Here’s what you’ll learn when you read this story:

• Japan’s Nankai Trough megasplay fault threatens millions of people, and has caused tsunamigenic earthquakes at regular, century-long intervals since time immemorial.
• Through the Nankai Trough Seismogenic Zone Experiment, scientists are better understanding the complicated mechanics of this immensely seismic region.
• In a new study, scientists detail how an additional variable could help us better understand why the region is so prone to earthquake-generating movement.

Japan is one of the most seismically active countries on the planet. The island nation lies at the meeting point of four tectonic plates—the Eurasian, Okhotsk, Pacific, and Philippine Sea plates—and because of this perilous location, Japan records more earthquakes than any other nation on Earth, according to the U.S. Geological Survey. This geography, located along the Ring of Fire, also means that Japan (specifically, its eastern coast) is home to a labyrinth of deep-lying trenches and shallower troughs that can make forecasting Japan’s seismic future extremely difficult.

In a new study published in the journal Geophysical Research Letters, geophysicists Matt Ikari and Alexander Roesner from the MARUM Center for Marine Environmental Sciences and Faculty of Geosciences re-analyzed the Nankai Trough megasplay—a fault that branches upward from a subduction megathrust, located where the Philippine Sea plate slides under the Eurasian plate. Although it was not responsible for the 2011 Tōhoku earthquake (which was instead caused by the Pacific plate subducting under the Okhotsk plate, a microplate often considered part of the North American plate), the Nankai Trough is to blame for many devastating historical megathrusts, which typically occur in pairs and often return every 90 to 150 years.

The most recent destructive duo—the 1944 Tonankai and 1946 Nankaido earthquakes—registered magnitudes of 8.1 and 8.3 (respectively) on the Richter scale. Some 80 years on, the fault is thankfully outside its seismic window, but scientists are still interested in its behavior. Ikari and Roesner are busy analyzing core samples gathered as part of the Nankai Trough Seismogenic Zone Experiment, intended to directly study this tsunamigenic region that’s harassed Japan for centuries.

“The Nankai Trough megasplay fault hosts diverse modes of fault slip, ranging from slow slip events to megathrust earthquakes, and is responsible for related phenomena such as tsunamis and submarine landslides,” the authors wrote in a study, published in the journal Earth, Planets and Space, back in 2022. “All types of slip events require some kind of frictional weakening process in order to nucleate and propagate.”

Now, in 2026, Ikari and Roesner have completed more friction experiments, using actual rock cores to better understand the mechanics of this seismic region. After dragging the rocks at different speeds and pressures to mimick the area’s geophysical environment, they discovered that the math behind the modeling of Nankai Trough needed some serious improvement.

“The movement of fault zones in the earth can be described with a set of mathematical equations, which may include 1 or 2 variables called ‘state variables,’” the authors wrote. “We observe that 2 state variables are needed to fit the friction measurements under low pressure, when the sliding can potentially generate an earthquake. Only one state variable is necessary at higher stress, when the sliding is stable.”

This second variable details a real process that requires rock to be porous enough to allow deformations along a simple slip plane, and these details are crucial for understanding how this megasplay fault behaves at shallow depths. Hopefully, future work will help scientists identify danger areas along the fault, while also improving prediction methods for when the next Nankai-generated earthquake(s) will inevitably strike.