Active Fault Outcrops and Fault Topography of the Shimoppara Fault in Shitada Village, Niigata Prefecture, Central Japan.

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  • Suzuki Ikuo
    Department of Geography, Faculty of Education, Niigata University

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  • 新潟県下田村，下原断層の断層露頭および断層変位地形
  • ニイガタケン シタダムラ シモッパラ ダンソウ ノ ダンソウ ロトウ オヨビ ダン

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Abstract

The landforms of central part of Niigata Prefecture can be divided roughly into two parts: high the Echigo Mountains in the east and low mountains, hill, terraces, and alluvial plains in the west. The boundary between these two is referred to as the Shibata-Koide Tectonic Line, which runs approximately trending NNE to SSW.<br>The Shinano River in Niigata Prefecture flows southwest toward northeast, and finally pours into the Sea of Japan. This region is known for having the shortest wave length of folding and the largest rate of folding in gradient change in Japan.<br>The studied area is located along the Ikarashi River, a tributary of the Shinano River, and is characterized by active folding and faulting trending NNE to SSW parallel to the Shibata-Koide Tectonic Line. In this area, the author discovered some outcrops of the faults cutting through terrace surfaces and deposits younger than 120-130ka. The most important of these faults is referred to as the Shimoppara Fault, which is a reverse fault and probably belongs to the Shibata-Koide Tectonic Line fault system. Such reverse faulting is the dominant mode of deformation in areas affected by compression. Left-lateral displacement is generally recognized in the northern part of the Shibata-Koide Tectonic Line. However, offset terrace scarps show that the Shimoppara Fault in the central part of the Shibata-Koide Tectonic Line was probably dislocated right-laterally.<br>Vertical and horizontal displacements amount to about 6.3m (or more) and 48.5m, respectively, in the recent 120-130ka. The mean rate of vertical displacement of the Shimoppara Fault is 0.05m/ka., and its average recurrence interval is estimated to be 25ka.

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