Unlike a true solid, the ground beneath our feet is primarily composed of granular materials such as sand grains or rock fragments. This also holds true for the fault lines within the Earth’s crust where tectonic plates converge. These disordered granular materials are inherently unstable, and their failure can have catastrophic consequences for those living on the Earth’s surface.
Predicting or controlling the point at which the friction forces holding a landslide or earthquake in check will no longer be sufficient to prevent movement is a daunting task. Fortunately, the principles governing these phenomena are analogous in smaller-scale systems that can be studied in laboratory settings. Physicists Kasra Farain and Daniel Bonn from the University of Amsterdam simulated an earthquake using a 1-mm thick layer of tiny spheres, each comparable in width to a human hair.
Their experimental setup allowed precise monitoring of how the granular material responded to external forces. To simulate conditions akin to those on steep mountain slopes or tectonic faults, they applied pressure with a rotating disc. By introducing a small seismic wave, generated by bouncing a ball near the setup, they observed rapid shifts in the granular material—a miniature earthquake had been induced.
“We discovered that even a minor perturbation, such as a small seismic wave, can cause a granular material to undergo complete restructuring,” Farain explains. Further analysis revealed a momentary transition where the granules exhibited liquid-like behavior rather than remaining solid. Once the perturbation subsided, friction reinstated itself, and the granules settled into a new configuration.
Similar phenomena occur during actual seismic events. “Earthquakes and tectonic activities adhere to scale-invariant principles, making our findings from laboratory-scale frictional experiments pertinent for understanding how seismic waves can remotely trigger earthquakes in much larger fault systems within the Earth’s crust,” notes Farain.
The researchers developed a mathematical model based on their experiments, which accurately explains how the 1992 Landers earthquake in Southern California triggered a subsequent seismic event 415 km away to the north. Additionally, their model effectively describes the increase in fluid pressure observed in the Nankai subduction zone near Japan following a series of small earthquakes in 2003.
The genesis of this research project can be traced back to Farain’s interactions with his colleagues: “Initially, my experimental setup was situated on an ordinary table, lacking the sophisticated vibration isolation required for precise measurements. I soon realized that mundane occurrences like someone walking by or the door closing could disrupt the experiment. I must have been quite a nuisance to my colleagues, always requesting quieter footsteps or gentler door closures.”
Inspired by how his colleagues’ movements affected his setup, Farain delved deeper into the underlying physics: “Eventually, I upgraded to a proper optical table, eliminating disruptions caused by people’s movements. But, true to my mischievous nature, that wasn’t the end of it. Later on, I returned to the lab with a loudspeaker to generate controlled noise and observe its effects on the experiment!”
