The Matsushiro earthquake swarm () was an

earthquake swarm In seismology, an earthquake swarm is a sequence of seismic events occurring in a local area within a relatively short period. The time span used to define a swarm varies, but may be days, months, or years. Such an energy release is different fr ...

that occurred near

Matsushiro Matsushiro may refer to: * Matsushiro Domain file:松代城(海津城) Matsushiro castle 2011.1.1 - panoramio (2).jpg, 300px, Matsushiro Castle file:Ryukoji05.JPG, Part of the Matsushiro domain's Edo estate, relocated to Kamakura and used as a ha ...

, a suburb of Nagano, to the northwest of

Tokyo Tokyo, officially the Tokyo Metropolis, is the capital of Japan, capital and List of cities in Japan, most populous city in Japan. With a population of over 14 million in the city proper in 2023, it is List of largest cities, one of the most ...

in 1965. The event is one of the best ever documented

earthquake swarms In seismology, an earthquake swarm is a sequence of seismic events occurring in a local area within a relatively short period. The time span used to define a swarm varies, but may be days, months, or years. Such an energy release is different fr ...

Overview

The Matsushiro swarm lasted from 1965 to 1967 and generated about 1 million earthquakes. The total sum of energy from all the tremors was approximately equivalent to an M6.4 earthquake. This swarm had the peculiarity to be sited just under a seismological observatory installed in 1947 in a decommissioned military tunnel. It began in August 1965 with three earthquakes too weak to be felt, but three months later, a hundred earthquakes could be felt daily. On 17 April 1966, the observatory recorded 6,780 earthquakes, with 585 of them having a magnitude large enough to be felt, which meant that one earthquake could be felt every 2 minutes 30 seconds on average. The phenomenon was clearly identified as linked to a

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

uplift, perhaps initiated by the

1964 Niigata earthquake The 1964 Niigata earthquake () struck at 13:01 local time (04:01 UTC) on 16 June with a magnitude of either 7.5 or 7.6. The epicenter was on the continental shelf off the northwest coast of Honshu, Japan, in Niigata Prefecture, about north of ...

which happened one year before.

References

1965 earthquakes 1966 earthquakes 1967 earthquakes 1965 in Japan 1966 in Japan 1967 in Japan Earthquakes of the Showa era Earthquake swarms {{Asia-earthquake-stub