Abstract
Near the mareographic station at Aburatsubo in the Miura Peninsula, Kanagawa Pref., the observation with the water-tube tiltmeter has been made by the Earthquake Research Institute of the University of Tokyo. Two tiltmeters are set in approximately west-east and north-south direction. In this peninsula the first order leveling surveys were repeated many times by G. S. I. in order to secure the leveling datum in Tokyo by connecting it with the mean sea level at Aburatsubo. Using this leveling data, the inclination of the ground near Aburatsubo can also be calculated. First the direction of the axis and the angle of the maximum inclination are derived by using the tiltmeter's record, where W-E and N-S tiltmeters are assumed to be set in the direction of the angle ψ from the east and ψ' from the north respectively (Section 2). Next, necessary equations of computing the components in the directions of two tiltmeters from the direction of the axis and the angle of the maximum inclination which are read from the equal vertical movement lines drawn by using the repeated leveling surveys (Fig. 3). The observed inclinations in two components (θ in W-E and θ' in N-S) with tiltmeters are shown in Table I. The calculated inclinations from the result of the leveling survey in the same directions as mentioned above are shown in Table II. Then the secular change in inclination, 40 and 40' in two ways are calculated and they are compared in Fig. 4. They are not in good coincidence. As the interval of bench marks in the first order leveling by G.S.I is about 2 km, the region of over 2 or 3 km is necessary in order to obtain reliable equal vertical movement lines. On the contrary, the inclination with the tiltmeter at a very point only corresponds to that in very small area. The reason of unsatisfactory coincidence mentioned above is seemed to be due to such opposite condition of two methods of observation.
