Abstract
Resurvey of the 3rd order triangulation in the Tanna area was carried out in 1973. This net is a part of area where the left-lateral fault of 2.-3 m displacements appeared at the time of the Kita-Izu earthquake (1930). The present survey used a method of trilatelation together with traversing by electronic distance measurements. Regardless of such technical methods, horizontal displacements during the period from 1933 to 1973 can be discussed. The past triangulation in Japan had no Laplace station. Besides, the concerned net is the 3rd order triangulation, so directly measured base lines are not involved. By these two reasons, displacement, which is calculated under assumptions of one fixed station and one fixed azimuth, usually shows pseudo rotation vector and pseudo convergent/ divergent vector. These pseudo signals can be successfully removed by applying a new method of signal coincidence. The corrected displacements show clear difference between east and west sides from the past fault line. The relative displacement is about 20 cm. At a glance, it may suggest continuation of movement along the slip surface of the fault. This idea, however, is denied by the fact of stable maintenance of the Tanna tunnel where displacement of 2.4 m appeared in 1930. Then, the horizontal displacements obtained at the present must be explained as follows: The crust of the east side of the fault is being displaced relatively to that of the west side. Accordingly, strain is accumulating in area along the past fault line. In order to obtain accumulated strain after the earthquake, some triangle must be chosen in which its vertex is located near the fault and both sides including the vertex angle are in perpendicular as long as possible with the fault line. The mean value of maximum shear strain is 3.7?105. The Yamakita area in the Kanagawa Prefecture can exactly be treated by the same way as in the Tanna area.
