What do a mountain, a tectonic fault and a pile of sand in the laboratory have in common? More than you might think. Because, as some Dutch physicists have demonstrated, a small jolt is enough to make the ground "liquefy" and trigger a mini-earthquake. A phenomenon which, on a small scale, perfectly reproduces what happens in the earth's crust during an earthquake. And that could help us better understand how to predict and deal with these catastrophic events. But let's proceed in order.
A mountain of grains
It all starts with an intuition from researchers Kasra Farain e Daniel Bonn of the University of Amsterdam. Their idea is clear: reproduce in the laboratory the conditions that occur on a steep mountain slope or along a tectonic fault, using a thin layer of tiny spheres the diameter of a human hair.
Why use grains instead of a nice block of rock? Because, as the researchers explain in the study that I link to you here, the ground we tread is far from a perfect solid. Indeed, it is more like a disordered mass of granules, whether grains of sand or fragments of stone. And the same goes for deep faults, where tectonic plates meet. In short, to understand how an earthquake is triggered, it is better to start from the foundations: the grains.
Dancing on the fault line: earthquakes in the laboratory
Using a disk pressed onto the surface of the grains and slowly rotated at a constant speed, the researchers simulated in the laboratory the forces that build up on a steep slope or along a fault. Then, with a simple bounce of the ball (literally) next to the experimental apparatus, they generated a small seismic wave. The result? The grains began to slide and rearrange, just like in a real earthquake.
But the real surprise came when the researchers analyzed the "dance" of the grains in detail. For a brief moment, in fact, these behave more like a liquid than a solid, losing friction and sliding over each other. Only after the passage of the seismic wave, the friction is felt again and the grains get stuck again, but in a different configuration.
From the test tube to the earth's crust
Sure, you might object, it's all very interesting, but what does a little pile of sand dancing in the laboratory have to do with it? with real earthquakes? More than you imagine. Because, as the researchers explain, seismic phenomena follow "scale invariant" laws. In short, whether we are dealing with tiny grains or entire kilometers-long faults, the basic physics is the same.
It is no coincidence that the mathematical model that Farain and Bonn deduced from their experiments is able to quantitatively explain how the 1992 Landers earthquake in California triggered a second seismic event remotely, 415 km further north. And not only that: the same model precisely describes the increase in fluid pressure observed in the Nankai subduction zone, near Japan, after a series of small earthquakes in 2003.
From the footsteps of colleagues to seismic waves
The story of this research also has an ironic side. Initially, in fact, Farain's experimental apparatus was positioned on a simple table, without all the sophisticated vibration isolation systems necessary for precise measurements. Result? Even the slightest movement of colleagues, from walking to closing a door, affected the experiment. A big headache for poor Farain, forced to beg for soft steps and gentle closures.
But as we know, sometimes annoyances turn into opportunities. Inspired by how his colleagues' movements affected his apparatus, Farain began to investigate the physics at work. And even after finally obtaining a table optimized for vibrations, he couldn't resist the temptation to return to the laboratory with a speaker, to generate controlled noises and study their effects.
Will we have a more predictable Earth thanks to mini earthquakes in the laboratory?
Tell me yes, I live in the Campi Flegrei. This research could have very serious implications for our understanding of earthquakes and the ability to predict them. We are still far from being able to predict with certainty where and when the next "Big One" will occur, but better understanding how even a small disturbance can trigger a seismic event is a fundamental step in that direction.
One day, perhaps, future generations will look back on these experiments as a turning point in our fight against one of nature's most devastating phenomena. A bit like how we today look at Galileo's experiments on the fall of bodies or Newton's on the orbits of the planets. Because even the most revolutionary science sometimes starts with a simple pile of sand. Or from a colleague who stomps his foot a little too much as he passes by a table.
