The island of Kyushu, Japan, is home to Mount Unzen – a volcanic group situated on the Shimabara Peninsula, about 25 miles east of Nagasaki. Unzen has several lava domes – mounds atop volcanoes, resulting from the accumulation of slow seeping lava, which cools and solidifies before flowing very far. On May 21st, 1792, a volcanic eruption caused one of those lava domes to fall into the sea, resulting in a tsunami and earthquake that caused considerable devastation and loss of life.
It began months earlier, in late 1791, with tremors and earthquakes on the western side of Mount Unzen, which gradually made their way up to one of its volcanic peaks. In February of 1792, one of those peaks began erupting, causing lava to flow for the following two months.
As the lava kept flowing from Mount Unzen, the earthquakes and tremors continued. On the night of May 21st, 1792, two big quakes hit. They were powerful enough to shake one of the lava domes loose, causing it to collapse down the eastern side of the mountain. That triggered a landslide, which swept through the city of Shimabara down below, and continued on to Ariake Bay.
When the landslide struck the water, it caused a mega tsunami, with waves nearly 70 feet high, rising up to 187 feet high in some places because of the seabed’s topography. The tsunami traveled across Ariake Bay, until it hit the city of Higo on the other side, where it caused widespread devastation. It then bounced back across the bay, and hit the city of Shimabara, where the dust had still not settled from the landslide that had swept through it and triggered the tsunami in the first place. About 15,000 people were killed in the disaster, making it Japan’s worst volcanic eruption.
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.
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