In earlier days most Japanese wanted eagerly to have means of earthquake prediction in order to escape earthquake disaster taking refuge in advance in open spaces since they were almost powerless in encountering the menaces. We are now enabled to long for the mitigation of the disaster by the advancement of science and technology, although we have not yet succeeded to predict the coming of a disastrous earthquake. Our safety thus depend entirely on our effort for the countermeasures against the calamity. But we have only limited allowances for such preparations economically and in time available. It is therefore inevitable to start the preparation from the most important and imminent localities in a country, from the national point of view. In Japan, according to the writer's witness on the imminence of Southern Kwanto District at the Diet in July, 1964, such preparation work for Tokyo Metropolis was started the next month by a new Earthquake Division of the Disaster Prevention Council of the Metropolis, and several important hazard possibilities of the imminent earthquake have been revealed with the result that similar committees have also been made in other prefectures and cities in the same area as well as in the Central Government of the country. In this paper the writer intended to give the ground for his assertion of the imminence of a disastrous earthquake giving proofs of the periodicity of such calamities on the statistical and historical analyses, and made some comments on the possible consequences and major problems in the countermeasures to cope with the conceived disaster as revealed by the studies made on the national and metropolitan levels. In view of the importance, interest and the space allowance, only the proofs of the periodicity in the recurrence of destructive earthquakes in the district concerned are summarised in this abstract. Historical earthquakes in and around Kwanto District as shown in the map (Fig. 2.1) were examined and those which were destructive at Kamakura or presumable as such from the standard intensity-distance-magnitude curve were selected and listed in Table 1.1. Periodicities of these earthquakes were analysed statistically. Similar analyses were also applied to the earthquakes experienced in Tokyo which are listed in Table 2.2. A definite period τ of 69 years (besides a few integral multiples of this period) was clearly found. The methods used in the above analyses were by means of (1) Fourier transforms (Fig. 1.1), (2) autocorrelation functions (Figs. 1.3 and 1.4) and (3) historical examinations of the number of cases when no destructive earthquakes took place within the standard deviation +√ ξ2 from the year tp when a destructive earthquake was expected to occur from the periodicity. Test of persistence of the period was also made by means of the theory of random walks consisting of Fourier amplitudes in successive periods (Figs. 1.5 a and b). Rayleigh-Schuster's criterion (Formula 1.3) that is the probability of obtaining the actual Fourier amplitude on the assumption of no periodicity or the resultant distance of above mentioned random walks gave the values of the probability equal to 0.06% and 0.03% for Kamakura and Tokyo respectively. In comparison to usual value of 5 or 10% for recognizing periodicity in geophysical phenomena, the smallness of the above values is more than enough to disprove the non-existence of the periodicity of 69 years in the recurrence of destructive earthquakes in the districts under consideration.
Recently, crustal movements in the Boso and Miura peninsulas have changed their mode of previous gradual sinking after the Great Kanto Earthquake to an upheaval tendency. The phenomena attracted much notice widely and increased the interest for earthquake prediction. At this opportunity, short sketch and discussion were made about the characteristics of recent crustal activities in the south Kanto district which had mainly obtained by means of geodetic repetition surveys such as levelling, tide observation, triangulation survey, distance measurement and geomagnetic survey. A picture for explaining the outline of the feature of crustal movement and a new geodetic survey planned by G.S.I. were also briefly discussed.
Based on seismic data and focal mechanism solutions, a study is made on the seismic activity in the southern part of Kwanto, central Japan, during the period from 1926 to 1968. In view of regularities of focal mechanism as well as a spatial distribution of major earthquakes, the area concerned in the present paper are divided into six blocks (cf. Fig. 5), and an interrelation of seismic activities among the six blocks are statistically tested. The tests suggest that the occurrence of earthquakes in each block is exclusive in time, and that the seismic activities in the blocks except zones 1 and 4 in Fig. 5 increase gradually year after year. A peculiar evidence is that only a few events were observed in the period concerned in the middle part of Boso Peninsula where an abnormal land upheaval of several centimeters were detected by means of geodetic surveys during the period from 1965 to 1969. It is also strange that the seismic activity is in a quite low level in the epicentral area for the great Kwanto earthquakes of 1923 in the past 43 years. In addition, a description is made the reliability of earthquake parameters given by the Japan Meteorological Agency and the earthquake detection capability of the seismological network of the Agency. The description will be useful in relevant people who conduct using the data given by the Agency.
The southern coast of Kanto District has no more than being attacked by a few destructive tsunamis, in compare with the coast of Kumano-nada and Sanriku. The tsunamis given damage on this coast have occurred in Sagami Bay, near Izuoshima, off the south-east coast of Boso Peninsula and in Enshu-nada, and the ones occurred in Kumano-nada and off Kii Peninsula give locally damage, as well as in case of the Chilean Tsunami. It must be noticed that two or three tsunamis had occurred at the same time along the offshore coast from Kanto District to Kinki one. There is no definite record that tsunami gave large damage on the inner coast of Tokyo Bay. From the height distribution of the Kanto Tsunami of 1923 and the Chilean Tsunami of 1960 in Tokyo Bay, the height of tsunami in the inner coast of the bay never increase, in order to possess large reflection and friction of sea bed in the bay. These facts can be also explained theoretically.
September 1, 1923, southern Kanto was devastated by a severe earthquake. The main shock of the events, which occurred at 11 h 58 m (JST), was felt all over Japan. The highest seismic intensity is 7 in JMA scale at the southern part of Kanagawa, Tokyo and Chiba prefectures. The epicenter is located on the Sagami-nada. Parameters of the main shock given by the Japan Meteorological Agency are as follows ; origin time : 11 h 58 m, September 1, 1923. epicenter : 35° 20' N, 139° 20' E magnitude : 7.9 The earthquake was accompanied by many fore- and after-shocks. The foreshocks occurred in Kashima-nada. It seems that the foreshock activities began in May 1923 and became vigorous in June 1923 before the occurrence of the main shock. The aftershocks occurred in Kanagawa, Yamanashi, Saitama, Chiba, Ibaraki prefectures and near seas, and the general trend in decrease of daily number of the aftershocks can be explained by Omori's generalized formula.