Deformation of the Rhombus Base Line at Mitaka

Abstract

The surveys of the rhombus base line at Mitaka, Tokyo, amount to 33 in number since 1916. From the results the followings are clarified.(1) The deformation of the rhombus at Mitaka is representative of the crustal movements in the southern Kanto district.a. Before and after the Great Kanto Earthquake The strain change in Mitaka rhombus base line (1922-1923);ε1=36X10^-6 (N5°W)ε2=-7X10-^-6 (Fig. 3)The strain change at Mitaka obtained from the geodetic data (189-.1925) Using the first and supplementary triangulations around Mitakaε1=25X10^-6 (N2°W)ε2=4X10^-6 Using the most likely fault model for the geodetic data in the southern Kanto districtε1=12X10^-6 (N8°E )ε2=-4X10^-6 (Fig.4 )b. After the Earthquake to the present (1925-1971)The strain change at Mitaka:ε1=7X10^-6 ε2=-20X10^-6 (N28°W) (Fig. 3) The strain change in the southern Kanto district;N-S contraction and NW-SE contraction prevail in Boso peninsula and around Sagami bay respectively, lε2l°2X10^-5 and ε1-ε2°3X10^-5 being in both areas. (Fig. 5 )(2) It seems likely that there is a strong correlation between the anomalous areal deformation at Mitaka and the occurrence of earthquakes with intensity more than V(>80 gal) at Tokyo, as shown in Figs. 6 and 7. There exist, however, the discrepancies of patterns in the areal deformation and the maximum shear, as shown in Figs. 2 and 8.