Vertical Land Movement in Japan as Deduced from Tidal Record, (1951-1978)

A computer program for the automatic processing of monthly tidal data is compiled to facilitate continuous monitoring of crustal activity. Its principle is briefly differential use of tidal stations in a region, so that the oceanographic disturbances of common causes may be eliminated from the sea level data. This method enables us to observe vertical land movements at tidal stations with sufficient precision. This program has processed tidal records from the 96 Japanese stations for the recent 30 years, where coast of the Japanese Islands are divided into nine regions, taking oceanographic conditions into account. General discussion on recent vertical movements, thus derived, has yielded several notable features as follows: (1) Rapid and stationary subsidence in the northeastern part of Japan, presumably of tectonic origin. (2) Tectonic subsidence of Setouchi region, southwestern part of Japan. (3) Complex pattern of uplift and subsidence in the zone parallel to the Suruga-Nankai trough, which seems to reflect subduction of the Philippine Sea plate against the Honshu block. (4) General trend of slight uplift in Kyushu district. Movements related to the earthquakes during the period 1951-1978 are also discussed, although few of the cases show clear cut premonitory movements