According to a study by researchers from the University of Texas at Austin and the University of Washington, scientists who drilled deeper into an underwater seismic fault than ever before found that the tectonic tension in the zone of subduction of Nankai in Japan was less than expected.
The results, published in the journal Geologyare a puzzle because the fault produces a large earthquake almost every century and it was thought to build another large one.
“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 Demian Saffer, director of the University of Texas Institute of Geophysics (UTIG) who co-led the research mission and scientist who drilled the fault. “It changes the way we think about stress in these systems.”
Although the Nankai Fault has been blocked for decades, the study shows that it is not yet showing major signs of pent-up tectonic stress. Saffer says that doesn’t 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 again in the future. over the next 50 years.
Instead, the findings will help scientists focus on the link between tectonic forces and the earthquake cycle and potentially lead to better earthquake predictions, both at Nankai and other faults. of megathrust such as Cascadia in the Pacific Northwest.
“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,” said Harold Tobin, a researcher at the University of Washington who is the first author of the article. “But I have some optimism that with more and more direct observations like this, 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 might help people prepare.”
Mega-thrust faults such as Nankai and the tsunamis they generate are some of the most powerful and damaging in the world, but scientists say they currently have no reliable way of knowing when or where the next big will hit.
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.
However, 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. Saffer and Tobin’s drilling expedition is closest to the scientists.
Their record attempt took place in 2018 aboard a Japanese science drill ship, the Chikyu, which drilled two miles into the tectonic plate before the borehole became too unstable to continue, one mile from 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 out. . 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 most recent large earthquake was close to zero, as if it 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.
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Harold J. Tobin et al, Direct stresses on the in situ stress state of deep drilling in the Nankai subduction zone, Japan, Geology (2022). DOI: 10.1130/G49639.1
Provided by the University of Texas at Austin
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