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Volume 27, Issue 2
Displaying 1-11 of 11 articles from this issue
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The Research Group for Explosion Seismology1974 Volume 27 Issue 2 Pages 95-103
Published: June 30, 1974
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSOn December 11, 1965, a 2.5 ton explosion was made in an abandoned tunnel of the Toyama mine, Kochi Pref. for the refraction survey of the crustal structure in Kiniki and Sikoku areas. In order to form a reverse profile to the Miboro explosions, 18 temporary stations were aligned to the NE direction from Kochi to the north of the lake Biwa with intervals of 20-30km. Three kinds of refracted waves with apparent velocities of 5.9km/sec, 6.5km/sec and 7.8km/sec were observed, although the seismograms were not high qualities. Some characteristic features with this explosions were as follows: 1) Pg waves with an apparent velocity of 5.9km/sec showed zero intercept time near the explosion site. 2) Most of wave energies were confined within Sikoku and were not propagated to long distances beyond the inland sea.View full abstractDownload PDF (621K) -
Harumi AOKI, Ikuei MURAMATU1974 Volume 27 Issue 2 Pages 104-109
Published: June 30, 1974
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSIn the Toyama explosion in 1965, which was made in order to continue the investigation of crustal structure in Kinki and Shikoku crossing the Median Tectonic Line, three kinds of refracted waves from the granitic and basaltic layers and from the upper mantle were detected. The seismograms obtained in the Miboro explosions, which formed reverse shots to the Toyama explosion, were, therefore, reexamined to detect waves from the basaltic layer, since they had not been reported up to the present time. An affirmative result was obtained although these waves were considerably masked by intense waves from the granitic layer.
Because of small number of data, only a rough picture on the crustal structure was discussed in this paper. The superficial layer with a velocity of 5.5km/sec varies from 0 to 2km in thickness and tends to thin out towards southwest in Shikoku. The first layer with a velocity of 6.0km/sec has an average thickness of about 20km and is underlain by the basaltic layer with a velocity of 6.6km/sec. The thickness of the layer may be about 10km at the center of the profile. The upper mantle is deep to the NE direction. The derived structure is concordant not only with gravity data along the profile but also with the crustal structure of the western part of Japan derived from the Kurayoshi and the Hanabusa explosions.View full abstractDownload PDF (514K) -
Harumi AOKI1974 Volume 27 Issue 2 Pages 110-119
Published: June 30, 1974
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSAccording to the plate tectonics in a subduction region, a lithospheric plate is bent downwards at a trench. The trench forms an arc because the earth's surface is not flat [FRANK, 1968]. If the descending slab is not rigid, Frank's theory does not hold quantitatively. A theoretical investigation on the relation between the dip of a descending slab and the curvature of an associated trench is made for a few models of lithosphere. The dip of the slab is approximately equal to the radius of curvature of the trench, or is about a half of the dip expected from Frank's theory. If the lithosphere consists mainly of forsterite, the slab is downward concave until it reaches to a depth of about 400km. Below the depth a phase change of forsterite gives rise to a large shear deformation or tearing of the slab. Otherwise the slab cannot penetrate into the mantle.View full abstractDownload PDF (757K) -
Takashi TADA1974 Volume 27 Issue 2 Pages 120-128
Published: June 30, 1974
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSCrustal movements caused by the Nemuro-Oki earthquake of 1973 were observed by means of leveling survey and tidal gauge measurment. It was found that the observed crustal movements were subsidence on the land and the maximum observed subsidence was about 7cm at Attoko from the above mentioned leveling survey.
In order to explain this subsidence, the low-angle reverse fault models were assumed and the vertical displacements were calculated by these models. Although it is impossible to obtain the unique solution because of a few data, the fault parameters obtained from a reasonable fault model are as follows,
fault dimension…100km×100km
dip angle…30°
direction of slip…N 60°W
dislocation…100cm (Ud:Us=2:1).
