Revised Estimation on Nankai Trough Megaquake

How likely is Nankai Trough Great Earthquake occurring within next thirty years? The answer of this question became extremely obscure after reviewing calculation for earthquake prediction. The answer is “60 to 90 percent” or “20 to 50 percent.” It is nothing more than saying that it is coming or not coming. The people wonder what they can do to the great earthquake based on that prediction.

The Earthquake Research Committee issued a long-term estimation for Nankai Trough megaquake with magnitude of 8 or 9 within next 30 years. The committee set the probability at 60 to 70 percent in 2013, a year after East Japan Great Earthquake, and raised it to 80 percent in January 2025. The government estimates that 298 thousand people may die with the quake and tsunami.

 

This estimation was established with a model based on past occurrence of great earthquakes in the region. There remains record of at least three megaquakes in Nankai Trough since 1700s. The experts calculated when next megaquake occurs there with evaluation of interval between past quakes and upheaval of a port in Muroto city, Kochi. However, there was a skepticism on the calculation relying on the data about only one sample.

 

Receiving the criticisms from experts, the committee revised its method of prediction. As its result, the committee concluded that the probability of megaquake in Nankai Trough within the next 30 years is “60 to 90 percent or more.” And the committee also adopted another simple model used for a megaquakes in other regions in Japan, which indicated the possibility as “20 to 50 percent.” The committee decided to use both conclusion as its estimation.

 

In the simple calculation model, the estimation was 20 percent at the time of 2013. But the committee did not emphasize that low-possibility model, considering negative effect to disaster prevention policy of the government of Japan. Although it was highly difficult for the geologist to predict when next great earthquake comes, the requirement of policy making section urged science to deliver conclusion with less credibility.

 

The national government has been requiring local communities to prepare for a megaquake. The local governments formulated disaster prevention plan with building evacuation facilities. Those disaster prevention measures demand each local government certain amount of financial spending. The national government has been supporting them with subsidies. As long as megaquake prediction is connected with execution of policies, the scientists cannot be independent from financial points of view.

 

Nankai Trough Great Earthquake is still categorized in “Rank III,” the highest possibility of occurrence. However, the revision of estimation proved that science has not advanced enough to precisely predict when next Nankai Trough megaquake happens. It is necessary for the government of Japan to prepare for next megaquake anywhere in Japan which is located at the place with concentration of edges of four tectonic plates.

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