Matsushiro Seismological Observatory

The Matsushiro Seismological Observatory (MAT) was established on May 1, 1947, taking advantage of the unfinished large-scale tunnel system planed and constructed by the Imperial Japanese Army. These tunnels, measuring about 3km in total length and 2.5 to 6m in diameter, locate in the depth 100m below the surface so that the internal air temperature changes little throughout the year. MAT is away from the noise sources such as railroad, automobile traffic, and the sea coast, which is at least 60km away. Those conditions were suit for the seismograph installation. (MAT is not an exception recently suffering from the artificial noises.)

MAT had been expanding its facilities such as Benioff seismograph installation in cooperation with IGY (International Geophysical Year), and followed by WWSS (World Wide Seismograph), ASRO (Abbreviated Seismic Research Observatory) system, IRIS (Incorporated Research Institutions for Seismology) system, and extensometer system composed of two-component 100m-long quartz tubes.

On August 3, 1965, three unfelt tremors were recorded at MAT. It was an inception of the Matsushiro earthquake swarm, which is peculiar and unique in that it started directly beneath a well-equipped observatory. Swarm activity had intensified to 100 felt shocks a day in early November 1965,and culminated in 6780 shocks a day on April 17, 1966 including 585 felt shocks with 3 shocks with JMA seismic intensity V.

In the initial phase of the earthquake swarm, the activity was within an elliptical area with N-S axis measuring 12 km and E-W axis measuring 7 km, and the corresponding focal depths were 2-8km. Accompanying the activity area had expanded with time, the frequency of shocks had been declining gradually. Although significant shock had seldom observed around Matsushiro since 1967, micro-earthquakes have been occurring in the bygone source area even at present.

The Matsushiro Earthquake Center (MEC) had been established in 1967 as a conference institute among government, the local organizations and the research institutes of universities committing Matsushiro earthquake swarm activity. MEC’s task is not only performing the datacenter for Matsushiro earthquake swarm, but also collecting the data concerning other earthquake swarms and earthquake disasters, contributing to the plans against earthquakes promoted by local communities, responding to the request by the public, and so on.

The MSAS consists of 8 seismic stations arranged in a circle, of which diameter is about 10km, and data processing units at MAT. The advantage of seismic array observation is such that:

(1) It is possible to reduce the interruption by background noise, which is called beamforming technique.
(2) The arrival direction and apparent velocity of the seismic signals can be determined by the arrival time difference at each array