Bulletin of the Geological Survey of Japan Top Page

Bulletin of the Geological Survey of Japan Vol.55 No.5/6 (2004)

Cover photograph | Table of Contents | Abstract

Cover photograph

Slope failure resulting in debris-flow disaster of Atsumari River Basin.

   A debris-flow disaster occurred in the Atsumari River Basin, Minamata City, Kumamoto Prefecture, in the early dawn on July 20, 2003. A slope failure of unstable and jointed andesite lava near a knick point in the midstream segment of the Atsumari River Basin immediately resulted in the debris flow. The slip occurred around the boundary between the andesite lava and the underlying weathered tuff breccia acting as a permeable layer and an aquiclude, respectively.

(Yusaku Taguchi, Tsutomu Nakazawa and Makoto Saito)

Table of Contents

Title	Author	PDF
Article
>Geologic and hydrologic background of slope failure and debris-flow disaster in Atsumari River Basin, Minamata City, Kumamoto Prefecture on July 20, 2003.	Tsutomu Nakazawa, Makoto Saito and Yusaku Taguchi (113-127)	55_05_01.pdf [4,251 KB]
Estimation of barometric responses of groundwater levels at observation stations of the Geological Survey of Japan, AIST in and around the Kinki district.	Yuichi Kitagawa, Naoji Koizumi, Makoto Takahashi, Norio Matsumoto, Tsutomu Sato,Yasuto Kuwahara, Hisao Ito, Akio Cho, Takashi Satoh and Eikichi Tsukuda (129-152)	55_05_02.pdf [3,676 KB]
Geochemistry of biogenic silica, carbonate materials and sea salts in the coastal marine sediments around the Japanese islands.	Shigeru Terashima, Noboru Imai, Ken Ikehara, Hajime Katayama, Atsushi Noda,Atsuyuki Ohta, Takashi Okai and Masumi (Ujiie) Mikoshiba (153-169)	55_05_03.pdf [1,108 KB]
Clinopyroxenite in serpentinite melange of the “Kurosegawa Belt” in the Izumi-Tomochi area, Kumamoto Prefecture, central Kyushu.	Makoto Saito, Kazuhiro Miyazaki and Hitoshi Tsukamoto (171-179)	55_05_04.pdf [1,178 KB]

Abstract

Geologic and hydrologic background of slope failure and debris-flow disaster in Atsumari River Basin, Minamata City, Kumamoto Prefecture on July 20, 2003.

Tsutomu Nakazawa, Makoto Saito and Yusaku Taguchi

   A debris-flow disaster occurred in the Atsumari River Basin, Minamata City, Kumamoto Prefecture, in the early dawn on July 20, 2003. The disaster was caused by a slope failure occurring near a knick point in the midstream segment of the Atsumari River Basin. This area is underlain by Cretaceous sandstone and mudstone of the Shimanto accretionary complex and Pliocene highly weathered tuff breccia and jointed, relatively fresh andesite lava, in ascending order. Among them, the unstable and jointed andesite lava was slipped and immediately resulted in the debris flow. As the slope failure occurred corresponding to the peak of heavy rainfall, it is considered that the slope failure resulted from a rapid increase of potential of groundwater in the andesite lava due to the heavy rainfall. The slip occurred near the boundary between the andesite lava and the tuff breccia acting as a permeable layer and an aquiclude, respectively.
   The debris-flow deposits along the Atsumari River are composed mainly of gravels and rather poor in muddy sediments. On the other hand, the 1997 Harihara debris-flow disaster resulted from a slope failure of highly weathered andesite lava after the peak of the heavy rainfall, and the debris-flow deposits are rich in muddy sediments. In the Harihara River Basin, the andesite lava was sheared by tectonic movements. Therefore, the remarkable difference of the mechanism between both two disasters is considered to be related to the degree of weathering of andesite lava caused by tectonic backgrounds in the slope failure areas.

Estimation of barometric responses of groundwater levels at observation stations of the Geological Survey of Japan, AIST in and around the Kinki district.

