Engineering news
Research from the California Institute of Technology (Caltech) and the École Normale Supérieure (ENS) in Paris has revealed how the ground can split open during quakes, potentially swallowing cars and people before quickly slamming shut.
Despite being a common image in disaster movies, scientists previously maintained that slabs of rock slid against each other during quakes, when the force of the pressure between them overcame the friction holding them in place.
However, by using a combination of specialist materials, high-speed cameras and precision instruments, the international team revealed the two sides of rock can twist away from each other, opening a gap of up to a few metres before snapping shut.
Previous computer models “have been programmed in a way that dictates that the walls of the fault cannot separate from one another,” said Ares Rosakis, a mechanical engineer at Caltech and one of the senior authors of the paper. “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.”
"They placed a small
nickel-chromium wire
fuse to simulate the
epicentre of the quake"
The new physical model in a Caltech facility represented a thrust fault, which occur in weak areas of the Earth’s crust where one slab of rock compresses against another, sliding up and over it during an earthquake. Thrust faults have been the site of some of the world’s largest quakes, such as the magnitude 9 Tohoku earthquake off the coast of Japan in 2011, which damaged the Fukushima nuclear power plant. According to the British Geological Survey, there are an average of 15 earthquakes every year with a magnitude of 7 and above.
At the Caltech facility, researchers used high-speed optical diagnostics to study how earthquake ruptures occur. To simulate a thrust fault earthquake in the lab, the researchers first cut in half a transparent block of plastic with mechanical properties similar to that of rock. They then put the broken pieces back together under pressure, simulating the tectonic load of a fault line. Next, they placed a small nickel-chromium wire fuse to simulate the epicentre of the quake. When they set off the fuse, the friction at the fuse's location was reduced, allowing a very fast rupture to move up the miniature fault.
The clear photoelastic material showed stress waves travelling through as light interference, recorded using high-speed cameras and captured by laser velocimeters, a type of particle speed sensor. When the rupture hit the surface, the plastic twisted open and snapped shut. The result was also seen in subsequent computer simulations, which removed the artificial rules against the fault opening.
The model shows how the hanging wall (right) of a thrust fault can twist away from the foot wall (left) during an earthquake. (Credit: Harsha Bhat/ENS)
With thrust faults happening both on land and underwater, the research has the potential to change the understanding of how tsunamis are generated.
"I am sure that now people will be much more careful in modelling earthquakes that reach the bottom of the sea or emerge to the surface," said ENS professor Raul Madariaga.
Harsha Bhat from the ENS said the research was a great example of collaboration between not only engineers, seismologists and tectonisists, but also between France and the US.
The British Geological Survey was contacted but declined to comment on the usefulness of the simulation.
The report has been published in the May 1 issue of Nature.