Journal of Physics of the Earth
Online ISSN : 1884-2305
Print ISSN : 0022-3743
ISSN-L : 0022-3743
Volume 45, Issue 2
Displaying 1-8 of 8 articles from this issue
  • Tameshige Tsukuda
    1997 Volume 45 Issue 2 Pages 73-82
    Published: 1997
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    Sources of luminescence and their buildup processes accompanying the 1995 Hyogo-ken Nanbu earthquake of M 7.2 are studied based on pieces of information obtained mainly by interviewing eyewitnesses. Gross forms of relatively large-scale luminous sources are roughly classified into four types: lightning with zigzag lines, a swelling shield-shaped source, an upward-extending fan-shaped source, and a belt of lights. The last one includes an arc-like source. Each source is predominantly in tones of either colorless-white, blue or orange-color. This paper presents 23 spottings, distributed as wide as 50 km from the epicenter of the mainshock near Kobe City. Along with these spottings, some local flashing events were reported. The upper limit of the height of several sources was able to be estimated as less than 200 m above the ground. The linear dimension of the horizontal extent ranged from about 1 to 8 km. The luminance was estimated to be more than an order of 103 cd/m2 for an arc-like orange colored source at the eastern part of the aftershock area. Not a few sources were glittering more intensely than this case. According to most of the yewitnesses, the luminosity started from ground level on land, suggesting that discharge processes of the polarized electricity in near-surface rocks may be the primary driving force of the luminescence. However, electricity charged in the air should be also responsible to some luminous phenomena, especially a kind of lightning above the sea. Fog or dust was observed in the air in the region around Nishinomiya City, east of Kobe, preceding the quake, which might have played the role of an effective electrical conductor in glow discharge.
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  • Hiroshi Fukuoka, Kyoji Sassa, Gabriele Scarascia-Mugnozza
    1997 Volume 45 Issue 2 Pages 83-90
    Published: 1997
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    The 1995 Hyogo-ken Nanbu earthquake caused not only casualties and damage to buildings and civil structures but also many landslides. Most of the landslides were triggered or reactivated in the northeastern part of the Rokko Mountains, Nishinomiya and Takarazuka Cities, and the northern tip of Awaji Island. Based on the analysis of aerial photographs coupled with field surveys, 674 landslides were mapped within an area of about 700 km2. The observed landslides were mainly rock slides, rock falls, and rock/debris avalanches. Debris slides, complex slides, and slumps were also found. A few debris slides showed low apparent friction angle during motion with long runout distance. The investigations of landslide typology and distribution show that, based on the relationship between landslide frequency and distances from the assumed fault rupture zone, an attenuation trend can be observed which shows a significant decrease within 3 km, while the maximum recorded distance was 10 km. While focused on the long runout mechanism in the Takarazuka golf course landslide, an undrained loading ring shear test on saturated golf course soil was carried out and succeeded to reproduce the low apparent friction angle mobilized in the landslide.
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  • Electromagnetic Research Group for the 1995 Hyogo
    1997 Volume 45 Issue 2 Pages 91-104
    Published: 1997
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    In association with the 1995 Hyogo-ken Nanbu earthquake, we observed changes in the telluric field by about 10 mV/km about 10 s after the origin time with some long-span electrical dipole network located at about 70 km from the epicenter. After the occurrence of the earthquake, electric and magnetic measurements such as DC resistivity, VLF-MT, self-potential, and the geomagnetic total intensity across the Nojima fault were made at several places in Awaji Island. The low resistive zone beneath the western side of the fault was found in the derived resistivity structure of the shallower portion of the fault in Hirabayashi. On the other hand, however, no significant anomaly associated with the fault structure was found from the measurements of total intensity and self-potential. Prior to the occurrence of the main shock, no geomagnetic or electric continuous stations existed near the focal area. Approximately within 1 week after the occurrence of the main shock, we set up 10 continuous observation sites for the geomagnetic total intensity in the focal area in order to detect geomagnetic changes due to stress change caused by the aftershock activity. No changes in the total intensity in association with the individual aftershock activity have been observed so far. However, systematic temporal trends in the geomagnetic differences have been observed, especially at sites in the northern half of the observation area.
