Types and Causes of Induced Seismicity - Induced Seismicity Potential in Energy Technologies - page 18

BOX 2.1
Conditions Leading to Seismic Slip on a Fault
Shallow earthquakes result from slip along a preexisting fault. The slip is triggered when the stress acting
along the fault exceeds the frictional resistance to sliding. The critical conditions are quantified by the Coulomb
criterion, which embodies two fundamental concepts, friction and effective stress. These two concepts can be
illustrated by considering the shearing of a split block (Figure 1). The block is subjected to a normal force F n
and a shear force F s , which can be translated into a normal stress σ = F n /A and the shear stress τ = F s /A acting
across the joint, with A designating the interface area of the joint. The joint (and possibly also the block if it is
porous) is infiltrated by fluid at pressure ρ .
According to the Coulomb criterion, there is no relative movement across the joint, as long as the shear
stress ( τ ) is less than the frictional strength μ( σ - ρ ), where μ is the coefficient of friction. The conditions for slip
are thus met when τ = μ( σ - ρ ). The term ( σ - ρ ) is called the effective stress; the presence of effective stress in
the Coulomb criterion shows that the fluid pressure ( ρ ) counterbalances the stabilizing effect of the normal stress
( σ ). The Coulomb criterion indicates that slip can be triggered by a decrease of the normal stress, an increase
of the pore pressure, and/or an increase of the shear stress (Figure 1b).
Note that the common concept that “injected fluids cause earthquakes by lubricating underground faults”
is not accurate because fluids do not decrease the coefficient of friction, μ. Rather, injected fluids (or extracted
fluids) cause earthquakes by changing the stress conditions around faults, bringing these stresses into a condition
where driving stresses equal or exceed resistive stresses, thereby promoting slip on the fault.
Within the context of slip on a fault, the normal and shear stresses acting across the fault, σ and τ , can
be directly expressed in terms of the vertical stress ( σ v ), the horizontal stress ( σ h ), and the fault inclination ( β )
(Figure 2). Prior to injection or extraction of fluid, the initial state is stable because the shear stress ( τ o ) is less than
the frictional strength μ( σ o - ρ o ), although the condition could be close to critical. Injection or extraction of fluid
could cause changes in the stress and pore pressure such that the critical condition expressed as τ = μ( σ - ρ ) is
met (Figure 2b is a graphical representation).
This box describes the simple case of a frictional fault. The more general case of a fault with cohesive-
frictional strength is treated in Appendix H.