A piece of rock the size of a mountain hiding under Japan directs earthquakes

An underground patch of rock the size of a mountain may have affected the tracks of major earthquakes in southern Japan.

The dense igneous rock, known as the Kumano pluton, lies about 3.1 miles (5 kilometers) below the surface under Japan’s Kii Peninsula. It is located in the crust of the Eurasian continental plate. Beneath this plate of continental crust, the Philippine oceanic plate dives toward the Earth’s mantle, a process called subduction. New research suggests that the heavy pluton within the Eurasian plate alters the slope of that dive, forcing the Philippine plate to slope more steeply.

The pluton is also near the foci of two major earthquakes in the 1940s, each of which traveled in opposite directions and did not rupture through the pluton itself.

“Ultimately, we don’t really know why these earthquakes don’t overlap in the pluton region,” said study co-author Dan Bassett, a marine geophysicist at New Zealand’s GNS Science, the Earth science research service. “It seems to play a really key role in eliminating these earthquakes and preventing them from joining in.” (An earthquake nucleus point is where the crust’s rupture begins.)

Although the pluton is located relatively close to the surface, it could have a significant impact on how water moves from Earth’s oceans into its mantle. The slope of the oceanic Philippine plate is twice the gradient under pluton pressure. This appears to cause more fractures in the subducting plate, allowing it to move more seawater downward toward the deeper crust and mantle. Then the water in the mantle drives things like volcanic eruptions.

The Kumano pluton in southern Japan appears as a red bulge (indicating dense rock) in the center of this new 3D visualization. The mountain-sized piece of rock is located in the crust of the continental Eurasian plate, under which the oceanic Philippine plate juts out toward the Earth’s mantle. (Image source: Adrien Arnulf) How crust breaks

The Philippine plate is grinding under the Eurasian plate off the coast of Japan at a rate of 1.78 inches (4.5 cm) per year. This process, called subduction, causes earthquakes and volcanoes. Scientists use seismic monitors to try to understand geological structures within subduction zones, but this is often an intermittent record, especially in offshore trenches where placing the equipment is not easy.

However, the coast of Japan is one of the best-watched places in the world, in terms of earthquakes. The Japan Agency for Marine Earth Science and Technology (JAMSTEC) has blanketed the Nankai Trough with seafloor monitors, and Japanese seismologists have assembled the densest set of borehole seismometers — earthquake monitoring equipment buried deep in the crust to reduce turbulence from earthquake seismicity — on the planet. .

“We realized we had this giant data set, which has been amplified for two decades and was really unique in that it would enable us to produce a really high-resolution 3D model of the entire subduction zone,” Bassett told Live Science. .

The team didn’t discover the Kumano pluton, known since around 2006, but they got the clearest picture ever of how this structure affects the subduction zone. What they found came as a surprise: Most research into subduction zones focuses on the structure of the plate that is sinking below the surface, but not at the plate above it. The new findings suggest that the cortical plate above the fused plate may be more important than anyone realized.

Wendy Bohon, a geologist with Research Institutes Incorporated for Seismology (IRIS), who was not involved in the study, said.

The findings, published in Nature Geoscience on February 3, raise new questions about the pluton’s role in earthquakes. In 1944, an 8.1-magnitude earthquake began at the edge of Pluton and shook the Earth toward the northeast. Two years later, a magnitude 8.6 earthquake began near the epicenter of the first but ruptured in the southwesterly direction.

“You have these points along the faults that are like little bumpy patches or, in this case, big bumpy areas and they can prevent the earthquake from slipping,” Bohon said, referring to structures like Kumano Pluton. “They can act as nucleation points, places where earthquakes start, or they can act as a buffer, where the earthquake can stop.”

Related Topics: Japan Earthquake and Tsunami: Facts and Information

It’s not clear what caused the pluton effect, Bassett said. The dense igneous rock may be exerting so much pressure on the subducting plate that it resists the kind of dramatic rupture needed to sustain an earthquake. Or, it could be because of the way the pluton changes the shape of the fused plate beneath it. In the pluton region, the slope doubles its steepness as it descends. This means that this oceanic crust is falling very deep and very fast. Earthquakes occur more easily at shallow depths where the crust is cold and brittle, so rapid steepness can limit the area of ​​crust capable of causing an earthquake.

running water

The sharp downward trajectory of the subduction plate imposed by the Kumano pluton has a clearer effect on the way water moves through the subduction zone. This water cycle is not directly related to earthquakes in the area, but is important for magma formation and large-scale mantle processes, said Donna Shillington, an earth scientist at Northern Arizona University, who was not involved in the research but was involved in the research. He wrote an accompanying article for News & Views about the findings.

For these processes, the pluton appears to be very important, Shillington told Live Science. The massive structure appears to be creating pressure that forces the impulse plate to incline. This steep gradient forces the sloping plate to twist and rupture, creating fissures into which sea water can seep. The course of a dive also affects where the water arrives and the minerals it can chemically interact with. The researchers found that seismic waves in this region slow down significantly, indicating the presence of an area rich in serpentine mineral water.

“These metals are stable up to a temperature of 400-600 degrees Celsius in the field [472 degrees to 1112 degrees Fahrenheit]”So it has to be somehow carried up before that plate gets hot enough to release that water,” Shillington told Live Science. “So it probably has a deeper impact.”

As in earthquakes, geologists have focused more on the subducting plate when trying to understand the water cycle deep within the Earth, Shillington said. The new study suggests that the top plate is also important.

“If we want to understand this water in the dish, we now have another variable that we need to think about,” she said.

The research team is now planning to build 3D models of the subduction zone in northeastern Japan where the 2011 Tohoku earthquake originated and the Hikurangi subduction zone off New Zealand’s North Island. These should be ready in a year or two, Bassett said.

“Being able to compare high-resolution 3D models of Earth’s structures across the three subduction zones should enable us to think a little more carefully about how the structure of subduction zones affects earthquake behavior,” he said.

Originally published on Live Science.

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