A period of slow, crawling movements, without any jerking, may be a necessary prelude to earthquakessuggests a new study.
The research, which looked at the fundamentals of material fracture, focused on cracks snaking through plastic sheets in the laboratory. But the experiments revealed some basic physics about how fractures work, particularly how a buildup of friction at the interface of two bodies turns into a sudden rupture. And these results apply to real earthquakes, the study author said. Jay Finebergphysicist at the Hebrew University of Jerusalem.
“It won’t matter what material the contact plates are made of,” Fineberg told Live Science. “The same physical process will take place in both cases: the explosive spring from the bent plates will release in the same way.”
Earthquakes form when two tectonic plates moving against each other become stuck, allowing the fault to build up stress. “The plates are increasingly stressed by the forces that try to move them, but get stuck at the fragile part of the interface between them,” Fineberg explained. This fragile section, which does not deform in response to stress, has a finite thickness and is what breaks during an earthquake.
“The fracture process doesn’t happen all at once. You have to create a crack first,” Fineberg said. When this crack reaches the limits of the fragile interface, it rapidly accelerates to speeds close to the speed of sound. This is what makes the earth shake.
“The question is, how does nature create the crack that then becomes an earthquake?” Fineberg.
Fineberg and his colleagues studied the question by combining theoretical mathematics and laboratory experiments. They reproduce earthquake-like fractures in the laboratory with blocks made of a thermoplastic called polymethyl methacrylate, better known as plexiglass. Researchers squeeze the Plexiglas sheets together and apply a shear, or lateral, force similar to those found in a strike-slip fault like the one in California. San Andreas Fault. Even though the materials are different, the fracture mechanics are the same.
Once a crack begins, it acts as a one-dimensional line tearing through the material. Fineberg and his team had already shown But before the crack forms, the material develops a kind of precursor phase called a nucleation front. These nucleation fronts – crack seeds – move through the material, but much more slowly than conventional cracks. It was unclear how this seed could quickly turn into a rapid fracture.
Fineberg and his colleagues were perplexed as to how this could happen. By combining laboratory experiments and theoretical calculations, they realized they needed a mathematical update: nucleation fronts should be modeled in 2D, not 1D.
Instead of thinking of a crack as a line separating broken material from unbroken material, Fineberg said, think of the crack as an area that begins in the plane where two Plexiglas “plates” meet. The energy required to break new material at the patch boundary is related to the perimeter of the patch: as the perimeter increases, the energy required for the new material to crack also increases.
This means that the area is moving slowly and is not yet causing rapid fracture that would create the seismic waves and shaking movements associated with an earthquake. While the rapid acceleration of a standard, fast crack releases kinetic energy into the surrounding material, the slow motion of the initial part releases no kinetic energy into its surroundings. Therefore, its movement is called “aseismic”.
Eventually, however, the zone expands outside of the weak zone where the two plates meet. Outside of this area, the energy required to break new material no longer increases with the size of the broken region, and instead of an energy balance, there is now excess energy that has to go somewhere .
“This additional energy now causes the explosive movement of the crack,” Fineberg said.
The results, published on January 8 in the journal Natureshow how a slow creep before a crack can quickly turn into an earthquake, he said. Theoretically, if one could measure aseismic movement before a rupture – on a fault line, for example, or even in a mechanical object like an airplane wing – it would be possible to predict a rupture before it occurs. produce. This can be complicated in the case of real faults, many of which experience aseismic creep over long periods of time. without triggering earthquakes.
Nevertheless, Fineberg and his team are now trying to detect signs of a transition from aseismic to seismic in their laboratory materials.
“In the lab, we can watch this thing unfold and listen to the noises it makes,” Fineberg said. “So maybe we can find out what you can’t really do in a real fault, because you don’t have any detailed information about what an earthquake does until it explodes.”