Zisin (Journal of the Seismological Society of Japan. 2nd ser.)
Online ISSN : 1883-9029
Print ISSN : 0037-1114
ISSN-L : 0037-1114
Volume 56, Issue 3
Displaying 1-8 of 8 articles from this issue
  • Yohei ITANI, Katsuhiko ISHIBASHI
    2003 Volume 56 Issue 3 Pages 231-243
    Published: December 25, 2003
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    In order to understand the active tectonics in the Izu collision zone, the Pacific coast of central Japan, we analyzed the daily coordinate data from GEONET (GPS Earth Observation NETwork), which is a nationwide dense network of continuous GPS observation operated by Geographical Survey Institute (GSI), in the Kanto-Tokai districts surrounding the collision zone for the period from January 1997 to January 2003. In the collision zone the Izu Peninsula, the northern cusp of the Izu-Bonin volcanic arc, is colliding against Honshu as the northernmost part of the Philippine Sea plate, and an active volcanic chain is crossing the collision boundary. For six successive one-year intervals we obtained distributions of principal axes of horizontal crustal strain rate, dilatation rate, and maximum shear strain rate in the Kanto-Tokai districts based on velocity vectors at every 0.1 degrees which were interpolated from observed velocity vectors. We examined the distributions of dilatation rate in particular, and found that all remarkable seismic and volcanic activities in the study area were detected in the distributions, proving the reliability of analyzed results. The notable feature in the distribution of dilatation rate is the existence of positive areas around Mt. Fuji, the most remarkable volcano to the north-northwest of the Izu Peninsula. One possibility for the cause of the positive dilatation is an inflation of magma chamber beneath Mt. Fuji. The other possibility is the regional tectonic effect due to the collision process. The crustal strain in the northern part of the Izu Peninsula is rather small, whereas that in the southern part is considerably large, which seems to reflect the present stage of the Izu collision process.
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  • Yuuichi KITAGAWA, Naoji KOIZUMI, Ryu OHTANI, Kunihiko WATANABE, Satosh ...
    2003 Volume 56 Issue 3 Pages 245-254
    Published: December 25, 2003
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    At the Yasutomi station of AIST located near the Yamasaki fault, a crustal strain field has been measured by a borehole strainmeter. Although the crustal strain rates were almost constant for the period before around April 2002, a conspicuous compressive strain change was observed for the period from June to around September 2002. The corresponding changes were too small to be detected from groundwater pressures and GPS measurement at the Yasutomi station of AIST, and extensometers at the Yasutomi and Osawa stations of Kyoto University. Moreover, the phenomenon was not detected at GPS stations of GEONET around the Yasutomi station, either. These observations indicate that the phenomenon was limited to be local and caused by a small source. Since it was not accompanied by earthquakes, it was possibly caused by unstable and aseismic slip occurring on the Yasutomi fault, which is a part of the Yamasaki fault system. Supposing that a left-lateral slip with 1mm on the shallow part of the Yasutomi fault, all observed results can be explained.
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  • Spatial Distribution of Mc East off Northeast Japan
    Hiroyasu MABUCHI, Masakazu OHTAKE, Haruo SATO
    2003 Volume 56 Issue 3 Pages 255-265
    Published: December 25, 2003
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    We investigated the spatial distribution of Mc value of the magnitude-frequency distribution proposed by Utsu (1971) for the region of east off northeast Japan. Mc is a model parameter that defines the upper limit of earthquake magnitude. We further examined the possibility of estimating the maximum earthquake magnitude based on Mc for a given region. The data we used are taken from the earthquake catalog of the Research Center for Prediction of Earthquakes and Volcanic Eruptions of Tohoku University for last 21 years. The magnitudes of M≥5 earthquakes in the catalog are replaced with those of the Japan Metrological Agency catalog to keep the continuity of magnitude scale. By using a moving window of 0.5°×0.5° for the analysis, we found a systematic pattern of the Mc distribution: the area of Mc>7 distributes off Aomori Prefecture, far off Iwate Prefecture, and off Miyagi and Fukushima Prefectures, while anomalously small value (Mc<6) prevails in a roughly 1°×1° area within 100km off the coast of Iwate Prefecture. Mc we estimated shows positive but weak correlation to Mcat, the maximum magnitude for last 115 years. By dividing the whole region into four zones based on the above result, we found excellent agreement between Mc and Mcat for each zone. This suggests that Mc may be used for predicting the maximum size of future earthquakes if appropriate zoning is done. Epicenters of past large earthquakes (M≥7) tend to locate in and near the high Mc areas, suggesting that the parameter Mc may reflect physical characteristics of earthquake generating field.
