Seismic Intensity Does Not Depend on Acceleration of Earthquake Motion

Abstract

In the latter half of the 19th Century, J. Milne thought that seismic intensity might be dependent on acceleration of earthquake motion. Later, in 1902, F. Omori added the value of acceleration to his list of seismic intensity, in 1932 M. Ishimoto added acceleration values to the C. M. O. list of seismic intensity, and in 1943, H. Kawasumi revised their values. The C. M. O. (Central Meteorological Observatory) list was made in 1908. This list has been used up to date. But, in the cases of natural earthquakes, seismic intensities are often not in accord with accelerations of ground motion. So the author was doubtful of the acceleration values of the C. M. O. list of seismic intensity. As the shaking table was made in the year 1970, the author made an experiment on the seismic intensity. We vibrated this table, on which a man sat, and measured the intensity of the vibration. The periods of the table were from 0.26 sec to 9.82 sec, and their amplitudes from 2.5 cm to 8.1 cm. The results of this experiment were tabulated and are now shown in Fig.1. The vertical axis shows the amplitude of the shaking table, and the horizontal axis its period. Intensities of these vibrations are shown by marks. If we classify them by straight parallel lines, we can draw solid lines as shown in Fig.1. Intervals of these lines are equal to each other. The broken lines are the long established intensity level depending on acceleration of ground motion. The result of this experiment does not agree with these broken lines. Fig.2 shows this disagreement. So the author thinks that seismic intensity does not depend on acceleration of ground motion. Fig.1 shows that the intensity of the vibration depends on velocity or the square of velocity and so on. Fig.3 shows the relation between intensity and velocity. Fig.4 shows the relation between intensity and the square of velocity. These relations show good agreement. R. Mallet has reported that intensity depends on the velocity of ground motion. But the author thinks that intensity depends on the energy of the ground motion. Because, in other physical phenomena (light and sound etc., intensity depends on energy. So we emphasize that seismic intensity depends on the energy which passes through a unit area per unit time. © 1972, Japan Meteorological Agency. All rights reserved.