Scientists who drilled deeper into an undersea earthquake fault than ever before have found that the tectonic stress in Japan’s Nankai subduction zone is less than expected. The results of the study led by the University of Washington and the University of Texas at Austin, published Sept. 5 in Geology, are a puzzle, since the fault produces a great earthquake almost every century and was thought to be building for another big one. Although the Nankai fault has been stuck for decades, the findings reveal that it is not yet showing major signs of pent-up tectonic stress. Authors say the result doesn’t alter the long-term outlook for the fault, which last ruptured in 1946, when it caused a tsunami that killed thousands, and is expected to do so again during the next 50 years. The findings will help scientists home in on the link between tectonic forces and the earthquake cycle. This could potentially lead to better earthquake forecasts, both at Nankai and other megathrust faults, like the Cascadia subduction zone off the coast of Washington and Oregon. “Right now, we have no way of knowing if the big one for Cascadia — a magnitude-9 scale earthquake and tsunami — will happen this afternoon or 200 years from now,” said lead author Harold Tobin, a UW professor of Earth and space sciences and co-chief scientist on the drilling expedition. “But I have some optimism that with more and more direct observations like this one from Japan we can start to recognize when something anomalous is occurring and that the risk of an earthquake is heightened in a way that could help people prepare. “We learn how these faults work by studying them all over the world, and that knowledge will directly translate into insight into the Cascadia hazard as well.” Megathrust faults such as Nankai and Cascadia, and the tsunamis they generate, are among the most powerful and damaging on the globe. Scientists say they currently have no reliable way of knowing when and where the next big one will hit. The hope is that by directly measuring the force felt between tectonic plates pushing on each other — tectonic stress — scientists can learn when a great earthquake is ready to happen. “This is the heart of the subduction zone, right above where the fault is locked, where the expectation was that the system should be storing energy between earthquakes,” said co-author Demian Saffer at University of Texas at Austin, who also co-led the scientific drilling expedition. “It changes the way we’re thinking about stress in these systems.” The nature of tectonics means that the great earthquake faults are found in deep ocean, miles under the seafloor, making them incredibly challenging to measure directly. Tobin and Saffer’s drilling expedition is the closest scientists have come. Their record-breaking feat took place in 2018 aboard a Japanese scientific drilling ship, the Chikyu, which drilled almost 2 miles, or just over 3 kilometers, into the tectonic plate before the borehole got too unstable to continue — 1 mile short of the fault. Nevertheless, the researchers gathered invaluable data about subsurface conditions near the fault, including stress. To do that, they measured how much the borehole changed shape as the Earth squeezed it from the sides, then pumped water to see what it took to force its walls back out. That told them the direction and strength of horizontal stress felt by the plate pushing on the fault. Contrary to predictions, the horizontal stress expected to have built up since the most recent great earthquake was close to zero, as if the system had already released its pent-up energy. The researchers suggested several explanations: It could be that the fault simply needs less pent-up energy than thought to slip in a big earthquake, or that the stresses are lurking nearer to the fault than the drilling reached. Or it could be that the tectonic push will come suddenly in the coming years. Either way, the researchers said the drilling showed the need for further investigation and long-term monitoring of the fault. “Findings like this can seem like they muddy the picture, because things aren’t as simple as our theory or models predicted they were,” Tobin said. “But that just means we’re gaining more understanding of how the real world works, and the real world is messy and complicated.” The research was funded by the Integrated Ocean Drilling Program and the Japan Agency for Marine-Earth Science and Technology, or JAMSTEC. Other co-authors are Takehiro Hirose at JAMSTEC and David Castillo at Insight GeoMechanics in Australia. For more information, contact Tobin at firstname.lastname@example.org or Saffer at email@example.com. Adapted from an article by the University of Texas at Austin..