Mountain-sized chunk of rock hiding under Japan is channeling earthquakes
An underground rock the size of a mountain could affect the paths of major earthquakes in southern Japan.
The dense igneous rock known as the kumano pluton lurks about 5 kilometers below the surface beneath Japan’s Kii Peninsula. It sits in the crust of the continental Eurasian Plate. Beneath this continental crustal plate, the oceanic Philippine Plate is subducting toward Earth’s mantle, a process called subduction. New research suggests that heavy pluton in the Eurasian Plate is changing the slope of this dive, forcing the Philippine Plate steeper down.
The pluton is also near the epicenters of two major 1940s earthquakeeach traveling in opposite directions and not breaking through the pluton itself.
“Ultimately, we don’t really know why these earthquakes didn’t overlap in the region of the pluton,” said study co-author Dan Bassett, a marine geophysicist at New Zealand’s GNS Science Earth Scientific Research Service. “It seems to play a really key role in creating these earthquakes and preventing them from combining.” (The nucleation point of an earthquake is where it begins to break up the crust.)
Despite being relatively close to the surface, the pluton can have a major impact on how water moves from Earth’s oceans into its mantle. The subduction of the Philippine oceanic plate is twice as steep under the pressure of the pluton. This appears to create more fractures in the subducting plate, allowing more seawater to be transported toward the deep crust and mantle. Water in the mantle then drives things like volcanic eruptions.
How the crust breaks
the Philippine plate grinding under the Eurasian Plate off the coast of Japan at a rate of about 1.78 inches (4.5 centimeters) per year. This process, called subduction, triggers earthquakes and volcanism. Scientists use seismic monitors to try to understand the geological structures within subduction zonesbut that’s often a spotty record, especially in submarine trenches where placing gear isn’t easy.
However, the coast of Japan is one of the best monitored places in the world, seismically speaking. The Japanese Agency for Marine-Earth Science and Technology (JAMSTEC) has equipped the Nankai Trough region with seafloor monitoring devices, and Japan’s seismologists have also assembled the densest array of borehole seismometers — seismic monitoring devices buried deep in the crust Minimize interference from other—earthquake vibrations—on the planet.
“We realized we had this gigantic dataset that had been swollen for a couple of decades and was really unique in that it would allow us to create a really high-resolution three-dimensional model of the entire subduction zone,” Bassett told Live Science.
The team did not spot the Kumano pluton, known since around 2006, but got the clearest picture yet of how this structure affects the subduction zone. What they found came as a surprise: most research on subduction zones focuses on the structure of the plate that dips below the surface but doesn’t consider the plate that sits above. The new results suggest that the crustal plate, which sits over the subducting plate, may be more important than anyone had thought.
“We think a lot about the angle of the downgoing plate and haven’t spent a lot of time thinking about how the properties of the upper crust affect the downgoing plate,” said Wendy Bohon, a geologist at Incorporated Research Institutions for Seismology (IRIS), who was not involved in the study.
The results published in the journal nature geosciences on February 3 raise new questions about the role of the pluton in earthquakes. In 1944, an 8.1 magnitude tremor began at the edge of the pluton and shook the ground to the northeast. Two years later, an 8.6 magnitude earthquake started near the epicenter of the first tremor but erupted in a southwesterly direction.
“You have these points along faults that are like little rough spots or, in this case, big rough spots, and they can prevent the earthquake from dissolving,” Bohon said, referring to structures like the Kumano pluton. “They can act as nucleation points, places where the earthquake starts, or they can act like a backstop, places where the earthquake could stop.”
Related: Earthquake and Tsunami in Japan: Facts and Information
It’s not clear why the pluton has this effect, Bassett said. It could be that the dense volcanic rock puts enough pressure on the subducting plate to resist the dramatic fracture required for an earthquake to continue. Or it could be due to the way the pluton changes the shape of the subducting plate below. In the region of the pluton, the subducting plate doubles the steepness of its descent. This means that this oceanic crust is sinking very deep very quickly. Earthquakes happen more easily at shallower depths where the crust is cool and brittle, so the rapid decay could limit the area of the crust that could generate a quake.
Flowing water
The sharp downward trajectory of the subducting plate, forced by the Kumano-pluton, has a more pronounced impact on the way water moves through the subduction zone. This water cycle isn’t directly linked to earthquakes in the region, but it’s important for magma formation and large-scale mantle processes, said Donna Shillington, an earth scientist at Northern Arizona University, who wasn’t involved in the research but who wrote one accompanying News & Views article on the results.
The pluton appears to be extremely important to these processes, Shillington told Live Science. The huge structure appears to be creating the pressure that is forcing the subducting plate into a steep dive. This steep dive forces the subducting plate to buckle and fracture, creating fractures that seawater can penetrate. The trajectory of the dive also affects where the water lands and what minerals it can chemically react with. Seismic waves in this region are slowing down dramatically, suggesting an area of the richly hydrated mineral serpentine, the researchers found.
“These minerals are stable to somewhere in the 400-600 degree Celsius range [472 degrees to 1112 degrees Fahrenheit]so it has to be carried down quite a bit before that plate heats up enough to release that water,” Shillington told Live Science. “So that’s probably going to have a deeper impact.”
Similar to earthquakes, geoscientists have focused more on the subducting plate when trying to understand the deep-Earth hydrological cycle, Shillington said. The new study suggests that the parent plate is also important.
“If we want to understand this water in the plate, we now have another variable to think about,” she said.
The research team now plans to build three-dimensional models of the subduction zone in northeastern Japan, where the 2011 Tohoku earthquake formed and the Hikurangi subduction zone off the North Island of New Zealand. These should be ready within a year or two, Bassett said.
“Being able to compare high-resolution 3D models of Earth structures across the three subduction zones should allow us to think a little more deeply about how the structure of subduction zones affects earthquake behavior,” he said.
Originally published on Live Science.
Mountain-sized chunk of rock hiding under Japan is channeling earthquakes Source link Mountain-sized chunk of rock hiding under Japan is channeling earthquakes
