This work is concerned with the relationship between hydraulic fracturing injection into a fault and the possibility of a seismic slip. The results of this study show that the nucleation of dynamic slip on a fault with slip‐weakening friction is only weakly dependent on the magnitude of the stress perturbation ahead of the propagating hydraulic fracture (HF), or the HF propagation regime, and is mainly controlled by the hydraulic fracture length (i.e., the size of the fully unloaded fault segment at a given time). The growth of the fault slipping patch remains stable when the background shear stress τ0 is smaller than the residual fault strength τr under ambient conditions. Otherwise (τ0>τr), nucleation of dynamic slip takes place when the hydraulic fracture grows to the critical size ℓc, which is vanishingly small ∝τp−τ0 for critically stressed faults (i.e., when the background stress approaches the fault peak strength, τ0→τp) and is diverging as ∝1/(τ0−τr) when the stability boundary is approached ( τ0→τr). Our solution for the critical HF size allows to infer the corresponding fluid injection volume that may lead to slip instability on a fault with given frictional properties and background stress.

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