The direction of fault motion is the same one which occurred in this region, and then, this earthquake is interpreted as the low-angle reverse faulting, with the Pacific plate underthrusting against the Continental (the Asian) plate.View full abstractDownload PDF (541K) -
On Relations between Magnitude and Period Corresponding to the Maximum Trace AmplitudeHikaru WATANABE1974 Volume 27 Issue 2 Pages 129-140
Published: June 30, 1974
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSRelations between magnitude (M) and observed period corresponding to the maximum trace amplitude (T) were investigated by using the data from different types of seismometers. The formula proposed by GUTENBERG and RICHTER (1942), logT=αM+β, is ascertained to hold well for shallow earthquakes of focal distances less than about 200km, where the seismic body waves give the maximum trace amplitude and the corresponding period coincides well with the peak of the observed spectrum. As a matter of course, there is much difference in the coefficients of the formula among the different types of seismometers.
With the aid of this formula, the dependence of magnitude on the period can be normalized, then the deviation of plots from the normal relation in the M-T diagram is found to represent a significant feature of the observed wave form. It is also certain that the feature does not essentially vary with focal distance, radiation pattern deduced from the initial P waves, and attenuation factor and structure of the crust. Thus, the deviation in the plot, named “s” value in the text, may be considered as a specific character of the earthquake origin, showing any of the source parameters, such as the effective stress, stress drop or strength of the source region.
From this point of view, we studied how the rupture process of the main shock related to the subsequent aftershock activity, taking, as an example, the aftershocks of the earthquake of the Central Part of Gifu Prefecture in 1969 (M=6.6). A crude analysis leads to a conclusion that the aftershocks with soft wave forms (s>10) occur at the very beginning of the sequence surrounding the focus of the main shock, whereas the events with hard wave forms (s<-10) are delayed for a while and located along the fault trend distant from the focus of the main shock.View full abstractDownload PDF (3657K) -
Isamu AIDA1974 Volume 27 Issue 2 Pages 141-154
Published: June 30, 1974
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSNumerical model to represent adequately a shallow water and bay region together with an open sea is constructed without an extreme increase of a number of grids. The computed values in this numerical model may be directly compared with data which are observed by a tide gage at the inside of a bay. As for the 1968 Hyuganada Earthquake, numerical experiments with the tsunami source computed from the dislocation of the earthquake fault are tried. The source model modified by about 10 degrees the dip angle of fault plane is fitter for the observed tsunami data than the one derived directly from seismic data. This model can reproduce the tsunami waves at 5 tide-gage stations in the error of ±20% in the height of the first crest of computed waves and of ±0.8min in the arrival time. The amplitude variation and the directivity of tsunami propagation, and the applicability of Green's law are examined. At the district of Hososhima, tsunami data are obtained at 3 sites, and it is found that the present numerical model is quite reliable to reproduce the variation of tsunami within a short distance.View full abstractDownload PDF (1141K) -
Takahumi MATSUI, Hitoshi TAKEUCHI1974 Volume 27 Issue 2 Pages 155-163
Published: June 30, 1974
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSInstantaneous phase and amplitude of a time dependent given function f(t) can be calculated by the method of Goodman (1960). It is interesting to see how the instantaneous phase and amplitude of ILS (International Latitude Service) and BIH (Bureau International de l'Heure) data change with time. These calculated instantaneous phase and amplitude are shown in Fig. 3, 4, 5, 6. In Fig. 3, 4 we find sudden phase changes in 1927 and 1942. Such a sudden phase change may be considered to be related with the excitation of the Earth's free damped nutations. It is, however, not clear what kind of natural phenomena are related with the excitation. Earthquake occurrences do not seem to be related with the excitation of the Chandler wobble.View full abstractDownload PDF (1090K) -
Precursory Time-Interval, Rise-Time and (Δρ/ρ)/(ΔL/L)Yoshio YAMAZAKI1974 Volume 27 Issue 2 Pages 164a-167
Published: June 30, 1974
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (429K) -
Hitoshi TAKEUCHI1974 Volume 27 Issue 2 Pages 164
Published: June 30, 1974
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (118K) -
Takashi TADA1974 Volume 27 Issue 2 Pages 167-169
Published: June 30, 1974
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (264K) -
1974 Volume 27 Issue 2 Pages e1
Published: 1974
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (321K)
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