Zisin (Journal of the Seismological Society of Japan. 2nd ser.)
Online ISSN : 1883-9029
Print ISSN : 0037-1114
ISSN-L : 0037-1114
Volume 41, Issue 1
Displaying 1-16 of 16 articles from this issue
  • Junji KOYAMA
    1988 Volume 41 Issue 1 Pages 1-8
    Published: March 25, 1988
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    The strong ground motions show azimuthal dependence due to finitely-propagating rupture. Since we may consider that the extreme value of strong ground motions determines seismic intensity, the azimuthal dependence of seismic intensity is derived on the basis of short-period seismic directivity. Unilateral faulting forms an asymmetric intensity distribution like an egg shape. The largest intensity is found in the direction of rupture propagation and the smallest in the opposite along the equi-epicentral distance. Bilateral faulting forms a symmetric intensity distribution like an elliptical shape. The smallest intensity is in the normal direction of fault strike and the largest in both the directions of the fault trend along the equi-epicentral distance.
    Seismic intensity distribution for the 1983 Japan Sea earthquake favors unilateral faulting, which matches with the rupture process estimated from the aftershock distribution. The distribution for the 1964 Niigata earthquake is consistent with bilateral faulting. The Niigata earthquake is known to be one of the largest bilateral faulting near Japan.
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  • Based on the Hypocenter Data by the Matsushiro Seismic Array System
    Makoto NISHIWAKI, Tadashi MAKI, Seiji NAGARE
    1988 Volume 41 Issue 1 Pages 9-15
    Published: March 25, 1988
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    Space-time variations of very-shallow (0-20km) earthquakes occurrence in the northern part of Nagano Prefecture are studied from the hypocenter data by the Matsushiro Seismic Array System (MSAS) and JMA. The very-shallow earthquakes are distinctively characterized by two active zones, or the one from Omachi to Nozawaonsen and the other from Sakakita to Suzaka including hypocenters of the Matsushiro Swarm Earthquake. A significant variation of very-shallow earhtquakes occurrence is recently observed along the Omachi-Nozawaonsen zone. After two aftershock activities from Dec. 30, 1986 in Ogawa and from Feb. 12, 1987 in Nozawaonsen, a very low seismic activity or gap between Ogawa and Nozawaonsen is now observed.
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  • Teruyuki ASADA, Susumu NISHIMURA, Narimitsu MATSUO, Naoji KOIZUMI
    1988 Volume 41 Issue 1 Pages 17-27
    Published: March 25, 1988
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    Between July 1985 and December 1986 we measured electrical conductivity, water-temperatures at different depths, and flow rate of the water of Moriyama hot spring in Shiga Prefecture. At the estuary of the old Yasu River near the eastern side of Biwako-Ohashi the observation well was drilled up to 1000m depth. Continuous measurements were also made of the chemical compositions of the bubble gas emanated from the hot spring. These data were carefully compared with the water level data of Lake Biwa and the meteoric factors, to check any possible relationship. During the observation period, the flow rate, the water-temperatures and the chemical compositions of the bubble gas anomalously changed twice. The first one can be the precursor to the earthquake of M5.0, which occurred on October 3, 1985 near the Hanaore fault. However, the second one is considered to be caused by the local heavy rainfall on July 22, 1986.
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  • Manabu HASHIMOTO, Takashi TADA
    1988 Volume 41 Issue 1 Pages 29-38
    Published: March 25, 1988
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    Displacements and strains during the recent 80 years in Hokkaido are investigated by the analysis of triangulation or trilateration data. According to the distribution of displacements and strains, Hokkaido can be separated into three blocks, eastern, middle and southwestern Hokkaido.
    In eastern Hokkaido, northwestward displacements and NW-SE contractions are dominant before the 1973 Nemuro-Oki earthquake, while seaward displacements and EW extensions are detected after the event. Summing up them, westward displacements are left, which suggests westward sliding of the outer Kurile arc along the volcanic front and colliding at the Obihiro Tectonic Line [TADA and KIMURA (1987)].
    The Ishikari Low Land, which is considered to be a relic trench, may be the boundary between middle and southwestern Hokkaido along which E-W contractions are prevailing. This observation suggests that northern Honshu would not belong to the North American or the Okhotsk plate.
