New Seismic Model Reveals Two Variables Needed to Predict Nankai Trough Quakes

Key facts

• Japan lies at the meeting point of four tectonic plates: Eurasian, Okhotsk, Pacific, and Philippine Sea.
• The Nankai Trough megasplay fault branches upward from a subduction megathrust where the Philippine Sea plate slides under the Eurasian plate.
• Historical megathrusts in the Nankai Trough occur in pairs every 90 to 150 years; the last pair was the 1944 Tonankai (M8.1) and 1946 Nankaido (M8.3) earthquakes.
• Geophysicists Matt Ikari and Alexander Roesner from MARUM re-analyzed core samples from the Nankai Trough Seismogenic Zone Experiment.
• Friction experiments on actual rock cores showed that two state variables are needed to model sliding under low pressure, but only one under high stress.
• A separate study in Science identified the 'stopping phase' seismic signature when a rupture hits a barrier, using data from 12 large earthquakes.
• The stopping phase was isolated for five strike-slip earthquakes; researchers expect it applies to thrust events but have not yet confirmed.
• Near-surface features like softer rock layers can amplify the stopping phase, leading to more severe ground shaking.

Deep Beneath Japan, a Fault Holds Clues to the Next Megaquake

Japan, the most seismically active nation on Earth, sits atop a collision of four tectonic plates—the Eurasian, Okhotsk, Pacific, and Philippine Sea plates—along the Pacific Ring of Fire. This geography creates a labyrinth of deep trenches and shallow troughs that make forecasting seismic hazards extraordinarily complex. Among the most dangerous structures is the Nankai Trough megasplay fault, a branching fault that rises from the subduction megathrust where the Philippine Sea plate dives under the Eurasian plate. While the Nankai Trough was not responsible for the devastating 2011 Tōhoku earthquake—which was caused by the Pacific plate subducting under the Okhotsk plate—it has generated numerous catastrophic megathrusts throughout history. These earthquakes typically occur in pairs, with a recurrence interval of 90 to 150 years. The most recent pair, the 1944 Tonankai and 1946 Nankaido earthquakes, registered magnitudes of 8.1 and 8.3, respectively. Some 80 years on, the fault is outside its seismic window, but scientists remain intensely focused on understanding its behavior.

Rock Cores Reveal a Missing Variable in Friction Models

In a new study published in Geophysical Research Letters, geophysicists Matt Ikari and Alexander Roesner of the MARUM Center for Marine Environmental Sciences and Faculty of Geosciences re-analyzed core samples obtained from the Nankai Trough Seismogenic Zone Experiment. This international project aims to directly study the tsunamigenic region that has plagued Japan for centuries. By dragging actual rock cores at different speeds and pressures to mimic the area's geophysical environment, the researchers discovered that the mathematical models used to describe fault movement require an additional variable. “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. They observed that two state variables are needed to fit friction measurements under low pressure, when sliding can potentially generate an earthquake. Under higher stress, when sliding is stable, only one state variable is necessary. The second variable describes a real process that requires the rock to be porous enough to allow deformations along a simple slip plane—a detail crucial for understanding how the megasplay fault behaves at shallow depths.

The Stopping Phase: A Seismic Signature of Rupture Barriers

Separately, a study published in Science investigates what stops large earthquakes and how that knowledge can improve hazard assessment. When a rupture propagating along a fault hits a physical barrier, it creates a seismic signature called the “stopping phase”—a shock wave traveling in the opposite direction of the main rupture. “When the rupture is going fast and encounters some barrier that suddenly makes it stop, it sends out a shock wave,” said study co-author Jesse Kearse, an Earth scientist at Victoria University of Wellington in New Zealand. A person above such a barrier would first feel the ground move in the rupture direction, then snap back, akin to a car braking suddenly. Kearse and his colleague Yoshihiro Kaneko, a geophysicist at Kyoto University, searched for this signature in seismic and geodetic data from 12 large earthquakes worldwide. They successfully isolated the stopping phase for five of those quakes, all of which were strike-slip events where two blocks of rock slide horizontally past one another. The team also found that near-surface features, such as softer rock layers above the stopping phase, can enhance the signal, leading to more severe ground shaking at the surface.

Barriers as Checkpoints: Why Some Quakes Stay Small While Others Grow

Every barrier a rupture encounters acts as a checkpoint. If the barrier holds, the earthquake stops, remaining a minor, localized event. But if the advancing rupture has enough energy to break through, it spills into the next fault segment, potentially cascading into a megaquake. “This demonstrates the extremely valuable role of near-field observations in understanding why earthquakes grow big or remain small,” said Yihe Huang, a geophysicist at the University of Michigan who was not involved in the study. Now that scientists can identify the stopping phase signature, they can analyze past earthquakes to map underground barriers, assess how much energy those barriers can absorb, and identify any amplifying near-surface features. “This new insight can potentially transform earthquake hazard analysis,” Huang added, by showing where an earthquake of a particular strength might be stopped and where it might be enhanced.

From Strike-Slip to Thrust: The Next Research Frontier

Kearse and Kaneko limited their study to strike-slip earthquakes because more data exist for that type. However, the recent magnitude 7.7 earthquake off northeastern Japan was a thrust event, where the ground moves up and down—a motion far more likely to generate a tsunami. “The obvious continuation of this work is to make it more general,” Kearse said. “But we expect this stopping mechanism is a common feature of the earthquake process that does apply to thrust events, too. We just cannot confirm that yet.” Similarly, the Nankai Trough research points to the need for refined models that incorporate the newly identified second state variable. Future work will aim to identify danger areas along the fault and improve predictions for when the next Nankai-generated earthquake or earthquakes will strike.

A Dual Path to Better Earthquake Forecasting

Together, these two lines of investigation offer complementary tools for understanding and ultimately forecasting earthquakes. The friction experiments on the Nankai Trough reveal that the behavior of shallow fault zones is more complex than previously assumed, requiring two variables to capture the potential for earthquake nucleation. The stopping-phase research provides a method to map the barriers that can halt or amplify a rupture, offering a direct observational check on model predictions. Both studies underscore the importance of direct, near-field observations—whether from rock cores or dense seismic networks—in unraveling the mechanics of fault zones. As researchers integrate these findings into hazard models, the hope is that communities in seismically active regions like Japan will gain more precise warnings of the next inevitable megaquake.

The bottom line

• The Nankai Trough megasplay fault requires two state variables in friction models under low pressure, where earthquakes can nucleate, but only one under stable sliding.
• The second variable depends on rock porosity allowing deformation along a simple slip plane at shallow depths.
• The stopping phase seismic signature occurs when a rupture hits a barrier and can be used to map underground barriers and assess their energy absorption capacity.
• Near-surface softer rock layers can amplify the stopping phase, increasing ground shaking intensity.
• The stopping phase has been confirmed for strike-slip earthquakes; its applicability to thrust events is expected but not yet proven.
• Both studies highlight the value of near-field observations—rock cores and dense seismic data—for improving earthquake hazard analysis.