A phase of gradual, subtle motion without any tremor might be an essential precursor to earthquakes, according to a recent study.
The investigation, which centered on the principles of how materials fracture, examined splits coursing through sheets of plastic in a controlled environment. The experiments disclosed some fundamental physics of how breaks occur — particularly how the accumulation of friction at the junction of two entities leads to a sudden failure. And these results are relevant to actual earthquakes, asserted study author Jay Fineberg, a physicist at The Hebrew University of Jerusalem.
“The substance comprising the contacting plates will be inconsequential,” Fineberg informed Live Science. “The identical physical mechanism will occur in both scenarios — the explosive release of the bent plates will unfold similarly.”
Earthquakes occur when two tectonic plates sliding against each other become lodged, permitting the fault to accumulate tension. “The plates are progressively stressed by the forces attempting to shift them, but are immobilized at the brittle region of the interface that distinguishes them,” Fineberg explained. This brittle area, which does not deform under stress, has a limited thickness and is what fractures during a seismic event.
“The fracturing mechanism does not transpire instantaneously. Initially, a crack must be established,” Fineberg stated. When that crack extends to the edges of the brittle interface, it then accelerates swiftly to velocities approaching those of sound. That is what induces the earth to tremor.
“The inquiry is how does nature initiate the crack that subsequently evolves into an earthquake?” Fineberg posed.
Fineberg and his associates examined this query using a combination of theoretical mathematics and laboratory trials. They replicated earthquake-like fractures in the laboratory with blocks made of a thermoplastic known as polymethyl methacrylate, commonly referred to as plexiglass. The researchers clamped sheets of plexiglass together and exerted a shear, or lateral, force, akin to those experienced at a strike-slip fault such as California’s San Andreas Fault. Although the materials differ, the mechanics of the fracture remain consistent.
Once a crack initiates, it behaves like a one-dimensional line tearing through the material. Fineberg and his group had previously demonstrated that prior to the formation of a crack, the material develops a precursor phase termed a nucleation front. These nucleation fronts — the precursors of cracks — traverse the material, albeit at a significantly slower rate than typical cracks. It remained unclear how this seed could swiftly transition into a rapidly propagating fracture.
Fineberg and his colleagues were baffled about how this phenomenon occurred. Through a blend of laboratory tests and theoretical computations, they recognized that they required a mathematical revision: The nucleation fronts should be modeled in 2D, rather than 1D.
Instead of conceptualizing a crack as a dividing line between broken and unbroken material, Fineberg suggested envisioning the crack as a patch originating within the plane where two plexiglass “plates” converge. The energy required to fracture new material at the edges of the patch correlates with the perimeter of the patch: As the perimeter expands, so does the energy needed for new material to fracture.
This indicates that the patch progresses slowly without immediately causing a rapid fracture that would generate the seismic waves and ensuing shaking linked with an earthquake. While the swift acceleration of a typical rapid crack releases kinetic energy into the surrounding material, the gradual movement of the initial patch does not discharge any kinetic energy into its environment. Consequently, its movement is termed “aseismic.”
Eventually, however, the patch extends beyond the brittle zone where the two plates converge. Outside this zone, the energy needed to fracture new material no longer escalates with the size of the fractured area, and instead of maintaining a balance of energy, there now exists excess energy that must dissipate somewhere.
“This surplus energy now incites the explosive motion of the crack,” Fineberg remarked.
The results, published on Jan. 8 in the journal Nature, illustrate how a gradual creep preceding a crack can quickly transform into an earthquake, he noted. Theoretically, if one could measure aseismic movement prior to a rupture — on a fault line, for example, or even in a mechanical object such as an airplane wing — it might be feasible to predict a break before it occurs. This may prove intricate in actual faults, many of which experience aseismic creep over extended periods without generating any earthquakes.
Nonetheless, Fineberg and his team are currently seeking to identify indicators of the conversion from aseismic to seismic in their laboratory materials.
“In the laboratory, we can observe this process unfold and we can hear the sounds it produces,” Fineberg stated. “Thus, perhaps we can discover what is not easily discernible in a real fault, as detailed information about an earthquake is not available until it erupts.”