Issues on the Japanese Earthquake Hazard Evaluation

The 2011 Great East Japan Earthquake forced the policy of counter-measurements to earthquake disasters, including earthquake hazard evaluations, to be changed in Japan. Before the March 11, Japanese earthquake hazard evaluation was based on the history of earthquakes that repeatedly occurs and the characteristic earthquake model. The source region of an earthquake was identified and its occurrence history was revealed. Then the conditional probability was estimated using the renewal model. However, the Japanese authorities changed the policy after the megathrust earthquake in 2011 such that the largest earthquake in a specific seismic zone should be assumed on the basis of available scientific knowledge. According to this policy, three important reports were issued during these two years. First, the Central Disaster Management Council issued a new estimate of damages by a hypothetical Mw9 earthquake along the Nankai trough during 2011 and 2012. The model predicts a 34 m high tsunami on the southern Shikoku coast and intensity 6 or higher on the JMA scale in most area of Southwest Japan as the maximum. Next, the Earthquake Research Council revised the long-term earthquake hazard evaluation of earthquakes along the Nankai trough in May 2013, which discarded the characteristic earthquake model and put much emphasis on the diversity of earthquakes. The so-called 'Tokai' earthquake was negated in this evaluation. Finally, another report by the CDMC concluded that, with the current knowledge, it is hard to predict the occurrence of large earthquakes along the Nankai trough using the present techniques, based on the diversity of earthquake phenomena. These reports created sensations throughout the country and local governments are struggling to prepare counter-measurements. These reports commented on large uncertainty in their evaluation near their ends, but are these messages transmitted properly to the public? Earthquake scientists, including authors, are involved in the discussion of these issues as committee members. However, we are wondering if the basis of these reports is scientifically appropriate. For example, there is no established method to evaluate the maximum size of earthquake, whose record is not known, in a specific area, but the committee made an estimate for the Nankai trough by extrapolating available knowledge. The Japanese policy makers further requested the probability of occurrence of such an event, which the committee had to decline because of the lack of knowledge. This example shows that Japanese earthquake scientists sometimes are involved in an important decision-making and are urged to go beyond the limit of earthquake science. We consider this difficult situation is formed on the basis of the history of the Japanese earthquake science and the 'myth of flawless of science' in the government and society, who often ask for a simple answer. Open discussion with people from other fields of science, such as social and human sciences, and the public would be an effective solution for the public to understand the complexity of the problems and to encourage appropriate counter-measures.