8 JULY 2023 WorldWide Drilling Resource® Analysis of Drilling into the Nankai Earthquake Fault Adapted from Information by University of Texas Back in 2018, scientists drilled deeper into an undersea earthquake fault than ever before - some two miles into the tectonic plate beneath the seafloor. Researchers at the University of Texas at Austin conducted a study on the data collected from the expedition and discovered the tectonic stress in Japan’s Nankai subduction zone is less than expected. The findings are puzzling because the fault is known to produce a great earthquake nearly every century and it was thought to be about time 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 locked for decades, the study indicates it is not showing major signs of pent-up tectonic stress. According to Saffer, this shouldn’t change the long-term outlook for the fault, which last ruptured in 1946. This led to a tsunami which killed thousands, something researchers expect to happen again during the next 50 years. The findings from this study will help scientists to focus on the link between tectonic forces and the earthquake cycle and could potentially lead to better earthquake forecasting for the Nankai, as well as other megathrust faults such as Cascadia in the Pacific Northwest. “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 Harold Tobin, a researcher at the University of Washington who is the first author of the paper. “But I have some optimism that with more and more direct observations like this, 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.” Megathrust faults and the tsunamis they generate are among the most powerful and damaging on the planet. Unfortunately, scientists currently have no reliable way of knowing when and where the next big one will hit. The hope is by directly measuring the force felt between tectonic plates pushing on each other - tectonic stress - scientists can learn when a great earthquake is about to happen. However, the nature of tectonics means the great earthquake faults are located deep in the ocean, miles under the seafloor, making them incredibly challenging to measure directly. Saffer and Tobin’s drilling expedition is the closest scientists have come so far. The 2018 record-breaking drilling project took place aboard a Japanese scientific drilling ship, the Chikyu, which was forced to stop drilling a mile short of the fault because the borehole became too unstable to continue. Yet, researchers gathered invaluable data about subsurface conditions near the fault, including stress. 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 the walls back out. This 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 since the most recent great earthquake was close to zero, as if it had already released its pent-up energy. Researchers have a few possible explanations: It could be the fault simply needs less pent-up energy to slip in a big earthquake than originally thought. Perhaps, the stresses are lurking nearer to the fault than the drilling reached. Or, it could be the tectonic push will come suddenly in the coming years. Either way, researchers said drilling revealed the need for further investigation and long-term monitoring of the fault. One of the dozens of risers used to drill into the Nankai earthquake fault. Photo by Demian Saffer courtesy of UTIG. ENV
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