Journal of Physics of the Earth
Online ISSN : 1884-2305
Print ISSN : 0022-3743
ISSN-L : 0022-3743
Volume 25, Issue 1
Displaying 1-6 of 6 articles from this issue
  • Kiyoji SHIONO
    1977 Volume 25 Issue 1 Pages 1-26
    Published: 1977
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    Focal mechanisms are investigated for 56 major earthquakes with magnitudes around 6.0 and greater that have occurred in southwest Japan since 1920's in order to clarify the relationship between seismic activity and the underthrusting of the Philippine Sea plate along the Nankai trough and the Ryukyu trench.
    Focal mechanisms of large earthquakes along the Nankai trough and in the Hyuganada are similar to one another, indicating a low-angled thrusting. This feature can be explained in terms of elastic rebound of the continental block which is dragged down by the underthrusting Philippine Sea plate.
    For very shallow inland earthquakes except those located around the Izu peninsula and in the Kyushu region, the P-axes tend to be oriented in the eastwest direction. This seismological evidence, together with geologic evidence, indicates that the E-W tectonic compression has prevailed in the crust of southwest Japan for one or two million years up to the present. The E-W compression has been interpreted to arise mainly from the subduction of the Pacific plate.
    Focal mechanisms of subcrustal earthquakes are characterized by systematic alignments of the T-axes. In the region from Iyonada to the southern Chubu district where the Philippine Sea plate does not penetrate into the asthenosphere but remains in the continental plate, the T-axes are parallel to the leading edge of the underthrusting plate (parallel extension). Under the Kyushu island and the northern Ryukyu arc, on the other hand, the T-axes dip westwards and are approximately parallel to the dip of the inclined seismic zone extending down to 150-200 km, suggesting down-dip extension characteristic of the intermediate-depth seismic zones. From these facts, the following tectonic model can be derived: While the leading edge of the underthrusting oceanic plate remains within the continental plate, subcrustal earthquakes with T-axes parallel to the leading edge occur as a result of interaction between the oceanic and the continental plates. As soon as the oceanic plate begins to penetrate down into the asthenosphere, subcrustal earthquakes with down-dip extension occur in the underthrusting plate by the gravitational pull.
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  • Haruo SATO
    1977 Volume 25 Issue 1 Pages 27-41
    Published: 1977
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    The elastic energy propagation in a three dimensional infinite elastic medium, in which scatterers are distributed homogeneously and randomly, is investigated by a statistical method. A single isotropic scattering process is investigated. The elastic medium is characterized by the wave velocity V and the distribution of the scatterers is characterized by the mean free path l. It is assumed that the elastic energy is radiated spherically from the source at a time t=0 in a short time duration. A space-time distribution of the mean energy density of the scattered waves is obtained as Es(r, t)=(W0/4πlVtr)1n((Vt+r)/(Vt-r)) for Vtr, where r is the distance from the source and W0 is the total energy radiated. A uniform spatial distribution is constructed far behind the wave front and near the source. The mean energy density Es is proportional to t-2 for t_??_2r/V and independent of r and W0. Several important properties of coda waves observed near the hypocenter are explained qualitatively by this solution when heterogeneities in the earth are interpreted as the scatterers and Es corresponds to the power spectrum of coda waves.
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  • Ryosuke SATO
    1977 Volume 25 Issue 1 Pages 43-68
    Published: 1977
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    Recent proliferation of large high-rise buildings makes it important to investigate long-period seismic particle velocities and accelerations besides displacements generated by the present rather simple dislocation source model.
    In this paper, expressions are derived for obtaining a displacement spectrum on the surface of a layered structure, when seismic waves due to a dimensional fault are incident at the lowest interface of the layer, The assumptions in the derivation are: (1) displacements at the base of the layer are taken as twice those due to the source, (2) only the base displacements in the distance range from r=r0-0.3λ to r=r0+0.3λ contribute to the surface displacements at r=r0, λ being the wave length considered, and the spectrum for this wave-length does not vary very much in this distance range, and (3) the layer effect is taken into account by numerically evaluating an integral with respect to the angle of incidence from 0 to π/2 instead of the rigorous limits from 0 to π/2+i∞ (complex angle of incidence). The numerical inverse Fourier transform is carried out by using the Filon formula to obtain displacements in the time domain. Particle velocities and accelerations can be found in a similar manner.
