The forces exerted by geophysical granular flows on Earth's surface, and the resulting seismic signals, can be used to monitor natural geohazards and understand their dynamic evolution and characteristics. Substantial research has focused on linking basal force fluctuations and seismic signals to granular flow dynamics. However, the mechanisms behind the generation and evolution of seismic signals remain incompletely understood. In this study, we conducted laboratory flume experiments to gain insights into the evolution and characteristics of basal force fluctuations and seismic signals and explored their relationship with the macroscopic properties of granular flows. Our results show that the shear and normal components of basal force fluctuations exhibit different behavior during flow evolution, which are related to variations in flow velocity fluctuations. As the granular flow moves downstream, shear basal force fluctuations decrease due to weakening velocity fluctuations, whereas normal force fluctuations increase. Similar to basal force fluctuations, seismic signals follow a generalized Pareto distribution. Basal force fluctuations and seismic signals are strongly nonlinearly related to the bulk flow properties, indicating that thicker, denser and faster flows generate stronger basal force fluctuations and more intense seismic signals. However, particle size significantly influences this relationship. We demonstrate that the inertial number, characterizing the macroscopic rheological properties of granular flows, can unify basal force fluctuations and seismic signals across different particle sizes, exhibiting a negative correlation on the temporal scale. This implies that the macroscopic rheological behavior of granular flows may provide critical insights into the mechanisms of generation and evolution of seismic signals.