Thursday, May 11, 2017

Mega-Quakes Can Cause Earth’s Crust to Rip Open and Snap Shut

[Live Science] Like a crocodile's jaw opening and snapping shut, Earth's crust can rip apart and then violently close back up during an earthquake, a new study finds. The discovery refutes previous claims that this kind of phenomenon was impossible, and the new research could potentially require that current seismic maps be redrawn.
The study focused on a particular paradox associated with thrust faults, a crack in Earth’s crust, where geologic forces are slowly pushing a huge slab of continental crust up and over an oceanic layer.
"For a long time, it was assumed that thrust faults, subduction zone faults being a class of such faults, could not have a large amount of slip close to the Earth's surface," said Harsha Bhat, a research scientist at the École Normale Supérieure (ENS) in Paris and co-author of the new study with California Institute of Technology graduate student Vahe Gabuchian. [The 10 Biggest Earthquakes in History]
The assumption was made because as the continental slab grinds over the oceanic one below, it scrapes off the soft surface clay and leaves it piled up in the subduction zone. Geologists thought that any energy generated from a seismic event within the thrust fault would peter out once it hit the soft clay and that a large slip wouldn't happen near the surface.
But clues from past earthquakes suggested otherwise, said Christopher Scholz, a professor of geophysics at Columbia University's Lamont-Doherty Earth Observatory in New York City. The San Fernando earthquake that occurred in 1971, for example, left behind an unusual pile of debris that anyone can still see today, said Scholz, who was not involved with the new study.
"It's right at the base of a mountain," he said. "The thrust comes out at a low angle, and it looks like [the earthquake] flapped the whole soil layer, just flipped it over below the fault."
How did the earthquake cause such a giant amount of material to flip over if the energy dissipated in the clay?
Geophysicist James Brune, then at the University of Nevada was the first scientist to attempt to answer that question in a 1996 study he published in the Proceedings of the Indian Academy of Science, Scholz said. Brune figured it was the result of some kind of torquing action in the fault. He conducted an experiment using foam rubber that showed the energy of a simulated earthquake propagating down a fault and flipping the tip — as if some large hand were cracking a whip. Read More