The possibility of a chemical decomposition of Mg₂SiO₄ at the pressure condition of the C-layer is discussed. The discussion is based upon an assumption that a gravitational separation due to density differences as well as chemical interactions among participating components controls the equilibrium condition of stable phases. It is concluded that Mg₂SiO₄ is decomposed into MgO and SiO₂ and SiO₂ is squeezed upwards at depths of about 600km. The gradual change in the character of the C-layer with depth is explicable in terms of the metastable state of Mg₂SiO₄. It is remarkable that the quantity φ of MgO is nearly equal to the observed φ of the D-layer. The squeezing of SiO₂ is discussed based upon a theory of thermodynamics of irreversible processes. The squeezing process has some relations to the terrestrial evolution.

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In 1955 Ultra Sensitive seismometers were temporarily set up at the observatory at Mt. Tsukuba by one of the present writers. The results of this experiment led to the conclusion that the equation, logN(M)=a-0.8M, holds for shocks of M from less than zero up to about 5. The present writers made temporary observations of near-by microearthquakes at Matsushiro with Ultra Sensitive seismometers in order to see whether or not this is the case in this region.
The writers found that the above mentioned equation holds good for shocks occurring in the vicinity of Matsushiro also. Microearthquakes of M≈0 are occurring about 20 times every day, and minor shocks of M, 4 or 5, are occurring 1 or 2 times a year in this region. The geographical distribution of foci of these microearthquakes agrees with that of minor shocks. Their foci are seated in the shallow part of the crust.
The E of the smallest shock recorded during the term of this experiment is about 10⁹ ergs.

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Not only the forms of amplitude distribution, but also relative sizes of amplitude corresponding to respective phases are taken into account. Amplitude-characteristics at various depths within the layers are also illustrated. The amplitude maximum can be fairly sharp near the surface of the earth, but is rather blunt at large depths.

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It is well known that the Rayleigh wave which is propagated along the surface of the superficial layer on a semi-infinite elastic solid is composed of two dispersive wave groups, and each wave group has infinite number of branch waves. These are named M₁₁, M₁₂, M₁₃…for one group and M₂₁, M₂₂, M₂₃…for the other. The range of existence of these branch waves has been investigated by several authors, but these studies are mainly concerned with M₁₁ and M₂₁.
In this paper the possible range of wave-length for a given phase velocity has been studied for the higher branches, M₁₂, M₂₂, M₁₃, M₂, M₁₄, M₂₄.
When λ′=μ′, λ=μ and ρ=ρ′, it has been found that any higher branch wave can exist, but the higher the branch is, the narrower is its range of existence.

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A simple method is proposed for solving the secular equation of the form (λI-a)x=0, where a is a known matrix, I the unit matrix, each of rank n, λ is the unknown eigen-value and x its eigen-function. In the present method, only one set of linear simultaneous equations of degree n needed be solved to get n; eigen values and their eigen-functions. Time and effort to solve this kind of problem will thus be greatly reduced by the present method.

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In the preceding paper, the characteristic frequency, power spectra, distribution of amplitudes and other characteristic features of waves in microtremors or short period microseisms at Hongo in Tokyo were discussed. In this paper, the results are described of the tripartite observations which were conducted at Hongo and Kokubunji in Tokyo, Kakioka in Ibaraki Prefecture and Matsushiro in Nagano Prefecture in order to investigate further in detail the character of propagation of microtremor waves.
The results obtained are:
1. Tripartite observations revealed phase velocities of 300m/s at Kakioka for vertical motion and 600m/s at Kokubunji for horizontal motion. The wave lengths 100m, and 250m were found respectively.
2. Short period microtremors with frequencies of more than 10c/s decrease notably during the night in amplitude.
3. Microtremors appear to be composed of waves which are generated from a number of surface sources and which have the character of surface waves.

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