房総半島における地殻変動(1)

抄録

The level-line that runs along the coast of Boso Peninsula was surveyed four times at the year of 1898, 1923-4924, 1930, 1965 respectively. Therefor we can compile the three charts showing the vertical deformation of the crust during the three different periods. From the results of these charts and considering the distribution of Bouguer gravity anomalies in this Peninsula together with other author's seismological, geomorphological and geological discussions, the following conclusions are obtained; (1) Under the region of Chiba City, down-buckling of the crust is observed and its thickening is calculated as about 12 km from the gravity anomalies. This amount is equal to the down of the upper surface of the distribution of deep earthquakes that are frequently occured at the upper mantle in this region. (2) The rate of the secular inclination of the Peninsula toward the centher of the down-buckling of the crust is about 0.01" 0.03" per year. This is the same order as geomorphological conclusion about the tectonics in this Peninsula. (3) Vertical movement of the B. M. 3880 that lies at the most southern part of the level-line in Peninsula shows that it was upheaved at the occurence of the great earthquake of 1923 and is gradually sinking after earthquake. (4) The region bounded by the Hota-Kamogawa line and the Enmyoji line shows the gradual subsidence relative to the surrounding region, and also the relatively negative gravity anomalies. A graben-like crustal structure is presumed from gravity anomalies analysis. We can recognize some active blocks in this region.