Rikuzentakata, devastated by a tsunami on March 13, 2011, in Rikuzentakata, Iwate Prefecture.
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The oft-cited figure—a 70-80% chance of a major Nankai Trough earthquake within 30 years—sounds precise. It isn't, according to Kenji Satake, professor emeritus at the University of Tokyo Earthquake Research Institute, who has spent decades studying the region's tsunami and earthquake history.
In an interview with Japan Forward, Satake explained that the number is built from roughly ten repeating earthquakes over the past 1,000 years, with intervals between them averaging 90 to 150 years.
Researchers use two different statistical models to turn that history into a 30-year probability, and the two don't agree. "One model gives 60% to 90% or higher, the other gives 10% to 50%," he said. "Because it's still unclear which model is more reliable, both sets of figures are published, despite the substantial differences between them."
Why Rupture Patterns Keep Changing
Part of the uncertainty comes from how unpredictably the Nankai Trough has ruptured in the past. The most recent sequence, in 1944 and 1946, broke in two separate earthquakes two years apart. The one before that, in 1854, ruptured in two events just two days apart. Before that, in 1707, the entire trough broke in a single massive earthquake.
"The pattern is a little different every time," Satake said, "and with current scientific knowledge, we don't know which pattern will happen next"—which is part of why the probability estimates carry such a wide range.
That unpredictability is also what shapes Japan's newest early-warning tool, the Nankai Trough Earthquake Extra Information system, first issued after a magnitude-7 earthquake off Hyuga-nada in 2024.
Satake broke down how it works. When a magnitude-8-class earthquake strikes and appears to represent one half of the trough rupturing, authorities issue a higher-level advisory. Historical records show that the other half can follow anywhere from two days to two years later.
A magnitude-7 event triggers a lower-tier caution notice instead, reflecting a smaller, if still elevated, probability that a larger rupture will follow.
A Shift Toward Worst-Case Planning
The 2011 Tohoku earthquake reshaped how Japan calculates both earthquake probability and expected damage, Satake said.
Before 2011, probability estimates relied on solid historical earthquake records. Afterward, scientists began incorporating older geological evidence—tsunami deposits, in particular—even where the dating is less certain, to capture events too old for written records. Damage forecasting shifted too.
That worst-case approach now operates on two tracks. "Level 1" covers earthquakes expected within a few decades and focuses on protecting both lives and property, primarily through hard infrastructure such as seawalls.
"Level 2" covers rarer, centuries-scale events—the 2011 disaster was one—and focuses on saving lives alone, since it isn't feasible to physically shield every home from such an event.
"That means evacuating sites and relocating critical facilities, such as hospitals, schools, and city (town) offices, to safer ground," Satake said, rather than trying to armor the coastline against the worst case.
What Old Documents Reveal
Historical research factors directly into that planning. Satake pointed to the Jogan earthquake of 869 as one example: it's built into current probability models, and the tsunami deposits it left behind help define hazard maps for low-frequency, high-impact events.
That kind of evidence is important because instrumental seismic data—readings from modern equipment—only goes back a matter of decades. "Anything older than that, you simply can't study without historical documents and geological data," he said.
Preparedness and Its Limits
Japan's reputation for earthquake readiness is well-earned but not absolute, in Satake's view. Since 2011, he said, the Tohoku region has changed substantially, and Japan is doing what it reasonably can.
He pointed to two features he considers distinctly Japanese. Damage estimates for major fault zones, including the Nankai Trough and the fault beneath Tokyo, are updated about once every decade as new data comes in. Meanwhile, nationwide disaster drills are held every year, keeping the memory of past disasters alive.
September 1—the anniversary of the 1923 Great Kanto Earthquake—is marked annually as national Disaster Prevention Day. "Ask any Japanese person when the Great Kanto Earthquake happened, and they'll know it was September 1," Satake said. "That people still remember the exact date, more than a hundred years later, is remarkable."
On the widely discussed scenario of a Nankai Trough earthquake triggering a Mount Fuji eruption, Satake noted that the mountain did erupt alongside the 1707 Hoei earthquake, so a repeat isn't implausible—though far from certain.
He described dramatic damage comparisons, such as one suggesting a joint disaster equivalent to 15 years of Tohoku-scale damage compressed into a single day, as reflecting the worst-case planning philosophy Japan adopted after 2011, rather than a firm prediction.
His message for less-prepared, earthquake-prone regions abroad was straightforward: major subduction earthquakes aren't confined to Japan—they strike around the Pacific "Ring of Fire" and beyond, including Indonesia. "Each country needs to scientifically study what could happen where they are," he said, "and then governments and scientists have a responsibility to communicate that risk clearly to the public."
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Author: Daniel Manning
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