Yuichi Kitagawa, Naoji Koizumi, Makoto Takahashi, Norio Matsumoto, Tsutomu Sato, Yasuto Kuwahara, Hisao Ito, Akio Cho, Takashi Satoh and Eikichi Tsukuda

   AIST has been measuring the groundwater level of wells in order to detect crustal movements. We analyzed the barometric responses of the groundwater levels to understand the hydraulic characteristics of the well and stratum. These information are useful to exactly evaluate detection capability of the crustal movements. The groundwater levels have been measured at 26 wells of 21 observation stations in and around the Kinki district mainly since 1996. We describe the three phenomena, which affect the barometric responses of the groundwater levels, and the characteristics of frequency dependence caused by each phenomenon. We calculated the frequency dependence of the barometric response of the groundwater level for each AIST observation well. The characteristics of frequency dependence of the barometric responses fall into several standard patterns. From the characteristics at each well, we estimated what phenomena affected the groundwater level. There are some wells of which the groundwater levels were largely affected by rainfall and the barometric responses could not be well estimated. We also compared the tidal response with the barometric response at each well. At some wells, the barometric responses of the groundwater levels were not consistent with the tidal responses of the groundwater levels. The possible causes of the inconsistency were discussed.

Geochemistry of biogenic silica, carbonate materials and sea salts in the coastal marine sediments around the Japanese islands.

Shigeru Terashima, Noboru Imai, Ken Ikehara, Hajime Katayama, Atsushi Noda, Atsuyuki Ohta, Takashi Okai and Masumi (Ujiie) Mikoshiba

   In order to characterize the geochemical behavior of elements in the biological and sea salts origin materials, the whole major elements together with the biogenic silica (Bio.SiO₂), carbon dioxide (CO₂), and water soluble chlorine (Cl) were analyzed for about 200 coastal marine sediments collected around the Japanese islands. The average abundance of Bio.SiO₂ is the most higher in sediments from off SE Hokkaido which have been affected by the Oyashio current. The sediments from off Tokai situated in the region of the Kuroshio current are clearly depleted in Bio.SiO₂. The Bio.SiO₂ and Cl are generally enriched in the fine sediment fraction than the coarse one, and positively correlated with water depth of the sampling stations. The CO₂ is dominant in the coarse sediment distributed in relatively shallower water depth. Although most representative carbonate mineral is CaCO₃, dolomite is occurred in some samples containing a large amount of CO₂. It is considered that all the heavy metals containing toxic trace elements are not enriched in the sediments of Bio.SiO₂, carbonate minerals and sea salts. In general, most heavy metals are contained more in fine fractions than coarse fractions of sediments, and those are sometimes enriched during the early diagenesis. The evaluation of biological and sea salts origin materials in the coastal marine sediments will be indispensable for estimation of geologic background in the catchment area.

Clinopyroxenite in serpentinite melange of the “Kurosegawa Belt” in the Izumi-Tomochi area, Kumamoto Prefecture, central Kyushu

Makoto Saito, Kazuhiro Miyazaki and Hitoshi Tsukamoto

   Clinopyroxenite blocks are sometimes included in serpentinite melange of the “Kurosegawa Belt” in the Izumi and Tomochi area, Kumamoto Prefecture, central Kyushu. They are up to 2 km in length and the largest body in Japan. The clinopyroxenite is composed of diopside with a small amount of serpentine. Diopside is usually 1 mm to 10 mm in length with a maximum length that reaches 10 cm and has many parallel partings. This clinopyroxenite site is important as a typical diopside (diallage) occurrence in Japan. The parallel partings generally develop as the diopside crystal becomes larger. A small amount of tremolite occurs along the partings and among the diopside crystals.
   By the electron probe microanalyzer (EPMA) analyses, there is no difference in the chemical composition among diopsides from three clinopyroxenite blocks. The chemical composition of diopside is almost same as those from other areas in the “Kurosegawa Belt”.
   As the clinopyroxene hornblende gabbro block in the serpentinite melange has a hornblende KAr age of 371±19 Ma, it is important to examine the age of the ultramafic and mafic rocks in the “Kurosegawa Belt”.