2023 Volume 23 Issue 1 Pages 1_59-1_78
After the Great East Japan Earthquake, the Japanese government terminated the “deterministic” earthquake prediction program for the Tokai area implemented in 1978. Instead, an “uncertain” earthquake information called “the Nankai Trough Earthquake Information” was introduced in 2017. The information is to be issued when the possibility of the occurrence of the megathrust Nankai Trough earthquake is assessed to be relatively higher than normal, but doesn't specify the time, place, and magnitude of the earthquake. To respond to this information, the Japanese government also published guidelines for proactive disaster mitigation and required municipalities that would be damaged by shakes and tsunamis to make disaster prevention plans. In this paper, we analyze government documentation to show two transformative and unprecedented operations following the information, which are completely different from conventional disaster mitigation measures for earthquake information. First, switching to so-called “alert mode” such as large-scale advance evacuation is to be conducted despite of the low accuracy of the Nankai Trough Earthquake Information in terms of prediction. In fact, according to a government's estimation for the case of “a partial area rupture”, which is one of the three cases to release the information, the chance of the subsequent large earthquake on the western side of the focal area triggered by a M8-class earthquake on the eastern side is estimated to be as low as 7%. Second, termination of alert mode is not based on seismological judgement; it is officially decided that when one week passes after the release of the information with no subsequent large earthquake, the government lifts pre-evacuation regardless of the situation of seismic activity and/or crustal deformation. This is because earthquake science is uncapable of informing whether warning status can be rescinded or not. These operations are the result of government's policy decision upon how to use uncertain seismological knowledge in disaster mitigation. We review the processes of implementation of the Nankai Trough Earthquake Information and discuss several issues in current disaster prevention measures.