    Displacements associated with four major earthquakes which occurred during the period between the first and the last surveys are computed. Results show that the 1940 Shakotan-Oki earthquake may not give much influence on Hokkaido. Theoretical displacements computed by models for the 1952 Tokachi-Oki (M8.1) and 1968 Tokachi-Oki (M7.9) earthquakes are inconsistent with the observation, which may require re-analyses of data. The model for the 1973 Nemuro-Oki earthquake can predict the observed displacements qualitatively. The observed displacements are greater than the computed ones by a factor of two or three, which means that old triangulation surveys might suffer from scale errors.
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  • Haruo SATO, Alisho M. SHOMAHMADOV, Vitaly I. KHALTURIN, Tachivana G. R ...
    1988 Volume 41 Issue 1 Pages 39-46
    Published: March 25, 1988
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    Coda decay of band-pass filtered seismograms were studied in relation to the occurrence of an earthquake of K=13.3 (ML=5.2) in Soviet Central Asia. The gradient of coda decay is characterized by coda attenuation Q-1C for each frequency band. Examining the temporal variation of log10(Q-1Cf) for small earthquakes which occurred in a restricted volume around the main shock focus during the period between 1979 and 1984, we found that the variances of log10(Q-1Cf) for forerhocks were significantly larger than those for aftershocks for the frequency bands of 0.62-5Hz and 18Hz. Especially at the 5 Hz band, log10(Q-1Cf) took a significantly lower value in the three-year period preceding the main shock compared with the preceding and following time periods. In the earthquake preparation stage, we may expect the formation of crack-clusters, which are composed of new cracks and reopened and connected existing cracks, in a fairly large region compared with the focal region due to the stress accumulation. The gradual appearance of inhomogeneously distributed crackclusters in the heterogeneous crust seems to be the most plausible mechanism by which the coda decay gradients are caused to scatter largely.
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  • Tameshige TSUKUDA, Megumi MIZOUE
    1988 Volume 41 Issue 1 Pages 47-57
    Published: March 25, 1988
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    Tremors due to avalanches were investigated for the purpose of discriminating them from natural earthquakes and understanding the nature of avalanches. In January, 1986, we have obtained many seismometric records of avalanches, small and large, which occurred near the seismic station NUJ, Nou Town, Niigata Prefecture.
    Among them, the powder-snow avalanche of January 26 at Shimamichi was the largest one that provided us with the direct information about the ground motion caused by a great snow stream nearby. Prominent phases on the avalanche seismogram are considered to be generated either by great changes in gradient of the slope or by the topographic asperities on the way. Based upon this interpretation, we have estimated the velocity of the snow stream and its impact on the ground surface. The final velocity is 45m/s, which is comparable with the fluid model of avalanche proposed by VOELLMY (1955). The force exerted on the ground is 1.2×1013dyn with a duration time of 0.033sec for the first prominent impulsive motion.
    Besides the above event, we have identified 22 avalanche events which occurred from January 22 to January 26. Time series of these events showed forerunning activities of small avalanches before a large one.
    Another typical avalanche occurred on February 6, 1984, about 4km away from the NUJ station. The waveform of this event resembles that of a teleseismic event with an S-P time being about 30sec. However, we are able to discriminate the records of avalanches from those of earthquakes since the former have the ambiguous first motion and the unusually short duration of coda waves.
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  • Sadaiku HATTORI
    1988 Volume 41 Issue 1 Pages 59-69
    Published: March 25, 1988
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    The following spectral ratios of strong motion seismograms due to the Mexico earthquake of September 19, 1985 were investigated: (1) Spectral ratios of observation sites in Mexico city to UNAM (Universidad Nacional Autonoma de Mexico), and (2) Spectral ratios of sites in the Foothill zone of Mexico city and sites in Guerrero Network to Caleta de Campos.
    UNAM is located on the Foothill zone according to microzonation of ground condition in and around Mexico city and so the strong motion of UNAM can be regarded as a seismic input on the base rock in the Transition and Lake zones. It was confirmed that the spectral ratios of sites in the Lake zone to UNAM were extremely high and the reason was due to large ground amplification of each site.
