PRECISE RELATIVE MFASUREMENTS OF GRAVITY IN KINKI DISTRICT, JAPAN

重力の経年変化を検出するために, 琵琶湖周辺地域において, 1971年以降, 重力測定を繰り返し実施してきた。さらに, 使用するラコステ重力計の特性実験を通じて, より精度の高い重力測定を実施するためには, ほぼ等しい重力値をもつ測定点を選んで実施する必要があることを認め, 基準点(京都大学理学部地球物理学教室, g=979.70775gal)とほぽ等しい重力値をもつ測定点を琵琶湖や紀伊半島を含む地域の水準線に沿て選び, 1972年以降, そこで精密重力測定を実施してきた。さらに, 測定点の空間的分布を考慮して, ほぼ979.686galの重力値をもつ測定点も選び, そこでも同様の精密測定を実施してきた。重力測定結果の精度につい, さまざまな観点から検討した。その結果, 精密重力測定の精度は標準偏差で10μgal程度と考えられる。重力測定により得られた結果を, 水準測量, 琵琶湖の水位測定, 検潮, 地下水位および三角測量の結果と比較した。得られた重力変化は, 水準測量から得られた上下変動とはあまり相関を示さなかったが, 湖水位や検潮記録から得られたそれとはかなりよい相関が認められた。短距離の範囲で, 上下変動を検出するには, 精密重力測定は水準測量に比べ精度面で劣る。しかし, 精密重力測定は, 遠く離れている2点間においてもそれらを直接結合できるという利点がある。水準測量ではこれが不可能であるが, 湖水位や検潮は重力測定と同じ利点をもっている。今回の結果は, これらに共通した利点の現われかもしれない。一方, 紀伊半島の先端部において重力の増加が観測された。この一部はフィリピン海プレートの押しによる陸側の地殻の圧縮により説明できるであろう。Gravity measurements have repeatedly been carried out in the area around Lake Biwa since 1971 in order to detect the secular change of gravity. In addition, through the examination on characteristics of LaCoste & Romberg gravimeters, we proposed the method of measurement at the stations with almost equal gravity value in order to carry out the gravity measurement more accurately. And therefore, another precise gravity measurement has been carried out at stations where the gravity value is almost equal to that of reference station (Geophysical Institute of Kyoto University; 979.70775 gals), along the levelling route in the area covering Lake Biwa and the Kii Peninsula since 1972. Besides them, the precise gravity measurement has also been carried out at stations with the gravity value 979.686 gals in due consideration of their distribution. The accuracy of the results was investigated in many points of view, and it was asc.ertainecl to be usually about 10 μgals in standard deviation. The results obtained the gravity measurements were compared with the results obtained by levelling surveys, by water level observations of the lake, by oceanic tidal observations, by underground water level observations and by geodetic control surveys. The gravity change was not in a good correlation with the vertical movement obtained from levelling surveys, but it was with the movements obtained from the observations of water level and oceanic tides. The precise gravity measurement is less accurate than the levelling survey when the vertical movement is to be obtained in a short distance. However, it is a very great advantage that the precise gravity measurement can directly connect two stations which are located far apart. The levelling survey cannot realize this advantage, while the observations of water level and oceanic tides have this advantage. The results of the present investigations may be ascribed to this advantage. On the other hand, the gravity increase was found at the tip of the Kii Peninsula, and this may be partly explained by the areal contraction caused by the push of the Philippine Sea Plate.publishe