The 1847 Zenkoji earthquake (善光寺地震, Zenkōji jishin) occurred at about 21:30 local time on 8 May at Nagano, Japan.

19 days later, the failure of this earthquake dam caused a flood that destroyed many more houses and killed 35 people.

This period of extensional tectonics caused the development of rift basins on the western side of Honshu, including the northern part of the Fossa Magna.

When the back-arc spreading stopped during the late Miocene, the rifted area started to shorten and the basins became inverted.

The largest of these dams was formed at the foot of Mount Iwakura from a landslide with an estimated volume of 20 million cubic metres that blocked the Sai River.