In Sect. 2.1, we first focus on the possibility for fault zones to develop a ductile rupture behavior within the brittle upper crust. This is happening to some fault zones affecting clay-rich sedimentary caprock layers. More generally, the brittle-ductile behavior of fault zones has potential strong implications on how fluids can penetrate and circulate within an activated fault. Indeed, a brittle fault will tend to dilate at rupture favoring hydraulic connection between the dilating pores resulting in the creation of fluid flow paths (and leakage). A ductile fault will tend to contract at rupture favoring “closing” of pores connections and preventing flow path creation. For these reasons, in Sect. 2.2 we explore how the brittle-ductile transition can be defined for a given fault zone depending on stress and on the uniaxial compressive strength of fault materials (UCS). In Sect. 2.3, we describe a rare attempt from Piau et al. (J Geophys Res Solid Earth 125, 2020) to adapt a modified Cam-Clay constitutive law to “simulate the effective behavior of fault zones of any maturity, thickness and complexity”. Finally, in Sect. 2.4, we apply Piau et al. (J Geophys Res Solid Earth 125, 2020) constitutive law to a scenario of fault reactivation relevant to a CO2 storage context.