Photo/Illutration Naoshi Hirata, chair of the Earthquake Research Committee, speaks at a news conference on Sept. 18 in Tokyo. (Takahiro Takenouchi)

The government’s latest probability figures for a long-expected Nankai Trough megaquake show that even Japan’s best scientific minds still cannot accurately predict earthquakes.

The Earthquake Research Committee on Sept. 26 announced a significant revision of its long-term forecast for a Nankai Trough earthquake, which is expected to hit off the Japan’s southern Pacific coast.

It had previously said the probability of a magnitude 8-9 class quake occurring in the region within the next 30 years was around 80 percent.

The panel now lists the likelihood as either “60 to 90 percent or higher” or “20 to 50 percent,” depending on the calculation method used.

The vagueness, acknowledged even by committee members, reflects both the limitations of current seismic forecasting and efforts to communicate scientific accuracy and integrity as transparently as possible.

Committee Chair Naoshi Hirata, professor emeritus of seismology at the University of Tokyo, said the public should be prepared because the question is not about if the megaquake will occur, but when it will strike.

“These are the best calculations science can offer at this time,” Hirata said. “The numbers show that a Nankai Trough earthquake could occur at any moment. We urge immediate action to prepare for it.”

The Nankai Trough runs offshore from Shizuoka Prefecture to near the main island of Kyushu. The long, narrow depression on the seafloor is known for generating major seismic events every century or two.

This is the first revision of the calculation method for a Nankai Trough earthquake in 12 years.

The physical conditions at the earthquake source remain unchanged, and only the statistical models have been updated. The projected scale and impact zones are the same.

The dual estimate does not alter the government’s worst-case scenario, which projects up to 298,000 deaths in a major Nankai Trough earthquake.

INFLATED PROBABILITY?

Until now, the 80-percent probability was based on a “time-predictable model,” which used uplift measurements from two historic Nankai Trough quakes in the 18th and 19th centuries and the intervals between them.

However, a research paper published last year questioned the reliability of that model, citing uncertainties in tide level data and varying interpretations of old documents.

Critics argued that the high probability figure for the Nankai region may have been inflated, citing lower estimates for other regions calculated using a different model.

The issue was also raised by lawmakers in the Diet, prompting the committee to review its methods.

In response, the committee updated the time-predictable model to incorporate a range of potential measurement errors and uncertainties, revising the estimate to “60 to 90 percent or higher.”

At the same time, officials added a second model--widely used for other regions--that yields a lower estimate of “20 to 50 percent.”

The committee acknowledged that neither model can be deemed scientifically superior.

Both estimates fall within the highest risk category under Japan’s three-tier scale, defined as 26 percent or higher.

The panel recommends continued vigilance based on the higher estimate, especially for disaster preparedness planning.

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