The 1984 Nagano earthquake hit the western part of Nagano Prefecture, Japan on September 14, 1984, at 08:48 local time (September 13, 1984, at 23:48 UTC). Registering a magnitude of 6.3, the earthquake destroyed Otaki (Japanese: 王滝(おうたき)), and triggered major landslides. The earthquake left at least 29 people dead or missing, making it the deadliest earthquake in 1984.

Geology

Although the epicenter was only 2 km deep, no visible fault appeared. The Japan Meteorological Agency estimated that two faults, one 15 km long and one 5 km long had ruptured simultaneously.

Relation to other earthquakes

Seismologists including Akeo Yoshida state that the 1948 Fukui earthquake, a 7.0 earthquake in Gifu Prefecture, 6.6 earthquake in Gifu Prefecture in 1969 and this earthquake were earthquakes that occurred in a cycle in small period of time in the same area.

Main shock

Since, there was no seismometer in the area Japan Meteorological Agency made an estimate intensity Shindo 6. Some unofficial estimates put it to Shindo 7. There are reports that rocks and pieces of wood flew in the air near the epicenter, due to ground accelerations faster than gravitational acceleration in the 5 Hz~10 Hz shaking range. A seismometer at Makio Dam, 4 km away from the epicenter observed very strong shaking but could not record more than 0.3 g which was the limit.

Damage

Due to torrential rains in the area before the earthquake, many landslides occurred.

Incidents

Precursors

Response

Nine check dams were built on Mount Ontake in four years after the earthquake.