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\author[1]{Matthew Arran}%
\author[2]{A Mangeney}%
\author[3]{J De Rosny}%
\author[4,5]{R Toussaint}%
\affil[1]{Affiliation not available}%
\affil[2]{Université de Paris, Institut de physique du globe de Paris, CNRS}%
\affil[3]{Institut Langevin, ESPCI Paris, PSL University, CNRS}%
\affil[4]{Institut Terre et Environnement de Strasbourg, UMR 7063, Université de Strasbourg, CNRS}%
\affil[5]{The Njord Centre, Department of Physics, SFF PoreLab, University of Oslo}%


\vspace{-1em}



  
  \date{June 08, 2024}


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\begin{abstract}
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, Arran et al.
(2021,~\url{https://doi.org/10.1029/2021JF006172}) (A21) compared the
predictions of pre-existing physical models to the forces exerted by
laboratory-scale flows. However, A21 was unable to establish whether the
laboratory flows exhibited basal slip, and the conditions for applying
its results are therefore unclear. Here, we describe discrete-element
simulations that examined the fluctuating forces exerted by steady,
downslope-periodic granular flows on fixed, rough bases that prevented
basal slip. We show that, in its absence, A21's results do not hold:
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 inter-particle
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. 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 A21, but also the relative
magnitudes of the high and intermediate-frequency force components.

\textbf{Plain Language Summary}

Any geophysical granular flow-such as a landslide, rockfall, or debris
flow-exerts fluctuating forces that cause the ground to vibrate, in a
`slidequake' that can provide useful information about the flow. Here,
we examine simulated slidequakes: computer models of the individual
particles within idealized flows, the collisions between them, and the
rapidly fluctuating forces they exert on the flow's base. By recording
particle and collision properties throughout the flow, we examine
pre-existing models for the fluctuating forces; develop, test, and
simplify a new model; and relate ratios between forces to an `inertial
number' that characterizes different flows. Our results differ from
those of laboratory experiments that previously investigated
slidequakes, but the two sets of results can be combined to provide
information about real geophysical flows.%
\end{abstract}%



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