Abstract
A gravity survey was extensively carried out around Mt. Ontake as a part of the 1986 Joint Seismological Research in Western Nagano Prefecture, central Japan (the JSR'86). Together with the overall revision of the existing data sets compiled by the Nagoya University, we completed a Bouguer anomaly map around Mt. Ontake with a high resolution of about 1 mgal. It delineates detailed gravity features in the area surrounding the focal region of the 1984 Western Nagano Prefecture Earthquake. A triangular crustal block around Mt. Ontake and a linear anomaly belt along the Atera Fault mark specific tectonic features. A characteristically strong gradient belt of Bouguer anomaly is outstanding in the middle of the triangular crustal block. The seismic fault plane of the 1984 Western Nagano Prefecture Earthquake runs along this belt and dips steeply northward. The fault plane of the largest aftershock exactly coincides with the western end of the gradient belt of Bouguer anomaly.
