Matsushiro Seismological Observatory, Seismological research facility in Matsushiro, Nagano, Japan

Matsushiro Seismological Observatory is a Meteorological Agency facility at the southern foot of Mount Maizuru (510 m) in Matsushiro, Nagano. Its main seismographs sit in the underground tunnels of the former Matsushiro Imperial Headquarters, about 100 m below the summit, in stable diorite and black shale. The site was chosen for its distance from the coast and from large cities.

The facility was founded in 1947 by repurposing military tunnels left from World War II into specialized measurement chambers. This transformation from wartime infrastructure to scientific research marked a significant turning point in Japan's approach to earthquake understanding.

The observatory's name appears in the nickname "Matsushiro Magnitude", used for earthquake magnitudes determined from data received via the USGS LISS and IRIS BUD servers. Under the Comprehensive Nuclear-Test-Ban Treaty its seismometers feed data online to the Vienna data centre, where underground nuclear blasts are distinguished from natural quakes by waveform. Much of that signal reached Matsushiro at night, Japan Standard Time, allowing finer readings.

The observatory is at 3511 Saijo, Matsushiro-machi, Nagano. From Nagano Station take a bus to Matsushiro High School (about 40 minutes), then walk about 20 minutes; from Yashiro Station the bus takes about 25 minutes. By car it is about 10 minutes from Nagano Interchange on the Joshinetsu Expressway. The facility was unmanned in 2016, so check access in advance.

The Matsushiro earthquake swarm began on 3 August 1965, the same day strainmeter observations started at the observatory. The instruments in the tunnels under Mount Maizuru recorded the sequence from its first shocks, giving seismologists a continuous local record of an earthquake swarm that lasted for years.