    Caleta de Campos of Guerrero Network is located on the source region of main shock. The strong motion of Caleta de Campos, therefore, can be regarded as a standard one which is reflecting in source spectrum. The followings were observed: (1) Spectral ratios of sites along the Pacific coast to Caleta de Campos decreased with the epicentral distance and (2) Spectral ratios of sites in the land area and with the epicentral distance of around 400km, were higher than those of sites in the coastal area and with almost the same epicentral distance.
    Mexico city is located at the epicentral distance of around 400km, but the spectral ratios of CU 01, TACY, SXVI in the Foothill zone and Transition zone, especially in the peried of 1.5 to 10.0sec, were very high and nearly equaled to those of the source region. This means that the seismic input between 1.5sec and 10.0sec on the base rock in and around Mexico city was very large in spite of the epicentral distance of 400km. This fact must be added to one of major causes of earthquake damages in Mexico city.
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  • A Fractal Source Model and Seismic Spectra
    Toru OUCHI
    1988 Volume 41 Issue 1 Pages 71-78
    Published: March 25, 1988
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    A fractal model of the earthquake source and the spectral structure of seismic waves are considered. This study generalizes conventional spectral models of seismic waves and also takes account of the fractal nature in the focal process. Presented source model consists of numerous unit point sources, whose spatial distributions show the fractal structure like dendritic and Levý flight type. Interestingly, the fractal structure or power law type frequency dependence ωh is obtained in the amplitude spectra of seismic waves from this source model. Here the exponent value h characterizes physical properties of the earthquake source. More high frequency abundant spectra are expected from this fractal model than conventional source models, which explains observed behavior of high frequency components of seismic waves that cannot be well explained by the conventional models. Analysis also shows the absence of corner frequencies in the amplitude spectra in the case of the fractal source that represent the characteristic space-time scales, although they have been generally expected in the ordinary spectral models.
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  • Junji KOYAMA, Yasuo IZUTANI
    1988 Volume 41 Issue 1 Pages 79-87
    Published: March 25, 1988
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    Azimuth dependence of strong motion amplitudes has been studied. The component waves of short period strong motion are characterized to be random phases, since they are generated by random fractures of small-scale fault heterogeneities (fault patches) and propagating real media through multiple reflection, refraction and scattering. Such the waves donot show coherent azimuth dependence like as one due to a point source radiation pattern. However, they show short period seismic directivity effect. The effect comes from azimuthal variation of energy radiation due to the finite-moving source. Azimuth dependence of maximum acceleration and rms acceleration has been derived for a unilateral faulting as
    α·(mLr2)1/2(c/v-cos θ)-1/2
    where α denotes the whole path attenuation, m is a number density of fault patches per unit fault area, L is fault length, and r2 is the average seismic energy radiated from one fault patch. c and v stand for the velocity of shear waves and average rupture propagation, and θ is station azimuth measured from the direction of the rupture propagation. Strong motion accelerations due to the 1968 Tokachi-oki earthquake and the 1983 Japan Sea earthquake do show such the azimuth dependence for maximum and rms accelerations.
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  • Takahiro OHKURA
    1988 Volume 41 Issue 1 Pages 89-96
    Published: March 25, 1988
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    Focal mechanisms of small-size shallow earthquakes which occurred from 1976 through 1986 in San'in, Kinki, and Hokuriku districts, South West Japan, were determined. These mechanisms were obtained from the first motion of P waves recorded at the microearthquake observation stations operated by Kyoto and Nagoya Universities and as well as at the stations of Japan Meteorological Agency (JMA).
    Most of the mechanisms in this region are strike-slip fault type. However, in the vicinity of Lake Biwa, there are also some earthquakes of reverse fault type.
    The P axes trend mostly between ENE-WSW and ESE-WNW directions. They agree with those determined by earlier workers for small and large earthquakes in this region. In the region along the Japan Sea coast, the P axes of most of the earthquakes are directed ESE-WNW. On the other hand, in the inland region adjacent to the coast, the P axes are directed E-W to ENE-WSW. However no distinct boundary between these two regions could be deciphered. It appears that from the inland region towards the cost P axis alignment gradually makes a clockwise rotation.
