The findings, published in the journal Geology, are a puzzle because the fault produces a great earthquake almost every century and was thought to be building for another big one.

"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 Demian Saffer, director of the University of Texas Institute for Geophysics (UTIG) who co-led the research and scientific mission that drilled the fault. "It changes the way we're thinking about stress in these systems."

Although the Nankai fault has been stuck for decades, the study shows that it is not yet showing major signs of pent-up tectonic stress. According to Saffer, that 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.

Instead, the findings will help scientists home in on the link between tectonic forces and the earthquake cycle and potentially lead to better earthquake forecasts, both at Nankai and other megathrust faults such as Cascadia in the Pacific Northwest.