Deepest Scientific Ocean Drilling Effort Sheds Light on Japan’s Next ‘Big One’
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September 22, 2022
Scientists who drilled deeper than ever before into an undersea seismic fault found that tectonic tension 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 September 5 in Geology, are a puzzle because the fault produces a large earthquake almost every century and was thought to build some another.
Although the Nankai Fault has been blocked for decades, findings reveal that it is not yet showing major signs of pent-up tectonic stress. The authors say the result does not change 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 at again over the next 50 years.
The results will help scientists understand the link between tectonic forces and the earthquake cycle. This could potentially lead to better seismic predictions, both at Nankai and at other megathrust faults, such as the Cascadia subduction zone off the coasts of Washington and Oregon.
“At this time, we have no way of knowing whether the biggest for Cascadia – a magnitude 9 earthquake and tsunami – will occur this afternoon or 200 years from now,” the lead author said. Harold Tobin, UW Professor of Earth and Space Sciences and Co-Chief Scientist of the Drilling Expedition. “But I have some optimism that with more and more direct observations like this in Japan, we can start to recognize when something out of the ordinary is happening and the risk of an earthquake is increased in a way that could help people prepare.
“We are learning how these flaws work by studying them around the world, and this knowledge will also translate directly into insight into the danger of Cascadia.”
Megathrust faults such as Nankai and Cascadia, and the tsunamis they generate, are among the most powerful and damaging in the world. Scientists say they currently have no reliable way of knowing when or where the next big thing will strike.
The hope is that by directly measuring the force felt between tectonic plates pushing against each other – tectonic stress – scientists can tell when a big earthquake is ready to strike.
“This is the heart of the subduction zone, just above where the fault is locked, where the system was expected to store energy between earthquakes,” said the co-author. Demian Saffer at the University of Texas at Austin, who also co-led the scientific drilling expedition. “It changes the way we think about stress in these systems.”
The nature of tectonics means that large seismic faults lie deep in the ocean, miles below the seabed, making them incredibly difficult to measure directly. Tobin and Saffer’s drilling expedition is closest to scientists.
Their record was held in 2018 aboard a Japanese science drill ship, the Chikyu, which drilled nearly 2 miles, or just over 3 kilometers, into the tectonic plate before the borehole became too unstable to continue – 1 mile of the fault.
Nevertheless, the researchers collected invaluable data on subsurface conditions near the fault, including stresses. To do this, they measured how much the borehole changed shape when the Earth squeezed it from the sides, then pumped in water to see what it took to force its walls back. This told them the direction and strength of the horizontal stress felt by the plate pushing on the fault.
Contrary to predictions, the horizontal stress assumed to have accumulated since the last large earthquake was close to zero, as if the system had already released its accumulated energy.
The researchers suggested several explanations: it could be that the fault simply needs less pent-up energy than expected to slip through a large earthquake, or that the stresses are lurking closer to the fault than the drilling reaches. Or it could be that the tectonic push happens suddenly in the years to come. 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 to cloud the picture, because things are not as straightforward as our theory or our models predicted,” Tobin said. “But that just means we understand better 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 and Earth Science and Technology, or JAMSTEC. Other co-authors are Takehiro Hirose from JAMSTEC and David Castillo from Insight GeoMechanics in Australia.
Adapted from a article by the University of Texas at Austin.
Tag(s): College of the Environment • Department of Earth and Space Sciences • earthquakes and seismology • Harold Tobin