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  • Yoshihiro OKUBO, Nobuhiro FURUSE, Yoshiteru KONO
    1988 Volume 41 Issue 1 Pages 97-102
    Published: March 25, 1988
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    Crustal structure models from the Noto Peninsula to the Hida mountains area crossing the Atotsugawa fault were estimated based on both detailed gravity anomaly and explosion seismological data. The results are as follows;
    (1) Minor changes of short wavelength Bouguer anomalies were observed across the Atotsugawa fault. This is due to small density differences of rocks around the fault. However, a locally high anomaly zone remained along the fault.
    (2) Variation of long wavelength Bouguer anomalies across the fault was about 12 to 20 mgal per 10km.
    (3) Estimated crustal structure models indicated that the deeper subsurface structure changes suddenly under the fault. It suggests that the Atotsugawa fault is situated at the border of large scale variation of crustal structure between the Hida mountains and the Noto peninsula regions.
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  • Masayuki TAKEMURA, Tomonori IKEURA
    1988 Volume 41 Issue 1 Pages 103-113
    Published: March 25, 1988
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    It has been indicated that the inhomogeniety of faulting strongly influences the excitation of short-period seismic waves, while the excitation of long-period seismic waves can be described by the average displacement or the average stress drop over the fault plane. A semi-empirical method has been developed to synthesize seismic waves of large earthquakes in both the long and the short period ranges. According to this method, the displacement on the fault plane of large earthquake is divided into two parts. One is the average displacement over the fault plane and the other is the deviation from the average value. An element earthquake should be selected, of which hypocenter is in and around the focal region of the large earthquake. The size of fault area of the element earthquake should be consistent with the fault patch size of the large earthquake. By using the observed seismograms of the element earthquakes, a seismic wave SL(t) due to the average displacement and a seismic wave SS(t) due to the deviation from the average value are synthesized separately, and the synthesized seismic wave can be expressed as a sum of SL(t) and SS(t).
    In this study herein, this method is applied to the strong ground motions of three Japanese earthquakes with magnitude 7.4 to 7.9, in the period range from 0.1 to 7sec. After comparing with observed and synthesized records, the applicability of this method is made clear for the estimation of the strong ground motions. The results of comparison with the spectral amplitudes SS(t) and SL(t) in the present study and in other studies indicate that SS(t) is larger than SL(t) in the short period range, while SL(t) is larger than SS(t) in the long period range. The boundary of the period ranges is almost consistent with the rise time of the average displacement over the fault plane of large earthquake.
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  • Yasukuni OKUBO
    1988 Volume 41 Issue 1 Pages 115-123
    Published: March 25, 1988
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    The aeromagnetic anomalies over the Izu-Oshima Island and its surroundings were interpreted using 3-D magnetic inversion analysis.
    A magnetic positive anomaly extends from the south of Oshima to the neighboring off-shore area trending northwest by west. Taking the previous rock-magnetic studies into consideration, the magnetic anomaly in the Oshima Island is caused by igneous rocks having high remanent magnetizations. Calculation shows that the observed anomaly can be interpretedwell by the effect of topography over the Oshima Island except for neighboring off-shore area.
    The thick magnetic layer trending NWW-SEE direction around the Oshima Island was suggested by the iterative inversion. The distribution of the thick magnetic layer coincides with the NW-SE trending dike distribution and the NWW-SEE trending alignment of submarine volcanoes. This indicates that the observed magnetic anomaly is associated with the subsurface volcanisms of the island. According to the comparison between the observed anomalies and the calculated anomalies caused by topographic relief, and the geometry of thick magnetic layer, the high magnetic anomaly around the Oshima Island includes the effect not only of surface volcanics but of a number of high magnetized concealed intrusives underneath.
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  • Shigeru NAKAO, Azusa NISHIZAWA, Akio TAKAGI, Takashi IIDAKA, Kiyoshi S ...
    1988 Volume 41 Issue 1 Pages 125-128
    Published: March 25, 1988
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
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  • Hideo TSUNAKAWA, Toshi ASADA
    1988 Volume 41 Issue 1 Pages 129-131
    Published: March 25, 1988
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
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  • Ichiro NAKANISHI
    1988 Volume 41 Issue 1 Pages 133-144
    Published: March 25, 1988
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    This paper reviews recent advances in our knowledge and understanding of the deep structure of the Earth. Subjects reviewed are mantle discontinuity, lateral heterogeneity, upper mantle anisotropy, aseismic slab, core-mantle boundary, and core.
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