A model earthquake on a lab bench shows that a basic assumption of introductory physics doesn’t hold up at small scales. The finding could have a wide variety of implications for materials science and engineering, and could help researchers understand how earthquakes occur and how bad they might be.
“Our group has recently devised a way to look at laboratory generated earthquakes as they take place,” said physicist Jay Fineberg of the Hebrew University of Jerusalem. “On the way, we found out that many of the assumptions people have used historically in investigating this stuff… are basically wrong. They don’t work.”
For centuries, physicists have thought that the amount of force needed to start a book sliding across a table is equal to the force from friction that keeps book and table stuck together. That frictional force is determined by a number called the coefficient of friction, which is the ratio between the forces pushing sideways and pushing down (basically, how much the book weighs).
These laws were first described by Leonardo da Vinci in the 15th century, and re-derived by Guillaume Amontons and Charles Coulomb a few hundred years later. They’ve been the stuff of introductory physics textbooks for decades.
But when Fineberg’s student Oded Ben-David, first author of a paper in the 8 October Science describing their experiments, tried to reproduce them in carefully controlled lab experiments, the laws fell apart. Ben-David found that he could apply up to five times as much sideways force as the coefficient of friction predicted, and the book still wouldn’t move.
“Even in the lab, he couldn’t predict what was going to happen,” Fineberg said. “Small, crazy details made a really big difference. ”
The team’s experimental setup was “the stupidest system you could think about,” Fineberg said. The physicists pressed two 8-inch-long blocks of Plexiglass together with tons of force, and pushed the top block sideways until it started to move. Sensors called strain gauges measured the amount of stress on the blocks in every direction. A combination of lasers and high-speed cameras tracked the points where the two blocks actually touched.
“We’re the first people who ever tried this,” Fineberg said. “Technically it’s a bit difficult to use, but we figured it out and it’s no biggie, it’s doable.”
Although the blocks looked like they were smoothly connected along the whole area between them, in reality there were only a few hundred tiny contact areas. The forces on each individual contact could get much bigger than the coefficient of friction allows before the contacts ruptured and the block began to slide.
“These blocks are optically flat, we spent thousands of dollars to make them that way. Even then, small nuances entirely change the dynamics of what we’re going to see,” Fineberg said. “If this is in a lab experiment, imagine what happens along an earthquake fault, which is a really crappy experimental system.”
Breaking the contact sent waves similar to sound waves rippling through the blocks, which resemble a miniature earthquake, Fineberg says. The blocks represent two tectonic plates sliding slowly against each other, and when the sliding force is enough to pull the plates apart, it sends shock waves through the earth.
“It’s exactly the same system, just scaled up by factors of thousands,” Fineberg said. “We can watch how these things unfold and measure all the variables that might be actually relevant, that there’s no way you can get to under the earth.”
The team found that the waves come in three distinct modes: slow ruptures that move at speeds well below the speed of sound in the glass; “sub-Rayleigh” ruptures that travel at sound speeds along the surface, which Fineberg describes as “your garden variety earthquake;” and “supershear” ruptures that go much faster than expected. Which type of wave you get is determined by the stresses at the contact point, which provides a measure of how much energy would be released if an earthquake were to begin there.
An earthquake moving at supershear speeds would cause a sonic boom. Some think the earthquake that hit Izmit, Turkey in 1999 was this kind of deadly quake, but not everyone is convinced that supershear earthquakes actually exist.
“I think one of the most exciting things about this paper is the fact that they see a whole range of behaviours in one system,” commented geophysicist Chris Marone of Penn State University, who was not involved in the new work. “People had seen different parts of this in different experiments in different configurations. But this is one of the first, if not the first time that people have seen all the different ranges of behaviours in one place.”
It’s still impossible to make detailed measurements of the stresses along a real fault, Fineberg points out. But current technology can track stresses as an earthquake is under way, and one earthquake can affect the initial conditions for the next one. Once there are measurements for one earthquake, “I have a certain amount of predictive power,” Fineberg said. “I can tell you that if an earthquake happens today, at this point, it will progress until this point.”
Someday, Fineberg suggests, seismologists may be able to use this information to trigger small earthquakes that prevent large ones.
“That’s science fiction right now, very far in the future,” he said. “But if we have the essence now in our hands, then maybe we can play with it.”