Scientists who drilled deeper than ever into an undersea earthquake have found that tectonic stress in Japan’s Nankai subduction zone is less than expected, according to a study by researchers at the University of Texas at Austin and the University of Washington.
The findings, published in the journal Geologyare a puzzle because the debt produces a major earthquake almost every century and was thought to be built for another major one.
“This is the heart of the subduction zone, right 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 for Geophysics (UTIG), who led the research and scientific mission that drilled the flaw. “It changes the way we think about stress in these systems.”
Although the Nankai Fault has been stuck for decades, the study shows it doesn’t yet show major signs of pent-up tectonic stress. According to Saffer, that doesn’t change the long-term outlook for the fault, which last ruptured in 1946 — when it triggered a tsunami that killed thousands — and is expected to do so again in the next 50 years.
Instead, the findings will help scientists learn more about the link between: tectonic forces and the earthquake cycle and potentially lead to better earthquake predictions both at Nankai and other megathrust faults such as Cascadia in the Pacific Northwest.
“Right now we don’t know if the big one for Cascadia — a magnitude 9 earthquake and… tsunami-will happen this afternoon or in 200 years,” said Harold Tobin, a researcher at the University of Washington who is the paper’s lead author. “But I have some optimism that with more direct observations like these we can begin to recognize when something abnormal is happening and increasing the risk of an earthquake in a way that can help people prepare.”
Megathrust faults like Nankai and the tsunamis they cause are among the most powerful and damaging in the world, but scientists say they currently have no reliable way of knowing when and where the next big one will strike.
The hope is that by directly measuring the force felt between tectonic plates pushing each other – tectonic stress – scientists can learn when a major earthquake is about to happen.
However, the nature of tectonics means that the major earthquake faults are found in deep ocean, miles below the sea floor, making them incredibly challenging to measure directly. Saffer and Tobin’s drilling expedition is the closest scientists have come.
Their record attempt came in 2018 aboard a Japanese science drillship, the Chikyu, which drilled two miles into the tectonic plate before the borehole became too unstable to continue, a mile before the fault.
Nevertheless, the researchers collected invaluable data about the subsurface near the fault, including stress. To do that, they measured how much the borehole changed shape as earth squeezed it out the sides and then pumped water to see what it would take to force the walls back out. That told them the direction and strength of the horizontal stress felt by the plate pushing on the fault.
Contrary to predictions, the horizontal tension expected to have built up since the most recent major 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 accumulated energy than thought to be able to get into a large earthquakeor that the stresses are lurking closer to the fault than the bore has reached. Or it could be that tectonic pressures come suddenly in the next few years. Regardless, the researchers said the drilling showed the need for further investigation and long-term monitoring of the flaw.
Harold J. Tobin et al, Direct constraints on in situ stress state of deep drilling in the Nankai subduction zone, Japan, Geology (2022). DOI: 10.1130/G49639.1
University of Texas at Austin
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