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(a)
(b)
no slip
Figure 1
(a) Shearing of a jointed block subjected to normal force F
n
and shear force F
s
, with fluid inside the
joint at pressure
ρ
. Slip along the joint is triggered when the shear stress
τ
is equal to the frictional strength
μ(
σ
-
ρ
), where (
σ
-
ρ
) is the effective stress and μ is the coefficient of friction. (b) Graphical representation of
the Coulomb criterion: there is no slip if the “point” (
σ
-
ρ
,
τ
) is below the critical line defined by slope μ.
(a)
(b)
O
(initial)
P
Figure 2
(a) The normal and shear stresses (
σ
and
τ
) acting across the fault depend on the vertical and hori-
zontal stress (
σ
v
and
σ
h
) and the fault inclination (
β
). (b) Fluid injection or extraction could induce changes in the
stress and the pore pressure; for example, fluid injection could move the initially stable “point” “O” in Figure 2b
to a new position “P” that is on the critical Coulomb line, thus triggering slip on the fault. The inclination of the
segment OP is a function of the poroelastic coupling described in Box 2.3.
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