Miles below the sea floor, scientists are collecting data on subduction stress

Scientists dug deep into the sea floor to see how tension is building inside the giant Nankai, a major subduction zone off Japan’s southeastern coast.

By Jay Barber, science writer (@JayBarber77)

Citation: Barber, J., 2022, Miles Under the Seafloor, Scientists Collect Data on Subduction Stress, Templor, http://doi.org/10.32858/temblor.287

Earthquakes can cause catastrophic damage and loss of life. However, no one can predict it. Earthquakes at subduction zones under the ocean are a particular disaster. In this setup, two tectonic plates meet, while the less buoyant plate sinks below the other. When the plates fail, the pressure builds. Eventually, the locked panels should release the pressure. If the launch is sudden, the result could be a massive earthquake, and the sudden movement could dislodge water, causing a tsunami.

Scientists who study subduction zones have long believed that if stress can be measured where the plates meet, the data can help them better predict such events. However, because much of any subduction zone is inhabited miles below the Earth’s surface (which is likely under miles of ocean water), physical measurements have been impossible – until now.

In research recently published in the Journal of Geology, scientists drilled nearly 2 miles (3,058 meters) below the sea floor, toward the massive rift in Japan’s Nankai Subduction Zone. The data they collected shows, somewhat unexpectedly, that tectonic stress does not appear to be building up where scientists expected to find it.

The Nankai Trough is a danger to many of Japan’s southern islands.

Data mining

Harold Tobin, a University of Washington seismologist and director of the Pacific Northwest Seismological Network, and his colleagues hoped to drill all the way to the boundary where the Philippine Sea Plate descends below the Amorean Plate in the Nankai region. But when they went down there, they found that they could not dig very deep. “It turns out that the accessible place of the fault is 2,000 meters (6,600 feet) out of the water and then another 5 kilometers (3 miles) below the ocean floor,” says Tobin. For the researchers to get to the fault, they would need to drill 7 kilometers (4.3 miles) below the ship, which was floating off the southeastern coast of Japan. So far, they’ve drilled about 1.6 kilometers (1 mile) of the fault. However, the subduction zone reveals some surprises.

Prior to this study, scientists had assumed that the horizontal stress — stress caused by the pressure of tectonic plates pushing together — would be greater than the vertical stress caused by the weight of the rock at that depth. If this is true, then the stress measurements could directly help scientists assess the stress produced by plate motion.

Because the last major earthquake in the Nankai subduction zone occurred nearly 80 years ago (1944), Tobin and colleagues expected to measure large stress buildup. In other words, the tension should have built up since the last earthquake, based in part on how the plates have moved toward each other since then, which scientists know from other measurements, such as the Global Positioning System (GPS). But the team’s measurements didn’t show the stress level they’d expected. Measurements showed much less. This result was contradictory to [expectation] What we thought [the stress] says Tobin.

The researchers drilled two miles below the seafloor at site C0002F/N/P. This cross-section through the subduction zone helps visualize how close they came. The surface labeled “megasplay bug” is the beginning of the board’s interface. Credit: Tobin et al. (2022)

One piece of the puzzle

Although the researchers noted less stress than they expected, their findings don’t mean the fault isn’t heading toward a major earthquake. In fact, they suspect that if they could dig deeper and get closer to the error, or if they could extend the duration over which they collected their measurements, they might find what they originally expected. “We know that the plates are really converging. It’s definitely a subduction zone,” Tobin says. “Either the pressure will change in the next few thousand meters [closer to the fault]and it is changing very quickly, or will change over time [in the future]. “

“It seems very likely that stress near the main subduction fault should push the fault in the direction of motion,” says Emily Brodsky, a seismic physicist at the University of California, Santa Cruz, who has studied the functioning of regions of the giant hole. “It is strange that this data shows that it is not.” She says the error may be at the beginning of his cycle, which is consistent with Tobin’s possible explanations. “It’s a head scratcher,” she says.

This research marks the first time that seismic data has been collected near the subduction zone plate interface. “we [historically] Make all of our observations using seismometers and GPS devices here on Earth’s surface,” Tobin explains. However, because earthquakes occur core deep below the Earth’s surface, “we need observations down near fault zones,” he says. Mars through the telescope in exchange for sending a rover to Mars.”

Because of this difference between where earthquakes begin and where scientists collect data, much of researchers’ current understanding of earthquakes relies on models of how they think the Earth shatters. Tobin says this new data will allow them to update those models. “This is one piece in the giant puzzle of how earthquakes work.”

Ideally, tools could be placed in the excavated hole which would allow long-term monitoring of stress buildup, Tobin says. The ultimate goal of this work will be to use the data from those instruments to understand whether signs or initial activity is present before an earthquake that researchers can monitor. If some measurable stress signal occurs before an earthquake strikes, scientists can better predict significant seismic activity.

References

Tobin, HJ, Safire, DM, Castillo, DA, and Hirose, T. (2022). Direct constraints on in situ stress condition from deep drilling at the Nankai subduction zone, Japan. Geology, 50 (11), 1229-1233.

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