The occurred at about 21:30 local time on 8 May at Nagano, Japan. It had a magnitude of 7.4 on the

surface-wave magnitude The surface wave magnitude (M_s) scale is one of the magnitude scales used in seismology to describe the size of an earthquake. It is based on measurements of Rayleigh surface waves that travel along the uppermost layers of the Earth. This mag ...

scale. It caused the destruction of many houses in Nagano and at least 8,600 people were killed. The earthquake triggered many landslides, the largest of which dammed the Sai River (a tributary of the

Shinano River The , known as the in its upper reaches, is the longest and widest river in Japan and the third largest by basin area (behind the Tone River and Ishikari River). It is located in northeastern Honshu, rising in the Japanese Alps and flowing g ...

). 19 days later, the failure of this earthquake dam caused a

flood A flood is an overflow of water (list of non-water floods, or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are of significant con ...

that destroyed many more houses and killed 35 people.

Tectonic setting

During the middle of the

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

period Japan was affected by the opening of the

Sea of Japan The Sea of Japan is the marginal sea between the Japanese archipelago, Sakhalin, the Korean Peninsula, and the mainland of the Russian Far East. The Japanese archipelago separates the sea from the Pacific Ocean. Like the Mediterranean Sea, it ...

as a

back-arc basin A back-arc basin is a type of geologic Structural basin, basin, found at some convergent boundary, convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found ...

in response to continuing

subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...

of the Pacific plate. This period of

extensional tectonics Extensional tectonics is concerned with the structures formed by, and the Tectonics, tectonic processes associated with, the stretching of a planetary body's Crust (geology), crust or lithosphere. Deformation styles The types of structure and the ...

caused the development of rift basins on the western side of

Honshu , historically known as , is the largest of the four main islands of Japan. It lies between the Pacific Ocean (east) and the Sea of Japan (west). It is the list of islands by area, seventh-largest island in the world, and the list of islands by ...

, including the northern part of the

Fossa Magna Fossa Magna is a great rift lowland in Japan. It is often confused with the Itoigawa-Shizuoka Tectonic Line. However, Itoigawa-Shizuoka Tectonic Line is a line; Fossa Magna is an area. Fossa Magna is Latin for "great crevasse". This name was gi ...

. When the back-arc spreading stopped during the late Miocene, the rifted area started to shorten and the basins became inverted. The western margin of the Nagano Basin, part of the Fossa Magna, is formed by an

active Active may refer to: Music * ''Active'' (album), a 1992 album by Casiopea * "Active" (song), a 2024 song by Asake and Travis Scott from Asake's album ''Lungu Boy'' * Active Records, a record label Ships * ''Active'' (ship), several com ...

reverse fault zone, the West Nagano Basin Fault. The 1847 earthquake was caused by rupture on part of this fault zone.

Landslides

The earthquake triggered a large number of landslides, due mainly the relatively unconsolidated Miocene sediments. At least seven of these formed dams on rivers in the surrounding area. 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. The water held back by the dam eventually broke through 19 days after the earthquake, flooding a large area downstream from the dam.

References

{{Authority control 1847 earthquakes 1847 in Japan May 1847 Nagano (city) Earthquakes of the Edo period Earthquakes in the Empire of Japan Landslides in Japan 1840s disasters in Asia 19th-century disasters in Japan