The “Nankai Trough” is a groove in the deep-ocean formed by plate subduction, which extends from off the coast of Shizuoka to off the coast of Miyazaki. In the vicinity of this area, large earthquakes have occurred quite frequently. Historically, there were cases in which Tonankai and Nankai earthqakes occurred consecutively, resulting in great disasters, such as the Ansei Earthquakes (1854) and Showa Earthquakes (1944 and 1946). In Japan, following the Great East Japan Earthquake, re-examination of earthquake long-term evaluation has taken place. In 2012, the Cabinet Office announced a new estimation based on experts’ opinions: if a gigantic earthquake would occur around the Nankai Trough in the future, it is possible that, in the worst case scenario, the earthquake could be maximum M9 class and the number of deaths could reach as large as 320,000 or even more.

In IRIDeS, young researchers volunteered to create a study group on the Nankai Trough earthquake and have been organizing monthly meetings by inviting experts of various fields to study and discuss pertinent isues. The central theme, above all, is how we can utilize uncertain forecast information to mitigate disasters. We cannot predict eartquakes in our current knowledge. However, seafloor observation networks have rapidly improved in recent years and have made it possible to capture oceanic plate movements and other phenomena, slow slips for example, that possibly trigger large earthquakes. Currently, in the Nankai Trough area, “the world's best observation network” that can acquire real-time data is in place.

With our current knowledge on earthquake physics, it is difficult to know if abnormal signals captured in the observation data indicate an impending earthquake. But we may know, with a large uncertainty, if the probability of an earthquake has increased or not. Can this type of information be utilized for disaster mitigation by allowing precautionary measures? Discussions have been going on among researchers, whose backgrounds are in different disciplines such as science, engineering, psychology, and volcanology, local government officials who have been taking part in disaster risk reduction plans in the Nankai Trough area, and journalists who have been involved in disaster reporting. Yo Fukushima, an associate professor of the Disaster Science Division in IRIDeS and the head of the study group organizers, states that "by continuing seminars and discussions for more than a year, we have been able to consider the problem from various aspects. We hope to continue working deeper on this theme through multidisciplinary discussions.”

Researchers can well analyze the past and present if data are available, but they cannot accurately predict the future. Yet, can we utilize available scientific information to build a better future, to mitigate the risk of Nankai Trough earthquakes? The researchers' pursuit continues.

A study meeting with an invited guest, Mr. Naoshi Kitagawa, a former trustee of Kochi Prefecture.

(Photo taken on January 22, 2018)

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