抄録
In a previous paper, one of the present authors estimates the rate of secular change of gravity at 0.817, cgal/year along the high gravity anomaly zone in South-Western Japan, with some discussions on the interaction of oceanic lithosphere of the Philippine Sea Plate and continental lithosphere of the South-Western Japan . In comparing this order estimation with actually observed gravity change, we must notice the possibilitythat the observed gravity change can be several times higher than estimated one, because the estimated change is the mean value for long time geological duration andthe contemporary change is interseismic one. This point is first discussed in the presentpaper based on the observed crustal deformation accompanied by plates interaction, and some attempts were made to detected secular change of gravity along the northernboundary of the Philippine Sea Plate by a comparison of old and new observed gravityvalues. After analysing the various kinds of errors included in the gravity surveys that were carried out by the Geographical Survey Institute, we can conclude that the ob erved secular change of gravity of +0.13+0 .18 mgals for past ten years at the southern parts of Kii Peninsula and Muroto Promontory is real one. Detected secular changes of gravity at each gravity stations in those regions become gradually higher as the stations locate more close to the southern point of promontory in concordance with the distribution of Bouguer anomalies. This fact means that the high gravity anomalies found in the peninsula or promontory along the northern boundary of the Philippine Sea Plate are being formed even at contemporary time. In conclusion, the authors emphasize that the more study of secular change of gravi y accompained by plates interaction should be tried as one of the most important problems of geodesy in tectonic area such as Japan Island.