    Surface displacements, velocities and accelerations in Tokyo due to a hypothetical Kanto earthquake proposed by KANAMORI (1974) are calculated assuming the surface structures beneath Tokyo metropolis obtained by SHIMA et al. (1975). The maximum displacement (um), velocity (um) and acceleration (üm), when the final dislocation is assumed to be 2m, are 44cm, 15 kine and 15gal, respectively. When evaluating these quantities, a Hamming frequency window is applied to the frequency range from 0Hz to 1Hz, to avoid the truncation effect of the spectrum. The apparent predominant period of acceleration is about 4sec or longer.
    If the epicenter of the hypothetical earthquake is located at a position about 50km northwest of the original (Kanamori's) position, um, um and üm in Tokyo are obtained to be 73cm, 18 kine and 19gal, respectively.
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  • PART I. MATHEMATICAL FORMULATION
    Mitsuhiro MATSU'URA
    1977 Volume 25 Issue 1 Pages 69-90
    Published: 1977
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    A reasonable and practical method of finding an optimal fault-plane solution and evaluating the degree of its non-uniqueness from a set of inaccurate geodetic data is developed by using the inversion technique.
    The static deformation fields are modelled by a multiple-fault system in an elastic half-space, which consists of several dimensional faults with various variables, such as the magnitude and direction of a slip, the length, width, depth and location of a fault, the dip-angle and strike direction. Analytical expressions for the elements of the coefficient matrix which relates a set of model parameters to a set of data parameters are given in section 3 for a case of surface displacement.
    In section 2, Jackson's formula on the discrete general linear inversion is followed and extended to a non-linear case in a systematic fashion by introducing a criterion for the convergence of the solution. As a measure of the reliability of the resulting solution, the "effective data number" is also introduced, which is defined as the number of effective data contributing to the determination of a model parameter.
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  • Michio HASHIZUME
    1977 Volume 25 Issue 1 Pages 91-101
    Published: 1977
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    Explosion seismic observations were made in the Sanbagawa high-pressure metamorphic terrane, Shikoku area, Japan. The profile was nearly in the direction parallel to the trend of the metamorphic belt. In this direction a considerably high seismic wave velocity for the upper crust had been expected due to a large amount of basic rock on the surface and predominant schistosity. The observed velocity for the upper crust, however, was found to be around 6.1km/s, a value that is commonly observed in the continental area. The crossover-distance between a travel-time curve for the granitic layer and that for the basaltic layer or the upper mantle was greater than 150km. This fact may be interpreted, combined with the gravity data, as indicating that the thicknesses of the granitic and the basaltic layers under the terrane do not differ significantly from those of the normal continental structure.
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  • Yasuhiro UMEDA
    1977 Volume 25 Issue 1 Pages 103-116
    Published: 1977
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    Some of the prevailing origins of the noise in the period range of several ten seconds were investigated using the Press-Ewing seismograph system at the Abuyama Seismological Observatory. A simultaneous observation of noises with seismometer, microbarograph and highly sensitive resistance thermometer was carried out in order to estimate the effect of the barometric or temperature variation on the noise. The vibration recorded on the vertical seismograms with a magnification of 800 at 15sec has a good correlation with the barometric pressure change; thus it is thought that this noise may be affected by the buoyant force of the pendulum of the vertical seismometer. The noise therefore can be almost completely eliminated by enclosing the seismometer in a pressure-tight vessel. on the other hand, the noises on the horizontal seismograms have no correlation with the microbarograms, so that these noises may be mainly caused by local inclination of the ground.
    Temperature variations seem to have little effect on the Press-Ewing type seismometers. Contrary to this, very high noise appears on the Galitzin type seismometers throughout the winter, or whenever the underground temperature is higher than the room temperature. A crude analysis on the fluid mechanics shows that the turbulence of air occurs inside a large case such as a Galitzin type seismometer. In order to prevent this turbulence, the size of the case should be made as small as that of a Press-Ewing type seismometer and/or the temperature distribution inside the case should be kept warm in the upper portion and cold at the bottom.
    Any noise caused by a barometric pressure change, turbulence of air and the inclination of the ground must have originated by the forces which act on the pendulum directly. In this experiment, the maximum sensitivity of the vertical seismograph against the barometric change, and that of the horizontal one against the ground inclination are estimated as 9cm/mb and 0.006"/cm at 150sec, respectively.
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