Ultra-deep drilling reveals mysteries of Japan tsunami

ITHACA, N.Y. – An international marine research team guided by Cornell University expertise has successfully completed an ambitious drilling project to investigate the plate boundary fault that ruptured during the devastating Tohoku earthquake in 2011.

At an extreme depth of 7 kilometers, the team drilled a series of deep boreholes, including a sub-seafloor borehole observatory that intersects the fault nearly 1 kilometer beneath the seafloor. They also conducted geophysical logging and coring, and reinstalled temperature sensors in a previous observatory well across the fault – a feat that had never before been attempted at such depths.

The team’s geologic and hydrologic testing will increase understanding of subduction zones and lead to better preparation for large earthquakes and tsunamis, according to Patrick Fulton, assistant professor of earth and atmospheric sciences at Cornell, who served as a co-chief scientist  on the project.

“The main technical challenge is that we’re working at a water depth of 7 kilometers, and then we go another kilometer underground, and so there aren't many ships that can operate in that extreme depth,” Fulton said. “It’s kind of like a NASA mission.”

The drilling expedition – officially known as the Tracking Tsunamigenic Slip Across the Japan Trench (JTRACK) project of the International Ocean Discovery Program (IOPD) – followed a similar effort in 2012, when Fulton and the IODP first ventured to the Japan Trench to study how and where the magnitude 9.1 earthquake – one of the largest ever recorded – originated.

In that rupture, the shallowest part of the fault slipped 50 to 60 meters, and “the seafloor itself jumped half a football field to the east over the course of a minute or two,” triggering a catastrophic tsunami that was much larger than anyone had expected, according to Fulton.

By studying the fault’s properties and conditions soon after the earthquake, the researchers sought to determine what caused such a significant rupture and why it had been so difficult to predict.

“Before we did this in 2012, no one had really tried to essentially drill a kilometer underground in such extreme water depth, let alone build an observatory or put geophysical tools down to try to characterize what’s in there,” Fulton said.

The observatory gave the team an up-close look at the conditions within the fault, revealing an anomalous heat signal caused by frictional heating on the fault during the earthquake. The unique data indicated that the fault was very weak. The observatory also revealed how, during aftershocks, faults and fractures opened up, with springs of water moving through them, which ultimately change the stress conditions within the system.

“So now, returning 12 years later, the down-going plate is still moving at about 10 centimeters per year. Has it stuck in certain spots? Has the fault started to build up stress, enough that maybe the shallow part could have another big earthquake? We want to see how those processes have evolved,” Fulton said. “All these things – coring, logging and observatory recording – we could just do little bits of 12 years ago, now we want to do a much deeper characterization of all the faults and fractures, and also look at the incoming plate before it subducts.”

The project concluded, in December, with the construction of a new, even deeper borehole observatory.

Fulton expects the data they’ve collected will generate a number of research papers in the coming years.

“It’s a big project. We don’t have the budget of [NASA's] Europa Clipper, but, I think, it’s in that kind of style,” Fulton said. “Lots of big earthquake science and earthquake physics knowledge came from our previous expedition. We’re back there, and we’re already learning a lot.”

For additional information, see this Cornell Chronicle story.

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