This caused the collapse of the southern flank of the Mayuyama dome in front of Mount Unzen, resulting in a tremendous tsunami, killing 15,000 people altogether.

On the night of 21 May, two large earthquakes were followed by a collapse of the eastern flank of Mount Unzen's Mayuyama dome, causing a landslide which swept through the city of Shimabara and into the Ariake Sea, triggering a great tsunami.

The tsunami struck Higo Province on the other side of Ariake Bay before bouncing back and hitting Shimabara again.

At the Osaki-bana point Futsu town, the waves locally grew to a height of 187 ft (57 m) due to the effect of sea bottom topography.

Its size was first 1 km (south-north) and 300m 400m (east to west), but the production of a water exit river made it smaller and it is now 200 m by 70 m. As a result of the destruction, Tsukumojima or 99 islets or rocks were distributed near Shimabara city.