Abstract
A gravimetric network for precise gravity measurements was established in the Tokai district, central Japan, which was known as one of the most active area of vertical crustal movement in Japan. The network consists of 57 gravity stations and the gravity values at 17 stations among them are selected very close within a difference of ±0.3 meals each other. The precise gravity measurements, which are aimed to detect a small gravity change associated with the crustal movement and to utilize them for the earthquake prediction in the area concerned, have been repeated three times a year since 1981. An unexpectedly large variation of the gravity changes is recognized at the stations whose gravity differences are large compared to that of the reference station. On the other hand, the variation at the station whose gravity values are very close to that of the reference station is very small. The gravity changes at the stations with gravity values close each other show similar patterns regardless of any local differences. This systematic and characteristic appearance in the patterns of the gravity changes suggests that it may be originated from the periodic irregularity of the composite gear system of the LaCoste and Romberg gravimeter employed for the present study. Although any effective method to correct this periodic errors has not been established yet at present, it is expected that a very small gravity change with a reliability of several μGals may be detectable.
