Scientists find a hidden mechanism that could explain how earthquakes ignite

A new study suggests that a period of slow, creeping motion without any shaking may be a necessary precursor to earthquakes.

The research, which was about the basics of how materials tear, focused on cracks that creep through sheets of plastic in the laboratory. But the experiments have revealed some basic physics about how fractures work, especially how the buildup of friction at the interface between two bodies turns into a sudden rupture. Study author Jay Feinberg, a physicist at the Hebrew University of Jerusalem, said these findings apply to real-world earthquakes.

“The material the communication panels are made of won't matter,” Feinberg told Live Science. “The same physical process will occur in both cases – the explosive spring of the bent plates will be released in the same way.”

Earthquakes are formed when tectonic plates move against each other, allowing the fault to build up stress. “The plates are increasingly stressed by forces trying to move them, but they are stuck at the fragile part of the interface that separates them,” Feinberg said. This brittle part, which does not deform as a result of pressure, has a limited thickness and is what breaks during an earthquake.

“The fracture process doesn't happen all at once. First, you have to create a crack,” Feinberg said. When this crack reaches the boundary of the brittle 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 a fault that then turns into an earthquake?” Feinberg.

Feinberg and his colleagues investigated the question with a combination of theoretical mathematics and laboratory experiments. They are reproducing earthquake-like fractures in the laboratory using blocks made of a thermoplastic material called polymethyl methacrylate, commonly known as plexiglass. The researchers bonded sheets of plexiglass together and applied a shearing, or lateral, force, similar to that found on a strike-slip fault like the San Andreas Fault in California. Although the materials are different, the fracture mechanism is the same.

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Once a crack begins, it acts as a one-dimensional line that tears the material. Feinberg and his team had previously shown that before a crack forms, the material develops a kind of initial phase called a nucleation front. These nucleation fronts – crack seeds – move through the material, but much more slowly than standard cracks. It was not clear how this seed could quickly transform into a fast-moving fracture.

Feinberg and his colleagues were puzzled as to how this could happen. Through a combination of laboratory experiments and theoretical calculations, they realized they needed to update the mathematics: nucleus fronts needed to be modeled in 2D, not 1D.

Instead of thinking of a crack as a line separating unfractured materials, Feinberg said, imagine the crack as a patch that begins inside the plane where two glass “sheets” meet. The energy needed to break down new material at the patch's boundaries is related to the patch's perimeter: as the perimeter grows, so does the energy needed to break down new material.

This means that the patch is moving slowly and is not yet causing rapid fracture that would create seismic waves and subsequent shaking motion associated with an earthquake. While the rapid acceleration of a fast scalar releases kinetic energy into the surrounding matter, the slow motion of a primary patch releases no kinetic energy into its surroundings. Therefore, its movement is known as “seismic”.

Eventually, the patch expands beyond the fragile zone where the two plates meet. Outside of this zone, the energy needed to break down new material no longer grows with the size of the broken zone, and instead of an energy balance, there is now excess energy that needs somewhere to go.

“This additional energy is now causing the explosive movement of the fault,” Feinberg said.

He said the results, published January 8 in the journal Nature, show how slow creep before a fault occurs can quickly turn into an earthquake. In theory, if one could measure non-vibrational motion before rupture occurs—on a fault line, for example, or even in a mechanical object such as an airplane wing—it might be possible to predict fracture before it occurs. This can be complicated on real-world faults, many of which undergo seismic creep over long periods of time without triggering any earthquakes.

However, Feinberg and his team are now trying to detect signs of the transition from seismic to seismic in their laboratory materials.

“In the lab, we can watch this thing unfold, and we can listen to the noise it makes,” Feinberg said. “So maybe we can figure out what you can't really do in the case of a real fault, because you don't have detailed information about what the earthquake does until it goes off.”

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