The University of Texas is leading an earthquake study off the coast of Japan

AUSTIN (KXAN) — Off the coast of Japan, one of the most dangerous fault zones on Earth has become a focal point for decades a research project conducted by the University of Texas and more than a dozen other countries.

“There’s a really long history of what we call large earthquakes,” said Demian Saffer, director of the Geophysics Institute at UT and leader of the project.

What Saffer and his team discovered could change the way we think about predicting earthquakes.

The fault zone, which should be near the end of its seismic cycle and a major earthquake, “is actually not as stressed as we thought it would be,” Saffer said.

Signs of stress are expected when an earthquake is approaching and is one of the factors scientists use to predict them. If this fault zone, which last produced a major earthquake in 1946, isn’t showing the signs it should.

“It kind of changes the way we thought about these systems,” Saffer said.

“Earth’s Biggest Earthquakes”

The team chose this particular fault zone for several reasons.

“The Japanese were very interested in it. For them. Understanding earthquakes is a matter of national security,” Saffer said.

The 1946 Nankai Earthquake was the last major earthquake to occur on the Nankai Slope.  (Courtesy: Public Domain)
The 1946 Nankai Earthquake was the last major earthquake to occur on the Nankai Slope. (Courtesy: Public Domain)

The fault zone is also a subduction zone similar to those near Alaska and the Pacific Northwest.

“Subduction zones are capable of very large magnitude nine earthquakes similar to the 2011 Tohoku earthquake.”

This 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 1946 Nankai earthquake killed more than 1,300 people and injured thousands. It registers between 8.1 and 8.4 on the moment magnitude scale.

Drilling five kilometers into an active fault zone

The Chikyu is a specialized scientific drilling vessel employed by the team.  (Courtesy: University of Texas)
The Chikyu is a specialized scientific drilling vessel employed by the team. (Courtesy: University of Texas)

To get their data, the team made several trips to the area and drilled into the fault zone. To reach the required depth, they have to make their drill go a mile and a half underwater, then about two miles into the actual land. The borehole was only a mile short of the actual fault line.

The team used techniques typically used by people who build oil rigs to drill that deep. They needed a massive ship, The Chikyu, which they leased from the Japanese government.

As the well was lowered, metal pipes were placed to keep the well open.

“It’s an active plate boundary, a tectonic plate boundary. So it’s a slightly more challenging environment to drill in some respects,” Saffer said.

Because of the cost (it’s expensive to charter a ship and drill underwater), the mission lasted nearly 20 years. The team would go out, dig a little, plug the hole and come back at a later date.

“This is such a significant undertaking that it can’t even be done in one season,” Saffer said. He participated in eight of the expeditions.

The drill was equipped with equipment that collected data as it went.

“What’s the temperature? What’s the pressure?” Saffer said they looked for cracks in the ground, distortion of the borehole itself and other signs of stress.

Detecting future earthquakes deep under the ocean

In 2018, the borehole reached its current depth. It was here that the team found that the stress was not as great as they expected. In fact, it was close to zero. This happens when there has been an earthquake recently.

“We expected that to happen, we would see signs of increasing stress that would be consistent with this kind of gradual build-up as the plates move past each other and that this fault line is locked until — until the next earthquake.”

  • The team prepares the drill for descent.  (Courtesy: University of Texas)
  • Deep underwater, the probe collected data as it descended.  (Courtesy: University of Texas)
  • Damian Saffer is director of the Geophysics Institute at the University of Texas.  He participated in eight of the expeditions exploring the fault zone.  (Courtesy: University of Texas)
  • Chikyu researchers believe the data they collected is changing the way we think about earthquakes.  (Courtesy: University of Texas)

The researchers came to several conclusions: either the fault did not have to be so stressed to cause a large earthquake, or the stress was happening at a much deeper level, closer to the actual fault line.

The final conclusion was most terrible. The stress came suddenly, just before a major earthquake.

Saffer said the data “throws a bit of a wrench in the way we’ve been thinking about these systems.”

The team hopes to return for a future expedition and dig deeper for answers.

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