Deepest scientific ocean drilling sheds light on Japan’s next major earthquake

The scientific deep-sea drilling vessel Chikyu, which in 2018 performed the deepest drilling of a seismic fault in the subduction zone. Credit: Satoshi Kaya / FlickR

Scientists digging deeper than ever into an underwater earthquake fault 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, published in the journal geologythey are an enigma because the fault produces a large earthquake almost every century and was thought to be building for another great earthquake.

“This is the heart of the subduction zone, just above where the fault is blocked, where the system was expected to store energy between earthquakes,” said Demian Saffer, director of the University of Geophysics Institute. Texas (UTIG) who co-led the research and scientific mission that pierced 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 still does not show any major signs of repressed tectonic stress. According to Saffer, this does not alter 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 next 50 years. years.

Instead, the findings will help scientists understand the link between tectonic forces and the earthquake cycle and potentially lead to better earthquake predictions, both in Nankai and other megathrust faults such as Cascadia in the Pacific Northwest.

Deepest scientific ocean drilling sheds light on Japan's next major earthquake

Harold Tobin of the University of Washington inspects drill rigs. The researchers used similar equipment during a record-breaking attempt to drill through Japan’s Nankai fault in 2018, led by the University of Texas Institute of Geophysics. Credit: Harold Tobin / University of Washington

“Right now, we have no way of knowing if the great 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 who is the first. author of the essay. “But I have some optimism that with increasingly direct observations like this, we can begin to recognize when something abnormal is happening and that the risk of an earthquake has increased in a way that could help people prepare.”

Megathrust faults like Nankai and the tsunamis they generate are among the most powerful and damaging in the globe, but scientists say they currently have no reliable way of knowing when and where the next big one will hit.

The hope is that by directly measuring the perceived force between tectonic plates pushing on each other – tectonic stress – scientists can learn when a major earthquake is ready to occur.

However, the nature of tectonics means that large seismic faults are found deep in the ocean, miles below the sea floor, making them incredibly difficult to measure directly. The Saffer and Tobin drilling expedition is the closest scientists have come.

  • Deepest scientific ocean drilling sheds light on Japan's next major earthquake

    Demian Saffer, director of the University of Texas Institute of Geophysics (UTIG), during scientific ocean drilling at the Nankai earthquake fault in Japan. Credit: Demian Saffer / University of Texas Institute of Geophysics

  • Deepest scientific ocean drilling sheds light on Japan's next major earthquake

    A drill rig aboard the Chikyu scientific drill ship. Dozens of risers have been linked together to reach a deeper than ever earthquake fault. Led by researchers from the University of Texas and University of Washington Institute of Geophysics, the science mission revealed that tectonic stress in Japan’s Nankai subduction zone was lower than expected. Credit: Demian Saffer / University of Texas Institute of Geophysics

Their attempt to break the record took place in 2018 aboard a Japanese scientific drilling vessel, the Chikyu, which drilled two miles into the tectonic plate before the well became too unstable to continue, one mile before the fault.

However, the researchers collected invaluable data on subsurface conditions near the fault, including stress. To do this, they measured how much the well changed shape when the Earth squeezed it from the sides, then pumped water to see what it took to force its walls 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 predicted from the most recent earthquake surge was close to zero, as if it had already released its pent-up energy.

The researchers have suggested several explanations: it could be that the fault simply needs less pent-up energy than is thought to slip into a large earthquake, or that the stresses are lurking closer to the fault than it reached the perforation. Or it could be that the tectonic push will come suddenly in the next few years. In any case, the researchers said the drilling showed the need for further investigation and long-term monitoring of the fault.


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More information:
Harold J. Tobin et al, Direct constraints on the in situ stress state from deep drilling in the Nankai subduction zone, Japan, geology (2022). DOI: 10.1130 / G49639.1

Provided by the University of Texas at Austin

Citation: Deepest Scientific Ocean Drilling Sheds Light on Japan’s Next Great Earthquake (2022, September 22) Recovered September 22, 2022 from https://phys.org/news/2022-09-deepest-scientific-ocean-drilling-japan .html

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