Abstract
Precise levelings across the Atera fault have been repeated every summer at Sakasita, Gifu-Prefecture since 1976. The leveling network consists of two loops with lengths of 1.3km and 2.6km. Loop closures of leveling were within 1.4mm. Geodetic tilt vectors are calculated from horizontal positions and annual elevation changes at bench marks by least squares method. If a residual of elevation change greater than the twice the standard deviation of the tilt vector is found at any bench mark, the calulation is repeated excluding the erroneous data. Significant ground tilts are observed four times, i. e., 1976-1977, 1981-1982, 1982-1983 and 1985-1986, and these tilt rates are 1.0-2.0μrad/y. Atera fault is geologically characterized by the uplift on the northeast side. However the present movement suggests northeast tilting during three epochs and southwest tilting at one epoch. The 1985 survey suggests that the western Nagano Prefecture Earthquake (ML=6.8; Sept. 14, 1984) about 30km to the north, caused co-seismic tilt of the ground.
