Origin of negative gravity anomaly landward of trench junction

抄録

Off Urakawa, Hokkaido district landward of a junction of Chishima trench and Japan trench, there is a large negative gravity anomaly of about 200 mgals. Off Miyazaki, Kyushu district landward of a junction of Ryukyu trench and Nankai trough, there is also large negative gravity anomaly.
If the oceanic plate uniformly moves, an excess mass is produced at the trench junction and the excess mass should be observed as a positive gravity anomaly. However, on the contrary, large negative anomaly is actually observed. The idea presented in this paper is that, the excess mass falls in the mantle, and this causes a depression of earth surface (free surface), and lack of mass at the surface produces negative free air gravity anomaly.
The problems of surface deformation due to falling ball which is rigid and denser than the surround viscous material, were discussed by Morgan in 1965, ^{1), 2)} and excellent papers were published. The deformations of the surface due to a rigid falling ball are divided into two parts. The one is due to direct attraction of the ball and gravity does not change though the surface sink in. The other is due to a current within the viscous fluid and the surface sink in and gravity changes. The amount of the change in gravity is roughly twice as large as gravity due to descending ball, with opposite sign.
The water contained in sediment on the ocean floor is transported to a deeper part of the mantle by a descending plate. The water is released into a mantle of continent at the depth of about 150 km and, lowers melting temperature there and negative buoyancy of the plate changes to buoyancy. Therefore the depth is a limit of negative buoyancy of the plate. According to the idea, the excess mass at trench junction off Urakawa and off Miyazaki were estimated and tried to explain origin of negative gravity anomaly landward of trench junction.

(Contributed by Yoshibumi TOMODA, M. J. A., Feb. 12, 2003)