On the United States Geological Survey’s (USGS) official website, a rather curious question can be found: “Can the ground open up during an earthquake?” it reads, evoking images of Hollywood movies where the ground spreads apart and swallows up cars and people whole.

“Faults,” the response begins, “do not open up during an earthquake. Movement occurs along the plane of a fault, not perpendicular to it. If faults opened up, no earthquake would occur because there would be no friction to lock them together.”

A rather stunning new study in the journal Nature, however, has provided evidence which refutes this. Not only can faults open up and close laterally during an earthquake, but they do so quickly and violently, like a lion's maw.

A team of engineers at Caltech and the École Normale Supérieure (ENS) in Paris took a look at the 9.0M Tohoku earthquake that generated a truly devastating tsunami back in 2011. In this case, a large volume of water was pushed forward by a thrust fault, wherein one massive slice of crust was pushed up above another.

With so much energy involved in this so-called “megathrust” earthquake, the team was wondering if it was possible that instead of just grinding across each other, the two segments of crust actually split apart at any point. Standard earthquake models explicitly prevent faults from opening like this, as it’s thought to be physically impossible.

In order to investigate this, they turned to Caltech’s “Seismological Wind Tunnel,” a high-speed advanced optical facility that can simulate and precisely observe laboratory-generated earthquakes.

Using realistically fragmented transparent blocks (to simulate the fractured fault lines) and compressional units (to replicate the tectonic pressure on the faults), the team placed a small fuse in the center. Upon its detonation – the trigger for their simulated earthquake – they watched the stress waves propagate through the blocks, and carefully tracked the blocks’ responses.

^{Illustration based on computer model shows how the hanging wall (right) of a thrust fault can twist away from the foot wall (left) during an earthquake. Harsha Bhat}

To their surprise, when the stress waves reached the surface of the fault, their high-speed camera clocked it splitting open before rapidly snapping shut. Similar computer simulations of the same type of thrust fault quake confirmed their suspicions – given enough energy, these particular faults can rupture open laterally, whether they are open to the atmosphere or buried deep underwater.

Although the open-and-close mechanism happens over incredibly short timescales – perhaps no more than a few seconds, or less – this suggests that cities living directly on fault lines are in greater trouble than previously thought.

When it takes place underwater, as in the Tohoku quake of 2011, this snapping would likely move even more water than just a regular thrust movement would, which would ultimately produce a more powerful tsunami.