ACTIVE FAULTS IN CENTRAL PART OF THE ITOIGAWA-SHIZUOKA TECTONIC LINE, CENTRAL JAPAN

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  • 中部フォッサマグナ西縁,富士見周辺の活断層
  • チュウブ フォッサマグナ セイエン,フジミ シュウヘン ノ カツダンソウ

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The purpose of this paper is to examine characteristic natures of active faults along the central part of the Itoigawa-Shizuoka tectonic line (I. S. T. L.) (Fig. 1).<br> Flights of terraced alluvial fans of different ages are developed in the study area. These terraces are classified into seven surfaces from I to VII in descending order, based on the relationship between the terrace deposits and overlaying tephra layers (Figs. 2, 3 and 4). Terrace I consists of Nirasaki mudflow deposits which were derived from Yatsugatake Volcano at about 300, 000 years ago. Terrace II is covered with paleo-soil and tephra layers which include Pm-I pumice bed of ca. 80, 000 y. B. P. originated from Ontake Volcano (Machida and Suzuki, 1971). Terrace III is conformably overlain by Pm-I. Terrace IV is covered with Pm-IV scoria layer of 40, 000 y. B. P. (Machida and Arai, 1979). Terrace V is overlain by an undifferentiated tephra layer without any distinctive key-bed. No covering tephra layers are observed on terraces VI and VII.<br> The study area is composed of two sections, Fujimi (at north) and Hakushu sections. In Fujimi section, a series of elongated tectonic bulges and three types of active faults (type A, B and C), which have the NW-SE trend, are observed (Fig. 15). The east facing scarp of the type A fault is a scarp of reverse fault and is located on the piedmont line. The type B fault is characterized by a well defined west-facing fault scarp which is located several hundred meters away from the piedmont line.<br> Each segment of these faults is short and straight, and it is arranged in left-hand echelon. A longitudinal profile of tectonic bulges associated with this type of faults is asymmetrical, steeper in nothern slope than the southern one. The above mentioned feature suggests a left lateral displacement along the I. T. S. L., although no systematic offset of streams and ridges has been recognized. The type C fault is presumed at the base of deformed terraces which slope down eastward, and it is probably a reverse fault.<br> In contrast, at Hakushu section, the strike of the active faults turn to N-S and only type A fault is observed.<br> In both sections, all the type of active faults show the progressive deformation. Presence of left lateral displacement in Fujimi section is probably attributed to the obliqueness of fault line to the axis of main compressional stress in central Japan.

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