The 4.6 earthquake that occurred on 17 August 2015 northwest of Fort St. John, British Columbia, is considered the largest hydraulic‐fracturing‐induced event in Canada, based on its spatiotemporal relationship with respect to nearby injection operations. There is a delay of this 4.6 mainshock from the onset of fluid injection at the closest well pad (W1). In contrast, other two nearby injection wells (W2 and W3) have almost instantaneous seismic responses. In this study, we first take a forward numerical approach to investigate the causative mechanisms for the 4.6 event. Specifically, three finite‐element 3D poroelastic models of various permeability structures and presence or absence of hydraulic conduits are constructed, to calculate the coupled evolution of elastic stress and pore pressure caused by multistage fluid injections. Our simulation results suggest that pore pressure increase associated with the migration of injected fluid is required to accumulate sufficient stress perturbations to trigger this 4.6 earthquake. In contrast, the elastic stress perturbation caused by rock matrix deformation alone is not the main cause. Furthermore, injection and seismicity at W1 may have altered the local stress field and brought local faults closer to failure at sites W2 and W3. This process could probably shorten the seismic response time and, thus, explain the observed simultaneous appearance of injection and induced seismicity at W2 and W3.