Deepest scientific ocean drilling sheds light on Japan’s next major earthquake

Scientists who have drilled deeper than ever into an underwater earthquake fault have found that tectonic stress in Japan’s Nankai subduction zone is less than expected, according to a study by researchers from the University of Texas at Austin and the University of Washington.

The results published in the journal geologyare a mystery because the fault creates a major earthquake almost every century and was thought to be building up for another major earthquake.

“This is the heart of the subduction zone, just above the fault, where the expectation was that the system should store energy between earthquakes,” said Demian Saffer, director of the University of Texas Institute of Geophysics (UTIG) who co-led the research – and science mission that drilled the fault. “It changes the way we think about stress in those systems.”

Although the Nankai Fault has been stuck for decades, the study shows that it is yet to show major signs of pent-up tectonic stress. According to Saffer, that doesn’t change the long-term prospects for the fault, which last ruptured in 1946 — when it created a tsunami that killed thousands — and is expected to do so again for the next 50 years.

Instead, the results will help scientists see the link between tectonic forces and the earthquake cycle, potentially leading to better earthquake predictions, both at Nankai and at other megathrust faults like Cascadia in the Pacific Northwest.

“Right now, we can’t know if the big one for Cascadia — a magnitude 9 earthquake and tsunami — will happen this afternoon or in 200 years,” said Harold Tobin, a researcher at the University of Washington who is the lead author of the Treatise. “But I’m optimistic that with more and more direct observations like this, we can start to see when something anomalous is occurring and that earthquake risk is elevated in a way that could help people prepare.”

Megathrust faults like Nankai and the tsunamis they generate are among the most powerful and damaging in the world, but scientists say they currently have no reliable method of knowing when and where the next major fault will strike.

The hope is that by directly measuring the force felt between colliding tectonic plates — tectonic stress — scientists can learn when a major earthquake is imminent.

However, the nature of tectonics means that the large earthquake faults are found in the deep sea, miles beneath the sea floor, making them incredibly difficult to measure directly. Saffer and Tobin’s drilling expedition is the closest scientists have yet witnessed.

Their record-breaking attempt took place in 2018 aboard a Japanese scientific drillship, the Chikyu, which drilled 2 miles into the tectonic plate before the well became too unstable to continue, a mile from the fault.

Still, researchers collected invaluable data on subsurface conditions near the fault, including stress. To do this, they measured how much the borehole changed shape as the earth pushed it from the sides, and then pumped water to see what it took to push its walls back out. That told them the direction and strength of the horizontal stress felt by the plate pushing on the fault.

Contrary to predictions, the horizontal stress expected since the recent major earthquake was near zero, as if it had already released its pent-up energy.

The researchers suggested several explanations: it could be that the fault simply requires less pent-up energy than thought to slip in a large earthquake, or that the stresses lurk closer to the fault than drilling reached. Or it could be that the tectonic surge will come suddenly in the coming years. However, the researchers said the drilling indicated the need for further exploration and long-term monitoring of the fault.

The research was funded by the Integrated Ocean Drilling Program and the Japan Agency for Marine-Earth Science and Technology. UTIG is a research unit of UT Austin’s Jackson School of Geosciences.