1999 Volume 52 Issue 4 Pages 45-54
Shimabara-Shigatusaku Earthquake occurred on 21 May 1792 (1 April, the 4th year of Kansei, Lunar calender of Japan) in the Unzen volcanic area in western Kyushu, during the last stage of volcanic activity of Mt. Fugen in the period between the end of 1791 and the beginning of 1792. A gigantic landslide was induced by the earthquake on the eastern slope of the Mayu-Yams lava dome of the Unzen volcanic area. Huge amount of soil and rocks rushed into the Ariake Bay generating a big tsunami. The tsunami hit both sides of the bay, and claimed the lives of approximately 15, 000 persons. This is the worst disaster in the history of volcanic hazards in Japan, called as “the Shimabara Catastrophe”.
Two old maps concerning the Shimabara Catastrophe prepared in the Tokugawa Era were discovered. One is a predisaster map titled as “Daisin-zu in Kansei 4”, as shown in Fig. 3 and the other one is a post-disaster map titled as “Big map of the Shimabara Catastrophe”, as shown in Fig. 4. Fig. 3 was prepared after flow down of the Shinyake lava flow occurred 25 March-21 April and another earthquake lasted for 2 days period of 21-22 April. Fig. 4 was prepared after Shimabara-Shigatusaku Earthquake and the Shimabara Catastrophe on 21 May.
It is supposed that these two maps were drawn exactly from the same place in the Shimabara Castle area. Fig. 5 is bird's-eye view of Mt. Mayu-Yama from the Shimabara Castle based on Digital National Land Information 1996. Photo. 1 is a picture of Mt. Mayu-Yama taken from the Shimabara Castle. These maps and photo show almost same objects in different times.
In the center of the post-disaster map as shown in Fig. 4, approximately 50 hummocky debris mounds are shown and on the left hand side which is southern part of the debris mounds, debris flow is depicted. Thus, the map shows that Mayu-Yama was broken down by two different forms of collapse, which caused the debris mounds and the debris flows. According to the map, the author assumes that these movements might not occurre at the same time.
