If you sat through movies like 2012 or San Andreas, you probably remember scenes where the ground cracks wide open, sending people, cars and buildings tumbling down into an abyss, before the gap snaps shut like the jaws of some huge hungry beast. It may seem exaggerated for the sake of a good visual, but a new study out of Caltech has demonstrated that this can actually happen, contrary to the beliefs of geologists.
Along a thrust fault line, two slabs of rock are pressing against each other, held in place by friction. As the pressure builds up, it can eventually become stronger than that friction and shift the plates, resulting in an earthquake. Until now, it was believed that in this type of fault line, the plates would simply rub against each other, moving a short distance before settling back down, and never opening a gap at the surface.
But the Caltech researchers found that the assumption didn't line up with real-world observations. When they investigated the site of the 2011 Tohoku earthquake – which caused a tsunami and in turn, the Fukushima nuclear disaster – off the coast of Japan, the team found that one side of the fault had slipped, in some areas as far as 50 m (164 ft). Their hypothesis was that the quake caused one chunk of rock to twist away from the other completely, creating a large gap that then allows one side to slip – a phenomenon normally thought impossible.
To try to recreate it in the lab, the researchers used a block of plastic that's designed to mimic the mechanical properties of rock. First, they simulated a fault line by cutting the material in half, then applying pressure to hold the two pieces together. Being transparent, the material allows the researchers to see the shock waves rippling through the plastic, and the whole experiment was captured on high-speed cameras and particle speed sensors.
The team set off a fuse at a given location along the fault line, which sent shock waves up through the fault to the surface. Sure enough, one half twisted and opened a crack, before quickly closing up again. These findings challenge the assumption that this reaction was impossible, a view so prevalent that it's built right into earthquake simulation systems.
"The models have been programmed in a way that dictates that the walls of the fault cannot separate from one another," says Ares Rosakis, senior author of the study. "The findings demonstrate the value of experimentation and observation. Computer models can only be as realistic as their built-in assumptions allow them to be."
When the team recreated the scenario using software that had been modified to remove those arbitrary limitations, they found that the data agreed with their observations. This could lead to improved understandings of earthquakes and how they spawn tsunamis – as well as maybe giving disaster filmmakers a little more credibility.
The research was published in the journal Nature.
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