Scientists who have delved deeper into an undersea earthquake fault than ever before have found that tectonic stress in Japan’s Nankai subduction zone is lower than expected, according to a study by researchers at the University of Texas at Austin and the University of Washington.
The results are published in the journal geologyis a mystery because the fault produces a major earthquake approximately every century and was thought to originate from another major earthquake.
“This is the core of the subduction zone, right above where the fault was locked, where the expectation was that the system should store energy between earthquakes,” said Damien Safire, director of the University of Texas Geophysics Institute (UTIG). He co-led the research and scientific mission that dug the bug. “It changes the way we think about stress in these systems.”
Although the Nankai Fault has been stuck for decades, the study shows that it has yet to show major signs of pent-up tectonic stress. According to Sfeir, this does not change the long-term view of the fault, which last erupted in 1946 – when it caused a tsunami that killed thousands – and is expected to happen again within the next 50 years.
Instead, the findings will help scientists learn about the link between tectonic forces and the earthquake cycle, and potentially lead to better earthquake predictions, both at Nankai and other faults such as Cascadia in the Pacific Northwest.
“Right now, we have no way of knowing if the big Cascadia earthquake – a magnitude 9 earthquake and tsunami – will happen this afternoon or 200 years from now,” said Harold Tobin, a researcher at the University of Washington. He is the first author of the paper. “But I have some optimism that with more and more direct observations like this, we can begin to recognize when something abnormal has occurred and that the risk of an earthquake is increasing in a way that can help people prepare.”
Giant confidence faults like Nankai and the tsunamis they generate are among the world’s most powerful and damaging, but scientists say they currently have no reliable way of knowing when and where the next major storm will strike.
The hope is that by directly measuring the felt force between tectonic plates pushing on each other – tectonic stress – scientists can tell when the Great Quake is ready to strike.
However, their tectonic nature means that large earthquake faults are found deep in the ocean, miles below the sea floor, which makes measuring them directly very difficult. The excavation expedition Saver and Tobin are the earliest scholars to have come.
Their record-breaking attempt took place in 2018 aboard a Japanese scientific drillship, the Chikyu, which drilled two miles into the tectonic plate before the well became too unstable to continue, a mile from error.
Nevertheless, researchers have collected invaluable data about subsurface conditions near the fault, including stress. To do this, they measured how much the shape of the well changed when the earth pressed against it from the sides, then pumped out water to see what it took to force its walls back down. This told them about the direction and strength of the horizontal pressure felt by the plate pressing on the fault.
Contrary to expectations, the expected horizontal stress created since the last great earthquake was close to zero, as if it had already released its pent-up energy.
The researchers suggested several explanations: It could be that the fault simply needed less pent-up energy than was thought to slip in a major earthquake, or that stresses lie closer to the fault than the craters have reached. Or a tectonic thrust could come on suddenly in the coming years. Either way, the researchers said, the drilling demonstrated the need for further investigation 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 in the Jackson School of Geosciences at the University of Austin.