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  • Shinichi MATSUSAKI, Yuki OHNO, Michiharu IKEDA, Yoshimitsu FUKUSHIMA
    2003 Volume 56 Issue 3 Pages 267-279
    Published: December 25, 2003
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    Using hypocentral data of JMA from 1997 to 2000, we determined upper surface shape of the Philippine Sea plate around Iyo-nada.
    The contour lines indicate that the Philippine Sea plate is spreading northward from Iyo-nada to Aki-nada in west Setonai-kai area. From 3-D bird's eye view, slight dent of the plate-upper surface, possibly due to buckling, can be seen at bound of the plate subducting around Iyo-nada area.
    It is considered that in addition to the plate bending, change of local tectonics from buckling to spreading may contribute to seismicity of the Iyo-nada area events. As well as this change may contribute to the source mechanism of small events and cyclic Geiyo earthquake.
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  • Points at Issue for Recognition of Concealed Fault under an Alluvial Plain
    Ryuichi SUGISAKI, Ken SHIBATA
    2003 Volume 56 Issue 3 Pages 281-296
    Published: December 25, 2003
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    A concealed fault named “the Gifu-Ichinomiya Line” along the eastern margin of the Nobi Plain was stratigraphically reexamined. This fault has been regarded as an active fault concealed by Cenozoic strata of the Nobi Plain, which runs in the NNW-SSE direction, and connects Gifu with Ichinomiya and Inazawa. However, the exact location of the fault has not been well defined. We focused our attention on the vertical position of a pumice bed of 80, 000 to 90, 000 years old that appears in bore-hole records around the site, because the pumice bed is a key to stratigraphic correlation. The correlation of 97 bore-hole records showed a throw of the pumice bed amounting to 5.6m in average. The strike of this throw parallels to the JR-Tokaido Line and runs about 1, 100m to the east of the line. The vertical displacements were also recognized for two gravel beds (G1 and G2) above and under the pumice bed. The throw for the deeper strata tends to be larger. We regard this throw as an active fault that moved at the Nobi earthquake of 8.0 magnitude in 1891 and other earthquakes repeated during the Quaternary.
    Recently, Aichi Prefecture carried out stratigraphic examinations of shallow well records and geophysical prospecting and reported that neither fault nor flexure is realized around the presumed site of the fault in question. On the basis of the results, Governmental Earthquake Research Committee concluded the fault to be nonexistent. However, the survey by Aichi Prefecture exploited exclusively the area to the west of the fault disclosed from our stratigraphic work and scarcely covered the site of the fault mentioned above. Therefore, the survey by Aichi Prefecture is meaningless in the examination of the Gifu-Ichinomiya fault, and their conclusion is erroneous.
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  • Masajiro IMOTO
    2003 Volume 56 Issue 3 Pages 297-302
    Published: December 25, 2003
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
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  • Makoto NAKAMUKAE, Hiroshi KATAO, Hiroo NEMOTO, Koichi NAKAGAWA
    2003 Volume 56 Issue 3 Pages 303-309
    Published: December 25, 2003
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
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  • Masaru KONO, David J. STEVENSON
    2003 Volume 56 Issue 3 Pages 311-325
    Published: December 25, 2003
    Released on J-STAGE: March 11, 2010
    JOURNAL FREE ACCESS
    Three-dimensional, fully nonlinear dynamo models were first presented in 1995 by two groups in Japan and USA. Since then, about ten groups have reported similar simulation results, and some important properties of the dynamos became known from these studies. Also in recent years, new observational results accumulated for the magnetic fields of planets (including satellites), and indicate very diverse dynamo and induction effects in these bodies. In this review, we first go through the method employed for the geodynamo simulations, and show how the magnetic field is generated and maintained by the convection system in the rotating spheres and spherical shells through a process similar to α-effect studied by kinematic models. We then review the observation of the planetary magnetic fields in the solar system, and investigate the conditions required for maintaining the dynamo process. Using these results, we examine the magnetic fields of the individual planets and consider how these fields are produced; by dynamo, induction, or remanent field.
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