AUSTIN (KXAN) — Off the coast of Japan, one of the most dangerous fault zones on Earth has become the focal point of a decades-long research project done by the University of Texas and more than a dozen other countries.
"It has a really long history of what we call great earthquakes," said Demian Saffer, director of the Institute of Geophysics at UT and a leader on the project.
What Saffer and his team uncovered may reshape the way we think about forecasting earthquakes.
The fault zone, which should be nearing the end of its seismic cycle and a major earthquake, is "not really stressed as much as we thought it would be," Saffer said.
Signs of stress are expected as an earthquake nears and is one of the factors scientists use to predict them. If this fault zone, which last produced a major earthquake in 1946, is not showing the signs it should.
"It throws a little bit of a wrench in the works in the way we were thinking about these systems," Saffer said.
The team chose this particular fault zone for several reasons.
"The Japanese were deeply interested in it. For them. It's a matter of national security to understand earthquakes," Saffer said.
The fault zone is also a subduction zone similar to the ones near Alaska and the Pacific Northwest.
"Subduction zones are capable of very large earthquakes, magnitude nine scale, similar to the Tohoku earthquake in 2011."
That earthquake caused a tsunami that swept across Japan, destroying part of a nuclear power plant.
The fault zone, known as the Nankai Trough, last produced a major earthquake 80 years ago. The Nankai Earthquake of 1946 killed more than 1,300 people and injured thousands. It registered between an 8.1 and 8.4 on the moment magnitude scale.
To obtain their data, the team took multiple trips to the area and drilled into the fault zone. To reach the required depth, they need to have their drill go a mile-and-a-half underwater, then about two miles into the actual ground. The drill was only a mile short of the actual fault line.
The team used techniques commonly used by people building oil rigs to drill that deep. They required a massive ship, The Chikyu, which they rented from the Japanese government.
As the drill descended, metal tubing was placed to hold the borehole open.
"This is an active plate boundary, a tectonic plate boundary. So it's a little bit more of a challenging environment to drill in some ways," Saffer said.
Due to costs (it's expensive to rent a ship and drill underwater) the mission took place over nearly 20 years. The team would go out, dig a little, cap the hole and return at a later date.
"It's such a significant endeavor that it couldn't even be done in sort of one season," Saffer said. He has been on eight of the expeditions.
The drill was outfitted with equipment that gathered data as it went.
"What's the temperature? What's the pressure?" Saffer said they looked for cracks in the ground, the warping of the borehole itself and other signs of stress.
In 2018, the drill reached its current depth. It's here the team found the stress wasn't as high as they expected. In fact, it was close to zero. This occurs when an earthquake has recently occurred.
"We expected that it would, we'd see signs of increasing stress that would be consistent with this kind of gradual build up as the plates are, are moving past each other and that that fault line is locked up until it — until the next earthquake."
The researchers came to several conclusions: either the fault didn't need to be as stressed to cause a major earthquake or the stress is occurring at a much deeper level, closer to the actual fault line.
The final conclusion was the scariest. The stress would come on suddenly, right before a major earthquake.
Saffer said the data "throws a little bit of a wrench in the works in the way we were thinking about these systems."
The team hopes to return for a future expedition and dig deeper for answers.
