關東地方大地震の餘震研究(熊谷にて)

Abstract

According to the observations made at Kumagai, the number of after-shocks accompanying the Great Kwanto Earthquake of September 1st, 1923. has amounted to 1689 during the interval ending at the end of January 1924. The frequency-time curve of the after-shocks may be represented by the formula(1) where y denotes the frequency, x the time elapsed since the noon of September 1st. This formula with the set of values b=603, a=0.0, c=1.6 fits well with the actual frequency curve for the duration from 1st till 19th of September, but for the succeeding period the values of constants must be changed to a=4.0, b=60, c=0.8 and even the formula can represent only the general decrease, but not the fractuation of frequency.On examining the mode of the fractuation, the author found that somewhat remarkable shocks occur periodically with a period of 19, 2 days and this period was repeated ten times during the said interval. In order to explain the mechamism of this periodicity, the author examined the distribution of seismic epicenters of these shocks and found that there are two principal zones of seismic activity in the vicinity of Kwanto Plain, one of which lies to the east of Tokyo Bay and the other to the west. Between these the seismic energy seems to be accumulated alternately, that is to say, when one of these regions is released from the stress on the earth strata by a remarkable earthquake occurring in this zone, then the other zone begins to be loaded with some seismic pressure which will gradually increase and after a certain period when the stress is accumulated enough to break the state of equilibrium, the second shock may occur, which releases the second zone and consequently the first must be put again under the seismic stress. In this way the oscilatory occurrence of earthquakes between two zones can be explained. Further more the author has found the following formula for the date X of the maximum frequency of the aftershocks felt at Kumagai where m is the integer representing the ordinal number of the occurrence of the maximum frequencies. By comparing this with actual date of the occurrence, he obtained a good accordance of the theory and fact that the maximum deviation between them was only 0.9 days. (S. F.)