Abstract
We analyzed data obtained by the water-tube tiltmeters and the horizontal pendulum tiltmeters installed at the OGA and NIBETU station of the Akita Geophysical Obser vatory for the period of 1968 to 1975. The distance between the stations is apart about 39 km and we perform a comparative study for ground tilt. Short and long period ground tilt characteristics are clarified using Chebychev approximation funciton. It is shown that ground tilt variation at the OGA is 5 to 10 times greater than that at the NIBETU by the end of 1973 when anomalous strain changes are recorded at many crustal movement observatories in Japan, and after that time ground tilt variation for both the stations becomes almost the same order. It is found that ground variations observed by the horizontal pendulum tiltmeters, the water-tube tiltmeters and the level ling show similar tendency, but the ground variation obtained by horizontal pendulum tiltmeters is about several times larger than that by the water-tube tiltmeters and ground movement obtained by water-tube tiltmeters is larger about one order than that by the levelling. It is also found that the OGA peninsula moves clockwise. It is noted that tilt move ments showed anomalous change before the earthquakes with magnitude 5.3 and 6.2 occurring at the distance 30 km from OGA and 80 km from NIBETU respectively.
