Experiments on Landquakes Generated by Free-Falling Granular Masses: Implications for Rockfall Impact Dynamics

The properties of rockfalls, such as the volume and geometry of the detached rock mass as well as the number and diameter of rock fragments, greatly affect their propagated behavior, and different strategies are required to mitigate rockfall hazards. Landquakes generated by rockfalls are regarded as good proxies for those properties. To explore the free-fall impact dynamics of rockfalls, a series of free-fall experiments on granular masses were designed and conducted with the characteristics of the generated landquakes versus initial variables being analyzed. The results show that the particle diameter is a dominant factor affecting landquakes, with the most significant effect on the mean frequency. The impact area is another key factor, with which both the maximum seismic amplitude and radiated seismic energy have strong positive correlations, while the mean frequency of landquakes shows a weak negative correlation. The maximum seismic amplitude appears to have a poor correlation with the numbers of layers of the free-falling granular masses, and it was proven that the maximum seismic amplitude is determined by only the lower part of the granular masses. Quantitative relationships between the number of particles and seismic signal characteristics indicate that disintegration needs to be accounted for when landquakes are used to investigate the characteristics of a rockfall. Moreover, the frequency of landquakes decreases with a decrease in the vertical velocity and increase in the horizontal velocity during granular mass collapse. The movement of a rockfall exerts forces on the ground over which it travels and generates outward-propagating seismic signals. These signals are considered valuable sources of rockfall information. However, the interpretation of the generated seismic signals may be challenged by the various effects of the properties of rockfall masses. Here, we conducted a series of controlled laboratory experiments on the impacts of free-falling granular masses on a flat plate with several different experimental conditions and seismic measurements. The power laws established allow us to further characterize the different effects of rockfall properties, including the rockfall volume, initial disintegration, and impact area, on the resulting seismic signatures. An important result is the dependency of the seismic signal frequency on the granular mass properties, which can be used to improve the inferences of rockfall characteristics from seismic signals. Particle size is a dominant factor affecting landquakes, with the most significant effect on the mean frequency The maximum seismic amplitude is mainly governed by the particle size and impact area rather than the numbers of layers of falling masses Links between rockfall dynamics and landquakes are studied, revealing a positive trend between the vertical velocity and seismic frequency