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This paper presents a three-dimensional fully hydro-mechanical coupled distinct element study on fault reactivation and induced seismicity due to hydraulic fracturing injection and subsequent backflow process, based on the geological data in H River Basin, Northeast British Columbia, Canada. The modeling results indicate that the maximum magnitude of seismic events appears at the fracturing stage. The increment of fluid volume in the fault determines the cumulative moment and maximum fault slippage, both of which are essentially proportional to the fluid volume. After backflow starts, the fluid near the joint intersection keeps flowing into the critically stressed fault, rather than backflows to the wellbore. Although fault slippage is affected by the changes of both pore pressure and ambient rock stress, their contributions are different at fracturing and backflow stages. At fracturing stage, pore pressure change shows a dominant effect on induced fault slippage. While at backflow stage, because the fault plane is under a critical stress state, any minor disturbance would trigger a fault slippage. The energy analysis indicates that aseismic deformation takes up a majority of the total deformation energy during hydraulic fracturing. A common regularity is found in both fracturing- and backflow-induced seismicity that the cumulative moment and maximum fault slippage are nearly proportional to the injected fluid volume. This study shows some novel insights into interpreting fracturing- and backflowinduced seismicity, and provides useful information for controlling and mitigating seismic hazards due to hydraulic fracturing.