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Volume 12, Issue 4
Displaying 1-6 of 6 articles from this issue
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Toshio SENSHU1959 Volume 12 Issue 4 Pages 149-161
Published: December 30, 1959
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSIt has been said that the number of occurrence of aftershocks in a prescribed time interval decreases according to the following formulae,
n(t)=A/t+c (1) or n(t)=kt-p (2)
where t means the time elapsed from the main shock occurrence, and A, c, k, p are constants.
On the other hand, it has also been known that there exists a number of big fluctuations on the observed number of aftershocks compared to the number calculated by means of the upper formulae.
In order to investigate this fluctuation, the time interval of the consecutive aftershocks has been adopted as a main variable.
In this paper, the writer described the frequency distribution of the time interval between consecutive aftershocks, and tried to illustrate it by means of the stochastic process.
The results obtained are as follows.
(1) Time interval (τ) distribution is empirically expressed by the following formula at the prescribed time interval T,
φ(τ)dτ=μe-μτdτ (3)
where τ is not so small.
This means that the occurrence of aftershocks, at a suitable time interval T, is random, i. e., an aftershock does not exert its influence on the consecutive one, namely, μ(τ), which means the non-conditional probability of aftershock occurrence at unit time interval, has no relation with τ,
μ(τ)=μ.
(2) In case of small τ, the time interval distribution is expressed by the formula
φ(τ)dτ=aτe-1/2aτ2dτ. (4)
This means that μ(τ) increases according to the time elapsed (τ) from the previous aftershock occurrence,
m(τ)dτ=ατ-βdτ.
This is known as TOMODA's distribution of the time interval using the name of the first investigator of this problem.
The writer showed that TOMODA's distribution can be derived using formula (3) or (4) and (2).View full abstractDownload PDF (729K) -
Hikaru WATANABE1959 Volume 12 Issue 4 Pages 162-170
Published: December 30, 1959
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSA new type of rotational strain seismometer is designed to observe the rotational strain component around the vertical axis.
As the rotational strain around the vertical axis arises from SH type waves only, not from P and SV type waves, we can pick up clearly SH waves and Love waves from the complicated seismograms as shown in Figs. 5 and 6.View full abstractDownload PDF (3362K) -
Tatsuhiko WADA1959 Volume 12 Issue 4 Pages 171-181
Published: December 30, 1959
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe physical properties of periclase (MgO) are discussed based upon modern solid state physics and the following results are obtained.
1) Calculated variations of the density and incompressibility with pressure agree fairly with those obtained by K. E. Bullen for the D-layer. By criticizing Born-Mayer's formula we find that the discrepancy between calculated value and Bullen's value is comparable to the error, which is due to using Born-Mayer's formula and increases with pressure. (Fig. 1, Table 1 and Fig. 2).
2) The electronic structure of MgO crystals is calculated by the tight-binding method. It is shown that the electric conductivity of MgO transforms to metallic one under about 1200, 000 bars (equivalent to 2, 600km depth in the Earth's mantle according to Bullen's table), and a new model of the core boundary is suggested: the metallic transition of electric conductivity occurs in the D-layer without density-jump and the metallic transition of the phase occurs at the core boundary with density-jump. According to N. F. Mott's opinion, in this model we can expect a sharp tansition of electrical conductivity from non-metallic state to metallic state in the D-layer. (Fig. 3 and Fig. 4).View full abstractDownload PDF (917K) -
Kennosuke OKANO1959 Volume 12 Issue 4 Pages 182-190
Published: December 30, 1959
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDuring I. G. Y. period the routine and tripartite observations of microseisms were made at Abuyama Seismological Observatory. And after I. G. Y. the particle motions of the ground were observed by the vector seismographs in UD-NS, UD-EW and EW-NS components.
These observations show that microseisms are generated by sea waves propagated from the disturbance source to the near coast of the Observatory. And it was made clear that amplitudes and periods of microseisms depend on the scale of the disturbance source and the distance from the disturbance source to the coast.
As the velocity of the microseismic wave 2.4-2.6km/sec. is reliable.
From the earth-particle trajectories it was confirmed that microseismic waves are Rayleigh-type, and that Love waves are scarcely present.View full abstractDownload PDF (2289K) -
Ryôsuke SATÔ1959 Volume 12 Issue 4 Pages 191-201
Published: December 30, 1959
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSIn the previous paper, the author investigated which method will be the best analyses for determining the crustal structure from the seismic surface waves propagating through it, which has no vertical discontinuities between an epicenter and a point of observation. In many cases, however, seismic surface waves do cross several vertical discontinuities before they reach observing points.
In the present paper, the author investigated dispersive waves crossing discontinuities taking a very simple model (two elastic plates are in contact in a plane perpendicular to them).
The observed group-velocity U is given by
Δ/U=Δ1/U1+Δ2/U2+…+Δn/Un
for a given period, where
Δ=Δ1+Δ2+…+Δn=total epicental distance,
Δ1=distance between a source and the first discontinuity,
Δm=distance between the (m-1)-th and the m-th discontinuities,
Δn=distance between the n-th discontinuity and a point of observation,
Um=group-velocity of surface waves under consideration in the m-th medium.
It must be noted that this relation holds only when waves are incident almost perpendicularly to the discontinuity planes.
The observed phase-velocity gives informations of the local structure in the vicinity of observing points.
In the latter part of this paper, the author deduced the position of discontinuity line between the Pacific Ocean and the Continent of North America, using the dispersion of Rayleigh waves observed by R. M. Brilliant and M. Ewing.View full abstractDownload PDF (920K) -
Takeo Matuzawa1959 Volume 12 Issue 4 Pages 202-203
Published: December 30, 1959
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (241K)
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