明応七年六月十一日（1498年6月30日）の日向灘大地震は存在しなかった

Abstract

A large earthquake (M7.0~7.5) in Hyuga-nada Sea on the east side of Kyushu along Nankai trough in Mi-no-koku (approximately 9-11 a.m.) on June 30th, 1498 on Julian calendar, which was based on the descriptions about serious earthquake damage in Kyushu in the war chronicle Kyushu-gunki, has been accepted by many seismologists as the 1498 Meio Hyuga-nada earthquake. Kyushu-gunki is one of the popular novels which was written more than 100 years after the 1498 event. The damage descriptions have been used to estimate the location of this earthquake without evaluating the reliability. In this study, in order to assess credibility of the description, we carefully examined writing process of Kyushu-gunki and damage descriptions in this war chronicle. Our findings are summarized as follows: (1) Kyushu-gunki was completed in the current Saga prefecture (northwestern part of Kyushu) in 1607, hence the writers’ experience of the contemporary 1596 destructive earthquake in northeastern Kyushu might have influenced the description of the damage of 1498 earthquake; (2) Damage descriptions of Kyushu-gunki were generic without location information and most of them were cited from descriptions of damage in Kyoto due to the large earthquake on Aug. 6th, 1185 in the war chronicle Gempei-josui-ki; (3) Origin time (Mi-no-koku) of this event is described only in Kyushu-gunki and it is close to that of the great Tokai earthquake on Sep. 11th, 1498 (Tatsu-no-koku: approximately 7-9 a.m.); (4) The chapter of the earthquake damage also includes serious famine in Kyushu in 1503 and pains of people by many disasters, making this chapter a stage setting for later stories. To make a story of the war chronicle more interesting, writers seem to have created fictitious story of serious damage due to a large earthquake in Kyushu based on old war chronicles and some records of the great Tokai earthquake. Therefore, we concluded that the 1498 Meio Hyuga-naga earthquake is a fake earthquake and it should be deleted from Japanese historical earthquake catalog. Other documents in Kyoto and Nara recorded an earthquake in Saru-no-koku (approximately 3-5 p.m.) on June 30th, 1498, but the location and magnitude are not clear.