Rock avalanches triggered by earthquakes frequently result in severe damages owing to their unpredictable hypermobility. Investigating the effects of seismic shaking on the mobility of this granular flow process is important for hazard risk management. In this study, the effects of seismic shaking on the mobility of granular free-surface flows are investigated using a series of discrete element method simulations, with a focus on the shaking frequency, magnitude, and particle size ratio. The results indicate that although the vertical (z-direction) seismic shaking slightly increases internal and basal energy dissipation, it supplies energy to the granular system and facilitates particle size segregation, increasing the mobility of size-bidisperse granular flows. As the shaking magnitude increases, the streamwise velocity and granular temperature increase, whereas the solid volume fraction decreases, and fluctuations in the effective friction coefficient become more distinct. The bulk streamwise velocity increases by 2.83%-42.45% as the vertical (z-direction) seismic magnitudes increase from 0.3 to 0.9 g. As the shaking frequency increases, the streamwise velocity and granular temperature increase but then decrease, whereas the effective friction coefficient and solid volume fraction remain largely unchanged. Granular temperature is a key parameter in establishing a rheological model for size-bidisperse granular flows under seismic shaking. Moreover, the limitations of this study and future research directions, including solid-liquid coupling, reciprocal feedback mechanisms between seismic shaking and rock avalanches, the complexities of real earthquakes, and the extension of granular rheology to polydisperse particles under seismic shaking, are discussed.