(4)Earthquakes on the Shimabara Peninsula (December 8, 1922, M 6.9, M 6.5)
These two earthquakes occurred in succession in less than 10 hours. The first was at 1:50 a.m. (M 6.9), and the second was at 11:02 a.m. (M 6.5). They occurred at shallow locations on land. The focal region for both earthquakes is thought to have been near Chijiwa Bay (Fig.9-16). The first earthquake caused extensive damage in the southern Shimabara Peninsula, particularly Kitaarima. The second earthquake caused substantial damage near Obama in the western part of Shimabara Peninsula. Combined, these two earthquakes resulted in 26 fatalities, 39 people injured, and the complete collapse of some houses. Stone monuments were also damaged in Amakusa and Yatsushiro in Kumamoto Prefecture. The aftershocks gradually decreased (Fig.9-17). An M 6.4 earthquake occurred on May 21, 1792 near the Shimabara Peninsula. This caused the partial collapse of Mt. Mayuyama (at that time called Mt. Maeyama), and a tsunami that resulted in destruction on the coast of the Ariake Sea (Ariake Kai). The tsunami is estimated to have been a maximum of about 9 m high. The tsunami killed 15,000 people and washed away more than 5,000 houses. The damage resulted in the greatest number of deaths ever caused by a volcano in Japan.