Rowe was one of more than 20 scientists aboard the research ship Chikyu when they successfully drilled into the clay, which the researchers think lines the fault responsible for the 2011 earthquake. The drillers pierced through more than 2,700 feet (800 meters) of seafloor and 4 miles (7,000 m) of ocean to reach the fault.
According to seismic surveys, the fault at the three drill sites is relatively flat; a classic shape geologists call a décollement, the studies report. (The plate boundary fault actually extends for hundreds of miles, diving down into Earth's mantle beneath Japan.)
When the scaly clay that marks the plate boundary fault appeared on deck, scientists gathered around and peered at it through a plastic casing, grinning at the sight. Later, in the shipboard labs, researchers simply stared at it in awe for a while before divvying up samples, Rowe said.
"It was superexciting," she said. "We knew we had crossed the plate boundary."
The lustrous clay is likely less than 16 feet (5 m) thick — the top and bottom were lost in retrieving the core — and the layer switches color back and forth from black to ochre. The scaly texture is common in seismically tortured clays. It's so slippery it feels like a lubricant, Rowe said.
Laboratory tests conducted at the University of Tsukuba in Japan, led by research scientist Kohtaro Ujiie, confirm the clay is weak under stress. These experiments simulated different types of earthquakes, such as small, moderate and large. The research revealed that the clay becomes even more slippery when it's wet and exposed to extreme friction, such as during the 2011 quake, Ujiie reported in Science.
How hot was it?
Another key measurement that confirmed the shallow fault was slippery and weak during the 2011 earthquake was the team's temperature probe. After the rock sampling finished, drillers installed temperature sensors in a borehole across the fault, which were then collected by a remotely operated vehicle after nine months.
Friction during earthquakes produces massive amounts of heat at faults, just as rubbing your hands together generates warmth. The Tohoku quake was hot because it slid so far, generating a residual heat anomaly of less than 0.5 degrees Fahrenheit (0.31 degrees Celsius), Fulton reported. [7 Craziest Ways Japan's Earthquake Affected Earth]
The heat signal translates to a coefficient of static friction of 0.08, according to computer simulations — the same as car tires on an icy road or 0.01 greater than a rubber shoe stepping on a banana peel. (The coefficient of static friction is a measure of the force needed to make an object to move.)
"This is a really, really small number — many times less than what we generally thought most rocks had a friction coefficient of [such as 0.6], and it tells us that the fault had very little to zero resistance during the earthquake," Fulton said. "It was very slippery."
This friction data will be a critical puzzle piece in better understanding earthquakes, he said. It's one of the only direct friction measurements ever obtained from a fault after an earthquake.
"Frictional resistance on faults is a fundamental parameter that controls how earthquakes start and stop, and grow into giant earthquakes," Fulton said. "We're all in the business of trying to know more about the physics of earthquakes and predict them if possible. To do that, we need to know what controls how earthquakes get big and how they start and stop. This puts a constraint on that, and is some of the first real robust measurements of those parameters, especially in a subduction zone."