ON SECULAR CHANGE OF GRAVITY

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ON SECULAR CHANGE OF GRAVITY 39 sures were adopted : taking the gravity value at the base station (Geophysical Institute of Kyoto University) as the coordinate origin, the gravity difference between each station and the base station as the abscissa, and plotting the difference between the gravity value of the present survey and that of the former one at each station as the ordinate, as shown in Fig. 2. For the former gravity values those of the survey in 195:.! were adopted by some reasons. In Fig. 2, it will be ~ mgol -r- • • 0.6 r-- • • • • ~ 0 0.2--: r-- . - r- g g • ~ .. ·5 - r-- • . ' '! • "' .. . • I '
40 T. ICHINOHE, I. NAKAGAWA AND N. SUMITOMO mgol 0.6 A .. 0 0 • Wotde~ Gro.vtft'le.::::l . o . Askmi•O Orov•meler _ ···l 1----0 -0+--+---+--+---1'--7~4---c-8--+-- i -0.2 • ~0 I ~~;' ~ 8 o- I -0.2 ~ o "I o I" I : J- t--- •o ~~: ~. c'. ·,_·-.+1.-.-'·'-11--+--+------+----- -o.•~ so .,, ~ D 0.2 - o .I -0,2 ~;' ~ E o- I -0.2 • I so . >O . I ' .,, ~ F ' 0.2 r---+----t--+--+--+1----". I' -o.: 'oo • • I ·'f--1 _;__~ j .--+-+ i.--;-: -+-I---- Fig. 3 Gravity change in each survey route. The abscissa represents the distance of each ob· servation station from the base station. changes in the routes A, B, and E, especially in the routes A and B it appears quite similarly on the both observed results by the Worden and Askania gravimeters. If it were merely originated from some instrumental or observational errors, it will be natural to attribute its origin, first of all, to the irregularity in the drift speed of the gravimeter used. If the drift speed were irregular, contrary to our assumption that drift had kept a uniform speed during one closing measurement, such gravity changes as observed in Fig. 3 should appear as the natural results of data analysis even if the fact were not so. It may be the change of temperature what produces the most effect on the irregularity of drift speed. But it was proved by the observations of weather stations that the atmospheric temperature of the days had been neither remarkably high nor low. It is also possible that intense solar radi· ation of the days might affect the drift speed. In this case, however, it is vEry strange that the two different types of gravimeters were affected by solar radiation in the quite same way, though it is not necessarily impossible. There may be the case that the upheaval or subsidence of the ground, as the results of the displacement of the observation stations, produced apparent gravity changes. But it is impossible that there has occurred during the last ten years so great ground subsidence that produced such gravity changes as shown in Fig. 3. There may be supposed the case that the change of underground water level produced some effects on gravity value. But it is also impossible in the present case, because it needs the change of water level reaching to about 25 metres to produce such gravity changes. Considering the above-mentioned conditions, it ought to be taken into account that the gravity changes shown in Fig. 3 possibly represent real gravity changes
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ON SECULAR CHANGE OF GRAVITY

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