Mini-earthquakes (as an experiment) took place at Science Park this week
Knowledge about earthquakes is crucial in high-risk areas such as Groningen. UvA-physicists created several in the lab at Amsterdam Science Park to better understand the underlying mechanisms and published the results in the scientific journal Science Advances.
The physicist Kasra Farain bounces a colorful ball on a steel plate in the physics lab at Amsterdam Science Park. The resulting shock wave causes a large number of microscopic grains to be set in motion in the setup on the table: a mini-earthquake in the lab.
The experiment shows how an earthquake elsewhere can lead to aftershocks or new quakes thousands of kilometers away. “The derived mathematical model explains, for example, how the 1992 Landers earthquake in Southern California triggered a second quake at a great distance away, 415 km to the north,” says Professor of Physics Daniel Bonn, who is also engaged in the research.
Scale
The contrast between a real earthquake and the experiment at Science Park is stark. The modest setup consists only of a steel table topped by a device that looks something like a table drill. Together, a plate and a moving cylinder mimic two tectonic plates. The layer between them consists of tiny white plastic balls of 40 micrometers in diameter, mimicking the layer of sand between the Earth’s tectonic plates.
According to the researchers, the scale and material of the setup doesn’t matter. “The measurements from the experiment are translatable one-to-one to measurements taken during real earthquakes,” Farain says. “That is, the same laws of physics apply. The action of an earthquake has never been simulated as small and as controlled as it is here, allowing us to measure all kinds of forces and changes in it that are difficult to observe in a real quake.”
That controlled setup wasn’t always there. “First, my experimental setup was on an ordinary table so that when someone closed a door or colleagues walked by, the table would vibrate and disturb my measurements, causing earthquakes in my experiment,” Farain laughs. “Meanwhile, we have a sophisticated setup that isolates vibrations so we can get very controlled and accurate measurements.”
High pressure
The researchers focused mainly on so-called grain flow, relevant to earthquakes because of the flow properties of sand or gravel under high pressure. Knowledge about this is limited but crucial to understanding earthquakes better. “You have to imagine that the layer of sand between two tectonic plates is under pressure from all kinds of forces like gravity and friction,” Bonn explains. “Those forces ensure that that layer of sand is arranged in such a way that it remains stable under that pressure and doesn’t move. A disruption of those forces, by, say, a shock wave from another quake or, in our case, the impact of the bouncing ball, can radically change that and cause the sand to behave like a liquid, triggering an earthquake.”
One of the changes the researchers have observed is that the solid granular material, the plastic balls, turns into a liquid state for a brief moment once the shock wave from the bouncing ball reaches the experimental setup. “Take the example of sand. When you walk on it, you would assume it is hard, in a solid state. But if you move your feet through the sand on the beach, would you still assume it to be solid? Or take sand that gets stuck in an hourglass. A small tap is enough to make the sand flow again. So sand can move like a fluid,” Farain explains. “Exactly the same principle that we demonstrate with our experiment applies to real earthquakes or landslides where the intermediate layer suddenly changes from a stable state to something that flows, resulting in an earthquake.”
Besides providing insight into the generation of earthquakes, avalanches, and other types of landslides, Farain and Bonn’s experiment also provides knowledge about soil stability. “This knowledge about grain flow is important in areas at high risk of earthquakes for all kinds of applications. Consider, for example, the construction of polders, the construction or repair of levees, or the construction of concrete buildings,” Bonn says. “Any place that involves land or soil requires an understanding of those flow properties. That’s where our mini-earthquakes contribute.”
