BULLETIN OF THE GEOLOGICAL SURVEY OF JAPAN
Online ISSN : 2186-490X
Print ISSN : 1346-4272
ISSN-L : 1346-4272
Volume 58 , Issue 9-10
BULLETIN OF THE GEOLOGICAL SURVEY OF JAPAN
Showing 1-2 articles out of 2 articles from the selected issue
Article
  • Shigeru Suto, Takayuki Inomata, Hisashi Sasaki, Sakae Mukoyama
    2007 Volume 58 Issue 9-10 Pages 261-321
    Published: December 14, 2007
    Released: May 15, 2014
    JOURNALS FREE ACCESS
    Supplementary material
    The whole total distribution map of the volcanic ash fall deposit in Japan was summarized with the data base under the program of the “Research on volcanic ash fall hazard assessment and risk management for industrial location” and the “Impact analysis on the volcanic ash-fall in the metropolitan area”. The heavy ash fall will cause the serious disaster of course, but well developed area also suffered heavy economical damage by even a thin deposit of volcanic ash. Around five hundred distribution maps of the volcanic ash fall deposits in Japan in these three hundred thousand years were collected. The first work is to redraw the isopach map completely for each unit, because major of the isopach map in the literature were not perfect in shape. The redrawing for each contour line was carried out mathematically to extend each semicircular curve to make a close circle automatically. The redrawn isopachs were reserved as a digital data and the thickness of the deposit between each contour line for around each one kilometer mesh, which is authorized by the third Digital National Land Information system in Japan, were calculated by logarithmical proportional allotment. The worked area was from 24 to 46 degree in the north latitude and from 123 to 149 degree in the east longitude. The name of the volcano, unit name and its alphabetical abbreviation, age of the eruption, and the references, of each ash fall unit were summarized as a data base for each mesh. So it is possible to show the thickness of each ash fall unit for each one kilometer mesh. For example Haneda airport and Narita airport were estimated to be suffered the 8.7 and 3.2 centimeters ash fall from Fuji volcano in 1707 respectively. And also it is easy to find the ash fall disaster history for anywhere and any municipality, cities and towns, which number is around two thousand, in Japan. For example the municipal office building of the Tokyo Metropolitan in Shinjuku has been suffered three, five, and seventeen times air fall ash in these one thousand, ten thousand, and a hundred thousand years, respectively, and the thickness of the deposit for each periods are also able to be shown using the data base.
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  • Shunso Ishihara
    2007 Volume 58 Issue 9-10 Pages 323-350
    Published: December 14, 2007
    Released: May 15, 2014
    JOURNALS FREE ACCESS
    Late Cretaceous plutonic rocks of 6 gabbroids, and 75 granitoids, all belonging to ilmenite series, were re-examined by polarized XRF method. The gabbroids are relatively abundant in the south of the studied region, implying more in-put of heat and mafic magmas from the upper mantle to the lower crust. The MgO/Fe2O3 ratio of the gabbroids reveal regional variation being MgO-rich to the south, which would reflect generation depth of the mafic magmas. The granitoids are classified into I type and S type. The I-type granitoids occur most widely and divided into five zones. Their average silica contents vary from the Median Tectonic Line to the north as follows: Shinshiro-Shitara zone (60.1 % SiO2), Asuke zone (64.2 %), Toyota-Akechi zone (70.0 %), Sanagesan-Obara zone (73.9 %) and Seto zone (75.2 %). This regional variation was considered to reflect chemical heterogeneity of the infracrustal source rocks, rather than the magmatic differentiation. Granitoids of the Okazaki-Busetsu zone, represented by garnet-bearing muscovite-biotite granites, occur in the highest metamorphic grade zone, and is felsic (average SiO2 70.2 %), peraluminous (A/CNK over 1.1), and thus be called S type. Because of the highest heat flow rate in the OkazakiBusetsu zone, even supracrustal rocks were converted to magmas and formed the S-type granites. Both I and S type granitoids are often heterogeneous in the foliated parts, as compared with massive granitoids of non-metamorphic terrains. The heterogeneity can be considered due to different original compositions of the source materials and/or formed by differential regional stress during the flow movement of the solidifying magmas. Lack of hydrothermal mineralization in the south where metamorphic roof-pendent still remain, can be explained by a deep emplacement level (15 km) and small degree of fractionation of these granitic magmas.
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