伊豆半島における異常隆起の推移とフィリピン海プレート最北端部の曲がり

抄録

Since the beginning of the anomalous crustal vertical movement in the Izu peninsula, Honshu, Japan, so many repeated precise levellings have been carried out by the Geographical Survey Institute. A special technique is required to adjust the results of these levellings, because they had been carried out for different epochs along each levelling route and also they had been affected by so rapid vertical crustal movement. We apply least square adjustment with velocity model in which corrections to the assumed height and linear rates of vertical deformation at any bench marks are assumed to be unknowns. We apply free net adjustment to this case. We can select several special bench marks in which height or velocity is given from any suitable results of tidal analysis then usual or free adjustment is applied according to the type of network adjustment.
The results show that the peninsula was inclined to the south-west as a whole during 1973-1986. Not only the north-east part of peninsula was upheaved as many investigators had pointed but also there occurred remarkable subsidence in the south-west of the peninsula. The detected rate of inclination is (2-8)×10^-6 rad/a and several times higher than the one during past several ten years. Thus inclination was proved to be accelerated.
The detected deformation is concentrated in the area where Nakamura (1980, 1984) pointed out the bending of the Philippine Sea Plate. The peninsula shows bending upward in the shallower crust along almost ENE-WSW direction before subduction. The detected crustal deformation covering the whole area of the peninsula is the direct evidence of the plate. The bending is asymmetric as detected deformation shows. It is remarkable in the eastern part of the plate and not so in the west.
When the plate bending is accelerated, extension becomes remarkable at the earths surface and contraction becomes remarkable at the deeper part of the subcrustal layer. This will be a cause of various kind of the contemporary crustal activities such as earthquakes in or near the Izu peninsula or volcanic eruption of the Oshima in 1974 and during 1986-1987.