Induced-failure criteria (pore fluid increase) and seismicity cloud and depth-time history (seismicity/hydraulic flow) of the 1993 stimulation. (a) (modified after ([Vilarrasa et al. 2013])) Induced failure criteria related to a pore fluid increase: 1) Friction criterion for hydro-shear initiation along a pre-existing fracture (Byerlee criterion - green); 1’) Fracture criterion for hydro-shear initiation in an intact rock (Mohr-Coulomb criterion - blue); and 2) Fracture criterion for hydro-frac intiation in an intact rock (Mohr-Coulomb criterion - red). (b) (modified after ([Cornet et al. 2007])) Top view of the seismicity cloud during the September 1993 stimulation (25,300 m 3 injected) for two different flow rates (below and above 18 l/s at two different depths: below 3,000 m where the seismic cloud is oriented N146°E consistently with a hydro-shear regime and between 2,850 and 3000 m where hydro-fractures develop with a cloud oriented N179°E along the direction of the regional maximal horizontal stress σ
). The angle between the orientation of the cloud during hydro-shear (1) and hydro-frac (2) defines the friction angle (ϕ≈33°) (c) (modified after ([Cornet et al. 2007])) Depth-time history of the seismicity and hydraulic flow during the 1993 hydraulic stimulation at Soultz-sous-Forêts.