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.)
On Examining photographs taken at Tatikawa, 35km apart from. Tokyo to the west, the author obtained the height of cumulus heads and the velocity of their ascent. The maximum and average height of cumulus produced by the fire of Yokohama was respectively 10850m and 5850m and that of Tokyo was 6560m and 5150m. The velocity of ascent of the heads ranged from 0.6 to 2.0m/s and 1.1m/s on the average for Tokyo and 0.9m/s for Yokohama. See Pl. 7 & 8.
After 45 minutes of the commencement of the great earthquake, our tide guage installed at the beach near this observatory traced an ascent of about 15cm which was succeeding by a descent of 24cm. of the sea level. The interval of time between the maximum and minimum of the height of the level was about 10 minutes. Such an oscilation of the level was then repeated a few times, meanwhile the average level was sinking gradually, and at last at 1h 19m pm. the lowest level was reached whose depth was 81cm. above the zero of the guage with an oscilation of the ascent of 49cm and a descent of 33cm. This was the one with the maximum amplitude and succeeded by complex oscilations for 3 hours. Along the coast from Sunosaki to Nozimasaki the intensity of Tsunami was greatest of these districts. Since January this year (1923) the water-stand of the ocean observed in this observatory was gradually rising. In May, June and July the average stand was higher than that in the last year by 18cm. In August the difference increased to 24cm and before the great earthquake it amounted to about 30cm. which was proved to have been maintained after the said phenomenon.
At the time of the great earthquake on September 1st the level of Lake Tyuzenzi situating toward NNW of Tokyo at the distance of about 120km had depressed about 2 feet at the shore near Tyugusi and recovered after about ten minutes. At the shore near Syôbugahama, the depression was about 0.5 or 6 foot and the time wanted for recovering was 5 minutes. Hence the water of the lake seems to have swung from NE to SW and then drew back. On the same occasion the water of six secondary cascades of Kegon Waterfall hanging at the end of the lake changed its colour and became turbid for about 22 hours, which seems to have been caused by some internal break of rocks at the boundary of the stratum of conglomerate mounting on the bed of quartz porphry. The water of the remaining six of the secondary cascades and the primary waterfall remained unchanged.