Observational relation of fault length to earthquake magnitude is tentatively illustrated by concept of the geometrical relation between earthquake focal region and earth's crust as is schematically shown in the inset of Fig. 2. Fault lengths thus interpreted seem to be in good accordance with concept of the voluminal storage of the earthquake energy. Proposed model can also explain the observational fact that percentage of the number of earthquakes of a given magnitude M accompanied by fault, to the total number of the earthquakes of that magnitude increases from 60% for 7.0<M<7.4 to 100% for 7.4<M.

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It is one of the most rundamental problems in seismology to determine the location of the focus, occurrence time and the propagation velocity of an earthquake using the arrival times of waves. It is clear, in this case, that there are good and bad distributions of observation points. For example, if all the points are distributed on a circle and an earthquake occurs on the center of this circle, we can determine neither the occurrence time nor the velocity. On the other hand, if all the points are on a half straight line with the epicenter at the origin of that line, then the velocity can be determined correctly though the focus and the occurrence time can hardly be calculated. Besides, according to the distribution of points, observation errors have large or small effect on the determination of these quantities. The present paper seeks optimum distribution points by means of Monte Carlo method.

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Several hundred microearthquakes observed with ultra-sensitive tripartite network at Matsushiro are analysed for the purpose of location of their origins. Attentions are drawn to the fact that lower limit of the observed apparent velocities shows a systematic tendency concerning the direction of the wave propagation. It varies broadly from 3.6km/s to over 10km/s. This tendency is interpreted due to the inclination of the surface layer boundary near the observatory. If the lower limit corresponds to the critical case of wave refraction, the expected inclination of the boundary is calculated to be 13 degrees under one of the crustal models proposed by Mikumo et al. (1961). Seventy five percents of analysed shocks are located in the western side of the fossa magna.

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It was pointed out by some seismologists that the investigation of occurrence of aftershocks might supply important clues for solving the mechanics of main shock occurrence itself. This paper is the preliminary investigation for this purpose, and deals with the regional concentration of aftershock energy and its relation with the crustal deformation caused by the main shock, chiefly making use of seismological data of the Japan Meteorological Agency (J. M. A.) during the period between 1926 and 1963, with some reference to other data.
First, the accuracy of seismological data of J. M. A. is examined, and according to the degree of accuracy, the following classification is introduced.
1) The first period (1926-1950) in which the average accuracy of epicenters may exceed several tenths of degree, and only great shocks of magnitude 8 and above can be treated for the present purpose.
2) The second period (1951-1960) in which the accuracy of many epicenters can be estimated about one tenth degree, and shocks of magnitude 7 1/2 can be treated.
3) The third period (1961-1963) in which the accuracy itself is shown in the seismological Bulletin of J. M. A. and the accuracy of many epicenters is less than 1 or 2 minutes. In this period shocks of magnitude 7 can be treated for the present purpose.
Then earthquakes in each of the three periods, such as the Nankaido earthquake of Dec. 21, 1946, the earthquake off Boso peninsula of Nov. 26, 1953, and the earthquake off Echizen-misaki of May 27, 1963, are treated.
It is found that aftershock energy is not distributed equally in so-called aftershock area, but there is regional concentration of energy. This energy concentrated area is near the epicenter of main shock, or on a line including the epicenter of main shock. This energy concentration is also found in the case of so-called earthquake swarms, such as the swarm off Ibaragi Pref. in January and February of 1961.
In the case where data of precise leveling are available before and after main shocks, the energy concentrated area or line corresponds to the zone of upheaval or the boundary between upheaval and subsidence caused by the main shock.
In the case of earthquakes of magnitude 6 1/2, such as the north Miyagi Pref. earthquake of April 30, 1962, the regional concentration of aftershock energy is not so clear, but it is found that the aftershock area coincides with the zone of upheaval caused by the main shock.
It is suggested that there seems to be the difference between the aftershock area determined from the routine observation and that from special observations of aftershocks.

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