Geodetic work in Japan with special reference to the Matsuchiro earthquakes



Geodetic surveys, first-order leveling, triangulation with electro distance measurements, and geomagnetic and gravity surveys have been conducted by the Geographical Survey Institute in and around the area of the Matsushiro earthquake swarm in the northern part of Central Japan during the period from 1965 to 1968. During this period leveling surveys were resurveyed every three months in the vicinity of Matsushiro and the other surveys were made at approximately six-month intervals. From the results of the surveys, the following facts have been found.

Extraordinary vertical deformations have continued near Mt. Minakami, with upheaval from October 1965 to September 1966 through the first and second peaks of seismic activities, followed by a subsidence. The maximum rate of the upheaval motion was about 50 mm per month. On the other hand, in the Omachi-Matsukawa region, which is about 30 km from the central area of the earthquake swarms, anomalous reversals of the pattern in the vertical deformation have occurred, for which the period also seems to be about six months. The mean rate of up and down motion was about 40 mm per six months. Such a reversible vertical deformation was also found in the large region along the first-order leveling routes through Nagano, Ueda, Matsumoto, and Omachi during the whole period of about three years, 1965–1968.