988; Chambon and Rudnicki , 2001] and investigate effects of hydromechanical properties on spatiotemporal slip evolution of a two‐dimensional (2‐D) vertical strike‐slip fault plane embedded in a homogeneous half‐space. More specifically, we apply the formalism of a single degree of freedom elastohydraulic model developed by Segall and Rice [1995] to the geometry of an extended 2‐D fault plane used by Rice [1993] and in other purely elastic models [ Tse and Rice , 1986; Ben‐Zion and Rice , 1995, 1997; Rice and Ben‐Zion , 1996].…”