A new study was conducted by a team from Hebrew University of Jerusalem and published at Nature Journal on January 8, about how a slow, aseismic movement transitions into rapid seismic rupture, providing insights into the physics of fractures and earthquake formation. The researchers suggest that a phase of slow, steady plate movement, even if the shaking is imperceptible, could serve as a critical precursor to earthquakes.
The research was based on laboratory experiments and studied the mechanics of material rupture, on cracks forming in sheets of plastic. After the experiments, the fundamental physics that explained how fractures occur were found. Specifically, it was explained how frictional buildup between two surfaces transform into sudden rupture.
It should be noted that earthquakes occur when two tectonic plates, moving in opposite directions, get stuck and accumulate stress at the fault line. Jay Fineberg, a physicist at The Hebrew University, elaborated that tectonic plates under extreme stress from opposing forces remain locked at the fracture-prone interface between them. Furthermore, he added that the principles that came up after the experiments are applicable to real-world earthquakes as the material type in the contacting plates doesn’t really change the outcome. In addition, he explained that in both cases, the physical process remains the same as the sudden release of stress stored in the bent plates will behave identically. Last, he added that the ultimate fracture doesn’t happen instantly as a crack needs to be initially formed. After the crack occurs, it will reach the edges of the fracture-prone area. When that happens, it will rapidly accelerate to near the speed of sound and will cause the ground to shake.
To conduct this experiment, the research team Fineberg combined theoretical modeling with laboratory experiments, to recreate earthquake-like fractures by using sheets of polymethyl methacrylate (plexiglass). After they clamped the sheets together and applied shear forces similar to those at strike-slip faults as is California's San Andreas Fault, they found that the fracture mechanics in plexiglass closely mimic those in real tectonic faults.
They noticed also that right before a crack will fully develop, the material faces a precursor phase involving a nucleation front. These nucleation fronts, which act as the seeds of cracks, move much slower than typical fractures. Additionally, the experiments helped them to update mathematical models and determine that nucleation fronts should be examined in two dimensions.
Therefore, they simulated the crack as a growing patch within the plane where two plexiglass sheets meet instead of a line separating broken and unbroken material, hence, the energy required to break new material increases with the perimeter of the patch. With this simulation they concluded that this slow expansion doesn’t release kinetic energy into the surrounding material, making it "aseismic."
In spite of that, they also noted that when the patch grows beyond the brittle zone where the two plates meet, the energy required to create new fractures no longer increases with the patch's size. Therefore, the excess energy that become available, is triggering the rapid acceleration of the crack and ultimate it creates the seismic waves associated with an earthquake.
Sources: livescience.com, gadgets360.com, natureworldnews.com
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