The seismic waves emitted during granular flows are generated by different sources: high frequencies by inter-particle collisions 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, dense, 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 collisions were analyzed, along with the intervals between collisions and the correlations in particles’ motion. The high-frequency seismic waves (1-50 kHz) were found to originate from particle collisions and waves trapped in the flowing layer. The low-frequency waves (20-60 Hz) were generated by particles’ oscillations along their trajectories, i.e. from cycles of dilation/compression during coherent shear. The profiles of granular temperature (i.e. the mean squared value of particle velocity fluctuations) and average velocity were measured and related to each other, then used in a simple steady granular flow model, in which the seismic signal consists of the variously attenuated contributions of shear-induced Hertzian collisions throughout the flow, to predict the rate at which seismic energy was emitted. Agreement with the measured seismic power was reasonable, and scaling laws relating the seismic power, the shear strain rate and the inertial number were derived. In particular, the emitted seismic power was observed to be approximately proportional to the root mean square velocity fluctuation to the power $3.1 \pm 0.9$, with the latter related to the mean flow velocity.

Geophysical granular flows exert basal forces that generate seismic signals, which can be used to better monitor and model these severe natural hazards. A number of empirical relations and existing models link these signals’ high-frequency components to a variety of flow properties, many of which are inaccessible by other analyses. However, the range of validity of the empirical relations remains unclear and the models lack validation, owing to the difficulty of adequately controlling and instrumenting field-scale flows. Here, we present laboratory experiments investigating the normal forces exerted on a basal plate by dense and partially dense flows of spherical glass particles. We measured the power spectra of these forces and inferred predictions for these power spectra from the models for debris flows’ seismic signals proposed by Kean et al. (2015), Lai et al. (2018), and Farin, Tsai, et al. (2019), using Hertz theory to extend Farin, Tsai, et al. (2019)’s models to higher frequencies. Comparison of our bservations to these predictions, and to predictions derived from Bachelet (2018) and Bachelet et al. (2021)’s model for granular flows’ seismic signals, shows those of Farin, Tsai, et al. (2019)’s ‘thin-flow’ model to be the most accurate, so we examine explanations for this accuracy and discuss its implications for geophysical flows’ seismic signals. We also consider the normalisation, by the mean force exerted by each flow, of the force’s mean squared fluctuations, showing that this ratio varies by four orders of magnitude over our experiments, but is determined by the bulk inertial number of the flow.

3D-printing techniques allow us to visualise geophysical concepts that are difficult to grasp, making them perfect for incorporation into teaching and outreach packages. Abstract models, often represented as 2D coloured maps, become more tactile when represented as 3D physical objects. In addition, new questions tend to be asked and different features noticed when handling such objects, while they also make outreach and education more inclusive to the visually impaired. Some of our most effective models are simply exaggerated planetary topography in 3D, including Earth, Mars and the Moon. The resulting globes provide a powerful way to explain the importance of plate tectonics in shaping a planet and linking surface features to deeper dynamic processes. In addition, we have developed a simple method for portraying abstract global models by 3D printing modified globes of surface topography, representing the parameter of interest as additional, exaggerated long-wavelength topography. This workflow has been applied to models of dynamic topography, the geoid and seismic tomography. In analogy to Russian nesting dolls, the resulting “seismic matryoshkas” have multiple layers that can be removed by the audience to explore the structures present deep within our planet and learn about the ongoing dynamic processes. While these 3D objects are easily printed on a cheap (<300 GBP, 400USD) desktop 3D-printer, the printing times still prohibit large-scale production. To ensure that there is sufficient material in a teaching setting, we have therefore also developed complementary paper equivalents. By projecting the coloured maps onto a dodecahedron, we developed cut-out-and-fold models to be handed out in a classroom setting to complement the 3D printed globes used for demonstration purposes. Together with animations, suggested questions and instructor “cheat-sheets”, these materials form a complete teaching and outreach package that is both interactive and inclusive.