That is, how much fluid can be injected or extracted from the reservoir without triggering the mechanical failure of the reservoir and surrounding rocks. In an enhanced geothermal system, such as Horstberg in North German Basin (NGB), an induced hydraulic fracture is created by massive water injection to enhance the wellbore productivity and to increase the surface area of heat transfer. A safe operation in Horstberg geothermal system requires that induced hydraulic fracture and surrounding faults mechanically stay stable. Particularly, the further extension of the induced hydraulic fracture and reactivation of the preexisting faults should be prevented. On one hand, fault mechanics and stability of the faults are traditionally described by Mohr-Coulomb theory where a frictional instability along a preexisting fault surface may occur (Ellsworth, 2013) and on the other hand, the onset of fracture extension is described by fracture mechanics and a critical stress intensity factor which often regarded as a material property, fracture toughness (Anderson, 1991).