A rock mass the size of a mountain lurks 3-12 miles below Japan's southern coast. This rock has the potential to become a magnet for megaquakes or powerful earthquakes, according to a study.
The rock, known as the Kumano Pluton, lies within the Nankai subduction zone, an area where the Philippine Sea Plate is subducting under the Japanese edge of the Eurasian plate.
Although it was first discovered in 2006, until now, its exact scale and impact has remained a mystery. In the new study, researchers from the University of Texas at Austin, United States (US) ran 20 years of seismic data through a supercomputer to produce the first complete visualization of the rock, which is about 43-78 miles wide.
The team said the rock diverted tectonic energy to points along its side, exactly where some of the region's biggest earthquakes originate.
In fact, large earthquakes with magnitudes higher than eight occurred along the side of Kumano Pluton in 1944 and 1946.
The findings could help research discover whether another major earthquake is likely to occur along the Nankai subduction zone in the near future. The study was carried out by geophysicist Shuichi Kodaira of the Japan Agency for Marine-Earth Science and Technology and his colleagues.
"We can't predict exactly when, where, or how big an earthquake will be in the future, but by combining our model with monitoring data, we can start forecasting processes in the near future," said Dr Kodaira. (19/5/2022).
"It will provide very important data for the Japanese public to prepare for the next big earthquake," he said.
In their study, the team used LoneStar5 from the University of Texas, one of the world's most powerful supercomputers, to combine millions of independent seismic records into one high-definition model of the Nankai subduction zone.
Their dataset includes readings from Japan's comprehensive network of thousands of seismic stations in the area, as well as other one-off surveys. Kumano's 3D reconstruction of Pluton reveals that the Earth's crust bends under Pluton's weight, and protrudes slightly above it.
The team also found that Pluton diverts groundwater in the wider subduction zone, allowing a pathway for liquid to reach the upper mantle. This in turn alters the tectonic forces that can trigger earthquakes, according to the researchers.
The findings are a major demonstration of the potential of so-called big data to revolutionize seismology, experts say.
"The fact that we can make major discoveries in areas that are already well-studied, I think, is eye-opening to what may await in less well-monitored areas," said paper author and University of Texas geophysicist Adrien Arnulf.
The same approach, he added, might be applied to create regional-scale images of the subsurface around other active subduction zones, such as in northeastern Japan, New Zealand and the US Pacific Northwest.