On September 26, the Earthquake Research Committee announced that it was revising its long-term forecast for a Nankai Trough megaquake. Yet rather than giving one clear forecast, it presented two ranges of probability: “60 to 90% or more” and “20 to 50%.” This caused confusion among the public and local governments. “There is a big discrepancy between the probability of occurrence calculated using the two methods, making it difficult for citizens to understand, and extremely confusing,” said Takashi Namba, the mayor of Shizuoka city. He also asked the national government to “provide a more understandable explanation in the future.”
Tatsuya Ito, the mayor of Tokoname in Aichi Prefecture, also voiced his concern, stating that “the widening range of the figures could lead to a decline in disaster prevention awareness among citizens.” Other city officials to express their bewilderment at the two ranges of possibility included representatives from Wakayama and Kochi prefectures. The former complained that, “It’s difficult to predict how residents will react, and we’re afraid that it will cause confusion,” while the latter said, “Listing multiple probabilities side by side makes it difficult to understand. We think we’ll focus on the higher probability.”
Different Models Used To Estimate Probability Rate for Nankai Trough Megaquake
Megaquakes have occurred along the Nankai Trough once every 90 to 150 years. The last one, which measured between 8.1 and 8.4 on the moment magnitude scale, struck on December 21, 1946. In 2013, the panel put the probability of another one occurring there within 30 years “between 60% and 70%.” Five years later, it became “between 70% and 80%.” In January of this year, it reached around 80%. These estimates are based on what’s known as a “time-predictable model.” This model, however, is considered inaccurate by some seismologists due to scientific concerns about its methodology.
They argue that the probabilities have been inflated, and cite lower estimates for other regions that have been calculated using a different model. The revised estimate of “60% to 90% or higher” incorporates various potential measurement errors and uncertainties. The second model, which doesn’t use uplift data and is widely used for other regions, estimates the probability between “20 and 50%.” While neither model can be deemed scientifically superior, the panel said, “the higher chances of a Nankai Trough quake should be highlighted” to raise local residents’ awareness of an imminent disaster.
The government estimates approximately 298,000 deaths if a magnitude 9 earthquake occurs along the trough. Economic loss, it says, would amount to around ¥292 trillion.
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