The cooperative microearthquake observation with seven portable seismometers and one fixed seismometer was carried out in the northeastern part (Tohoku) of Honshu, Japan, for twenty days in 1967. Twenty recording sites were used to locate the foci of about fifty microearthquakes during this period. One-half of the events were located in the zone (Morioka-Shirakawa tectonic line) between the western edge of Kitakami mountain and the volcanic front, the others were located on the westward dipping seismic plane. The crust bounded by these two seismic zones is aseismic.
The Morioka-Shirakawa tectonic line, which was inferred from the gravity anomaly, is a shallow seismic belt with a NS trend, however the level of the activity is not high; seven small shocks along this line in the surveyed area had been located by JMA network during the last 15 years. This tectonic line was characterized by the occurrence of small earthquake swarm.

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The Scandinavian region is uplifting and the Indian region is a stable area so that a negative gravity anomaly can be found in the former region and no systematic anomaly can be found in the latter. Against these expectations there is no systematic negative anomaly in the Scandinavian region and a negative anomaly in India if the anomalies are calculated on the basis of a normal gravity formula including only (n=2 and 4, m=0) terms in the spherical harmonic expansion. It may be, however, that processes responsible for the Scandinavian uplift and Indian geology occur in shallower depths (probably in the B layer, asthenosphere) and that mass anomaly responsible for the anomaly of rather long wavelengths (or smaller wave numbers n and m) exists deep (probably in the C and D layers, mesosphere) within the earth. If such is the case, in order to get gravity anomaly closely related to geology, we must take as the standard gravity not only (n=2 and 4, m=0) terms but also higher order terms in the spherical harmonic expansion. In short we must choose the standard gravity formula so as to get gravity anomaly which is agreeable with other geophysical and geological observations.
Studies are made on the basis of the above point of view. Gaposchkin and Lambeck's data on the spherical harmonic expansion of geoidal undulations are used. It is our tentative conclusion that by including terms up to (n=8, m=8) we get anomalies which are more agreeable with geology than the normal gravity formula.

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Following the way developed by Kostrov, the problem of propagation of longitudinal shear crack of finite length is discussed. It is assumed that the crack is developed at y=0 and its left edge is fixed. Assuming also constant stress drop τ₀ along the crack, the x coordinate of the right edge of the crack X(t) is calculated as a function of time t. Essential non-dimensional parameters in the problem thus defined are a=L₀/(βt₀) and t₀=πμT/(2βτ²₀), where L₀, β, μ and T are initial crack length, shear wave velocity, rigidity and surface tension, respectively. Making use of seismological data of the Chile earthquake (May 22, 1960), T of the order of magnitude 10¹⁰ erg/cm², an interpretation of which is given in the text, is obtained. After some additional calculations, it is concluded that, in order to stop the crack propagation abruptly, a geologically discontinuous surface, both sides of which have utterly no elastical connection, is necessary.

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An earthquake source mechanism is described from the standpoint that the dynamic characteristics of friction play a more important role in the dislocation motion than the boundary condition does. By considering an earthquake dislocation as a slip motion in a mechanical system involving friction, which depends upon velocity during slip, a displacement time function G(t)=1-exp(-t/τ)(1+t/τ) which satisfies the necessary continuity criterion is derived. The rise time of the source time function, which must be directly dominated by the physical conditions on the source region, is straightforward estimated from the time constant
τ=(π/8K)(υ/β)W/β
where K is (λ+μ)/(λ+2μ) for dip slip and 1/2 for strike slip, and λ and μLame's constants, υ the propagating velocity of dislocation, β the shear wave velocity and W the fault width. The values derived from the above formula agree quite well with those determined on the basis of the seismic wave data for all the earthquakes investigated.
Seismic wave energy and its efficiency are discussed in relation to the physical controlling factors in the source region. It is noteworthy that the seismic wave energy is related explicitly to the fault width, and that the seismic efficiency depends only upon the ratio of the shear or compressional wave velocity to the propagating velocity of dislocation, the ratio of the fault length to width, and the ratio of the stress drop to the in situ stress before or after dislocation. It is shown that the amount of the total seismic wave energy is very small compared with that of the elastic energy released by an earthquake dislocation. This suggests that an earthquake source may be a great heat source.

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It was ascertained that Love waves could be generated even by small explosions, Various features of the wave group which seemed to be Love waves, such as the direction of wave vibration, the dispersion, the amplitude characteristics and the spectrum, were investigated. It was also found that the energy ratio of Love waves to that of Rayleigh waves due to explosive sources was only a few percent.

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