Comparative Studies of River Terrace Deposits and Landslide Dam Deposits Caused by the 2004 Earthquake in Niigata along the Imokawa River, Niigata Prefecture

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  • 新潟県長岡市（旧山古志村），芋川の河岸段丘堆積物は土砂崩れダム（天然ダム）堆積物か？
  • ニイガタケン ナガオカシ キュウ ヤマコシムラ イモカワ ノ カガン ダンキュウ タイセキブツ ワ ドシャクズレ ダム テンネン ダム タイセキブツ カ

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Landslides triggered by the 2004 earthquake in Niigata formed dams and lakes along the Imokawa River. These lakes were rapidly buried with sediment, which was primarily composed of fine to medium sand derived from unconsolidated sedimentary rocks (Shiraiwa and Uono Formation), transported by the Imokawa River. Similar very fine sand deposits were observed in a river terrace situated 70 m above the Imokawa river bed. The objectives of the survey were : i) to compare present lake deposits behind landslide dams with terrace deposits distributed along the Imokawa River, and ii) to consider the effects of sediment load increases after the earthquake on river terrace-forming processes.<BR>Three types of sedimentary facies were observed in the landslide dam deposits : i) fairly wellsorted and obscurely stratified fine to medium sand deposit, with evidence of upward fining and a lack of silt and clay, ii) laminated deposit of silt and very fine sand, and iii) parallel bedding sand deposit with gravel particle lenses. Sedimentary facies i) and ii) are related to lake-enlarging stages, while facies iii) is related to Imokawa riverbed deposits after the lake filled with sediment.<BR>Eight terraces along the Imokawa River, divided into three groups (H1-H2, M1-M2, L1-L4), were classified and mapped. Their longitudinal profiles illustrated that terraces along the Imokawa River are consistent with those along the main stream, the Uono River. Most of these terraces are erosional, however, at three sites (Loc. 1, Loc. 4, Loc. 5) we observed well-sorted and obscurely stratified fine to coarse sand deposits of 2-3 m in thickness, with upward fining and a lack of silt and clay, that continued at times to laminated deposits of silt and very fine sand. At the top, coarser sand layers with gravel particle lenses were found. These facies are very similar to the present lake deposits behind landslide dams. Moreover, Carbon-14 dating for Loc. 4 and Loc. 5 deposits on a lower terrace plain estimated the occurrence of a landslide dam at ca. 2900 yBP. It is possible that this landslide event was triggered by a former large earthquake in Yamakoshi.<BR>Every few thousand years, well-sorted massive sand deposits behind landslide dams, which are triggered possibly by earthquakes, must have been formed along the Imokawa River; however, almost all of these deposits have been eroded away. We expected that the increasing sediment loads caused by earthquakes would have caused the river floodplain to widen, but such a river channel metamorphosis has not occurred along the Imokawa River as small amounts of gravel particles were supplied in the geological setting of the Shiraiwa Formation. Fluvial terraces along the Imokawa River have developed every few ten thousand years, while the riverbed height of the main stream, the Uono River, has remained almost constant.

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