「重力乱れ」と地球内部構造

抄録

This paper claims that ‘Gravity disturbance’ instead of free air anomaly should be used for deriving the subterranean structure of the earth from gravity anomalies. The free air gravity anomaly is affected not only by the underground density anomaly but by the undulation of geoid which is also caused by the same density anomaly. The gravity disturbance is a kind of free air anomaly reduced to the surface of the normal earth ellipsoid which is the reference surface to estimate the geoidal height. The geoid undulation that is caused by the disturbing potential is also affected by the reference surface on which the disturbing potential is evaluated. This effect has been examined, leading to a conclusion that the effect is apparent, but that its amount is negligible from the viewpoint of the influence on the density structure.
Bouguer reduction which is to be applied to gravity disturbance is also discussed. ‘Bouguer disturbance’ thus newly defined can be obtained by conducting the Bouguer reduction while replacing the elevation by the height measured above the normal ellipsoid in the case of land, or replacing the depth of sea by the depth minus the geoidal height in the case of sea.
Comparison between the free air anomaly and the gravity disturbance in and around the Japanese islands shows that the differences which are the averages within grids 1°×1° range from -0.3mgal to +14.5mgal. In order to examine how the density structure of substratum would change if the gravity disturbance is used instead of the free air anomaly the 2 dimensional structure across the northeast Honshu along a parallel 39°45′N that had been obtained by Segawa and Tomoda (1976) was reexamined. The result shows that the density of crust may change by the amount of +0.01g/cm³, and that the density of the low velocity zone in the upper mantle inferred to exist beneath the Sea of Japan may change by the amount of +0.0009mgal. The difference of the density obtained by reestimation seems to be small, suggesting that the difference between the free air anomaly and the gravity disturbance might not be so important. However, the author is of the opinion that the gravity anomaly that can be used equivalently to the height of geoid is not the free air anomaly but the gravity disturbance, and that even the minor difference in density estimated here would become more important in future when better information in the other field of geophysics becomes available.