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  • Hiroshi Katao, Naoki Maeda, Yoshihiro Hiramatsu, Yoshihisa Iio, Setsur ...
    1997 Volume 45 Issue 2 Pages 105-119
    Published: 1997
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    Earthquake focal mechanisms before and after the 1995 Hyogo-ken Nanbu earthquake have been investigated using seismic records from regional seismic networks. Before the mainshock, seismicity was very active at the Tamba Plateau, a neighboring area of the Hyogo-ken Nanbu earthquake rupture zone. In contrast, the seismicity along the Hyogo-ken Nanbu earthquake rupture zone was not so active. Most earthquakes in these regions had source mechanisms of E-W compression and were of the strike-slip or reverse-fault type. Most aftershocks along the Hyogo-ken Nanbu earthquake rupture zone have strike-slip solutions with P-axis in the E-W or ESE-WNW direction, which is compatible with the trend of aftershock distribution and the strike of active faults the same as the mainshock mechanism. Simultaneously, many other aftershocks were of the reverse-fault type with E-W compression. This area is still controlled by the regional stress field of E-W compression observed before the mainshock. Although, we could find various types of mechanisms in the aftershock sequence, some normal fault-type events were also observed in the mainshock rupture zone. We could find events of SE-NW compression, and this direction is nearly perpendicular to the trend of the mainshock rupture zone. Some aftershocks that occurred near the epicenter of the mainshock had solutions of N-S compression. The geometry of the active fault systems and/or local stress change induced by the mainshock may cause these complex features of focal mechanisms. After the mainshock, the focal mechanisms of earthquakes in the Tamba Plateau were approximately E-W compressional; the same as that before the mainshock.
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  • Messele Haile, Kazuoh Seo, Katsumi Kurita, Hideo Kyuke, Hiroaki Yamana ...
    1997 Volume 45 Issue 2 Pages 121-133
    Published: 1997
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    In this study, an attempt was made to examine the effect of site conditions on damage distribution in the Kobe City area, after the Hyogo-ken Nanbu earthquake of Jan. 17, 1995, using measured microtremors. A preliminary measurement of microtremors was done between Feb. 18 and 21, 1995. Based on the findings of the preliminary measurement, a more detailed investigation was made between March 4 and 6, 1995. In both cases continuous-stationary measurements and mobile measurements were made. The measured microtremors in the Kobe area showed site-dependent variation in time history, spectral amplitude, spectral ratio, and predominant period. Stiff soil sites showed lower values of amplitude both in time history and spectral amplitude, while soft and deep soil formations showed higher values. The predominant periods also varied in relation to the soil formation in some of the cases. The damage distribution of building structures, especially that of low-rise structures including wooden houses, showed good relation with the characteristics of measured microtremors.
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  • Yoshimitsu Fukushima, Kojiro Irikura
    1997 Volume 45 Issue 2 Pages 135-146
    Published: 1997
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    The peak horizontal acceleration and velocity of observed records from the 1995 Hyogo-ken Nanbu earthquake are compared with those predicted from empirical attenuation relations that were derived to be applicable to near-source areas. We found that the observed peak values matched well the empirical attenuation relations. The observed peak vertical accelerations were about half the peak horizontal accelerations when less than 100 cm/s/s. But, the observed peak vertical accelerations tended to be more than half the horizontal ones beyond 100 cm/s/s, and at some sites on soft soil, the peak vertical accelerations were greater than the peak horizontal accelerations. On the other hand, most of the observed peak vertical velocities were about 40% of the peak horizontal velocities independent of the peak amplitudes. At Port Island in Kobe City, remarkable non-linear behavior of soft soil was observed in the vertical array records. The high-frequency shear waves decreased due to liquefaction and the peak horizontal accelerations decayed at the surface. On the contrary, vertical ground motion was amplified by the strong contrast of P-wave velocity in the surface soils. This fact may be one of the causes for greater peak vertical acceleration than peak horizontal acceleration.
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  • Yutaka Mamada, Takashi Okumura, Hiroshi Takenaka, Sadaomi Suzuki, Seij ...
    1997 Volume 45 Issue 2 Pages 147-154
    Published: 1997
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
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  • Kiyoshi Yomogida, Takashi Nakata
    1997 Volume 45 Issue 2 Pages 155-165
    Published: 1997
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
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