新潟地震の震域における重力の変化
1966 年 19 巻 3 号 p. 200-216
詳細
1966 年 19 巻 3 号 p. 200-216
During Jun. 1964 and Mar. 1956, the Geographical Survey Institute carried out the resurvey of the gravity in the shock area of the Niigata Earthquake, 16 Jun. 1964 immediately after its occurrence. By using the results of these works, we can discuss the possibility of the gravity change in this area. Also summarizing the results of all the other re-surveys in Niigata Plain we can study the change in this plain. There are many level-lines where the re-survey of gravity was made at various chances.
In the study of gravity change, it is the main purpose to detect the change in gravity caused by the change in density of the underground mass. So we adopt the method of comparing the old with the new Bouguer anomaly value calculated by the G. S. I. system where the vertical change in gravity is assumed to be 0.3086mgal/m, the density of the mass above sea level is 2.67, and no topographic correction is applied.
By this comparison, the following two facts can be accepted;
(1) Along the level line from B. M. 6519 to B. M. 6493 that runs near the sea shore of Honshu opposite to Awashima island, it was found that the wave-form change of Bouguer anomaly during 1954 and 1964 with the amplitude of 0.25mgal and the half wave length of 52km.
(2) In the Niigata Plain along the level line from B. M. 6475 to B. M. 4430, also there was found the decrease of Bouguer anomaly during 1954 and 1964 with the amplitude of 0.25mgal and the half wave length of 50km.
In order to discuss the meaning of these phenomena, we assume the distribution of the observed change in gravity at earth surface as 0.25mgal·sinπ/52km·xkm in the case of (1). Then the corresponding mass distribution with 0.25gr/cm3 change in density is proved to be 108m·sinπ/52km·xkm at depth of 25km that is the mean depth of the after shocks of Niigata Earthquake. The corresponding change of vertical gradient of gravity is 1.5×10-5mgal/m·sinπ/52km·xkm.
In the case of (2), as the thickness of the deposit of low density that is presumed to be a cause of the negative Bouguer anomaly in Niigata Plain is 1800m at its maximum, we try to calculate how much growth of this deposit can explain the change in gravity. The observed distribution of gravity change is 0.25mgal·sinπ/50km·xkm, then the corresponding growth of the thickness of the deposit is 8.9m·sinπ/50km·xkm during 10 years. This value is too great by ten times as compared with the rate of crustal deformation obtained by the first order leveling.
