Of the roughly 15,000 killed by the Mount Unzen eruption, about 5000 were estimated to have been killed in the landslide that swept through Shimabara city. Another 5000 were estimated to have been killed by the ensuing tsunami when it reached Higo, across the bay from Shimabara.
The final 5000 were estimated to have been killed when the tsunami bounced back from Higo, recrossed the bay, and struck Shimabara. It did not go unnoticed that the eruption had occurred Mount Unzen, in the Shimabara Peninsula, but many deaths from the ensuing tsunami occurred in Higo, about 15 miles away across the Ariake Bay. That gave rise to a Japanese saying about things that happen in one place, yet impact those elsewhere: Shimabara erupted, Higo impacted.
Where Did We Find This Stuff? Some Sources and Further Reading