Geophysical granular flows generate seismic signals known as “slidequakes” or “landquakes”, with low‐frequency components whose generation by mean forces is widely used to infer hazard‐relevant flow properties. Many more such properties could be inferred by understanding the fluctuating forces that generate slidequakes' higher frequency components and, to do so, we conducted discrete‐element simulations that examined the fluctuating forces exerted by steady, downslope‐periodic granular flows on fixed, rough bases. Unlike our previous laboratory experiments, our simulations precluded basal slip. We show that, in its absence, simulated basal forces' power spectra have high‐frequency components more accurately predicted using mean shear rates than using depth‐averaged flow velocities, and can have intermediate‐frequency components which we relate to chains of prolonged interparticle contacts. We develop a “minimal model”, which uses a flow's collisional properties to even more accurately predict the high‐frequency components, and empirically parametrize this model in terms of mean flow properties, for practical application. Finally, we demonstrate that the bulk inertial number determines not only the magnitude ratio of rapidly fluctuating and mean forces on a unit basal area, consistent with previous experimental results, but also the relative magnitudes of the high and intermediate‐frequency force components.