Scientists who have delved deeper into an undersea earthquake fault than ever before have 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 are published in the journal geologyis a puzzle because Error It produces a great earthquake about every century and was thought to build for another major earthquake.
“This is the core of the subduction zone, right above where the fault was locked, where the expectation was that the system should store energy between earthquakes,” said Damien Safire, director of the University of Texas Geophysics Institute (UTIG). He co-led the research and scientific mission that dug the bug. “It changes the way we think about stress in these systems.”
Although the Nankai rift has been stuck for decades, the study shows that it has not yet shown major signs of pent-up tectonic stress. According to Sfeir, this does not change the long-term view of the fault, which last erupted in 1946 – when it caused a tsunami that killed thousands – and is expected to happen again within the next 50 years.
Instead, the results will help scientists get back to the link between them tectonic forces and the earthquake cycle, which is likely to lead to better earthquake predictions, in both the Nankai and other faults such as Cascadia in the Pacific Northwest.
“Right now, we have no way of knowing if it was the Great Cascadia earthquake — a magnitude 9 earthquake tsunami“It 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 research paper. “But I have some optimism that with more and more direct observations like this, we can begin to recognize when something abnormal has occurred and that the risk of an earthquake is increasing in a way that can help people prepare.”
Giant confidence faults like Nankai and the tsunamis they generate are among the world’s most powerful and damaging, but scientists say they currently have no reliable way of knowing when and where the next major storm will strike.
The hope is that the perceived force between them will be measured directly tectonic plates Pressing each other – tectonic stress – scientists can learn when the Great Quake is ready to strike.
However, the nature of tectonics means that great earthquake faults have been found ocean depths, miles below the sea floor, which makes measuring them directly very difficult. The excavation expedition Saver and Tobin are the earliest scholars to have come.
Their record-breaking attempt took place in 2018 aboard a Japanese scientific drillship, the Chikyu, which drilled two miles into the tectonic plate before the well became too unstable to continue, a mile from error.
Nevertheless, researchers have collected invaluable data about subsurface conditions near the fault, including stress. To do this, they measured how much the shape of the well changed when the earth pressed against it from the sides, then pumped out water to see what it took to force its walls back down. This told them about the direction and strength of the horizontal pressure felt by the plate pressing on the fault.
Contrary to expectations, the expected horizontal stress created since the last great earthquake was close to zero, as if it had already released its pent-up energy.
Researchers have suggested several explanations: The reason may be that the fault simply needs less pent-up energy than was thought to slip in Earthquake, or that the stresses are latent closer to the fault than the drilling has reached. Or a tectonic thrust could come on suddenly in the coming years. Either way, the researchers said, the drilling demonstrated the need for further investigation and long-term monitoring of the fault.
Harold J. Tobin et al, Direct constraints on the in situ stress condition from deep drilling in the Nankai subduction zone, Japan, geology (2022). DOI: 10.1130 / G49639.1
The University of Texas at Austin
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