Simulated Slidequakes: Insights From DEM Simulations Into the High-Frequency Seismic Signal Generated by Geophysical Granular Flows

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. 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. A new "minimal model" best describes the rapidly fluctuating forces exerted by steady granular flows on fixed, rough bases We recommend that one of two different models is applied in practice, depending on whether or not flows exhibit basal slip A bulk inertial number determines the relative magnitudes of mean, intermediate-frequency, and high-frequency forces