Mechanisms of induced seismicity
Shear-slip on fault planes is the most common earthquake source process. The factors involved in earthquake nucleation can be related by the “Mohr-Coulomb Theory”
τcrit ≥ μ(σn – P) + So
where τcrit is the critical shear stress needed for failure, μ is the coefficient of friction of the fault plane, σn is the applied normal stress across the fault, P is the pore fluid pressure in the fault zone and So is a constant related to the cohesive strength of the material or sliding surface. Below we summarise the most common anthropogenic induction mechanisms.
Increased pore fluid pressure
Increases in pore fluid pressure reduce the effective stress (σn – P) and consequently lower the shear stress needed for failure. Pore fluid pressure can be increased by the injection of fluid and the presence of surface water impoundments.
Reduced normal stress
The removal of overlying mass can reduce the normal stress across a fault. Since the normal stress can act to clamp a fault “shut”, reductions in normal stress reduce the shear stress required for failure. This mechanism has been proposed for quarrying, mining and fluid extraction.
Increased shear stress
Increases in vertical load (i.e. mass) can increase the shear stress on a fault plane. This increase in shear stress may exceed the critical shear stress needed for failure. Mass loading as an induction mechanism has been proposed for skyscrapers, coastal engineering and water-reservoir impoundments.