  The seismic waves emitted during granular flows are generated by different
sources: high frequencies by inter-particle shocks and low frequencies by
global motion and large scale deformation. To unravel these different
mechanisms, an experimental study has been performed on the seismic waves
emitted by dry quasi steady granular flows. The emitted seismic waves were
recorded using shock accelerometers and the flow dynamics were captured with a
fast camera. The mechanical characteristics of the particle shocks were
analyzed, along with the duration between shocks and the correlations in the
particle motion. The high-frequency seismic waves (1-50 kHz) were found to
originate from particle shocks and waves trapped in the flowing layer. The
low-frequency waves (20-60 Hz) were generated by the oscillations of the
particles along their trajectories, i.e. from cycles of dilation/compression
during the shear. The profiles of granular temperature (i.e. the square of
particle velocity fluctuations) and average velocity were measured and related
to the average properties of the flow as well as to the slope angle and flow
thickness. These profiles were then used in a simple steady granular flow model
to predict the radiated seismic energy and the energetic efficiency, i.e. the
fraction of the flow potential energy converted to seismic energy. Scaling laws
relating the seismic power, the shear strain rate and the inertial number were
derived. In particular, the emitted seismic power is proportional to the
granular temperature, which is also related to the mean flow velocity.
