We present a numerical investigation of the effect that static stress perturbations due to fluid injection have on a nearby active fault where the fluid does not come in physical contact with the fault. Our modelling employs a lattice Boltzmann pore diffusion model coupled with a quasi-dynamic earthquake rupture model. As diffusivities and frictional parameters can be defined independently at individual nodes/cells this allows us to replicate complex 3-D geological media in our simulations. We demonstrate the effect an injection can have on an active nearby fault. Compared with our control catalogue (identical to the original simulation but without the injection), the injection not only altered the timing of the next earthquake sequence, it also changed its size, producing a Mw 6.7 event, the largest observed earthquake on the fault. This large event pushes the fault into a subcritical state from which it took roughly 200 yr of continuous tectonic loading for the fault to return to a critical state.