Strike-slip Faulting of Late Quaternary along the Median Dislocation Line in the Surroundings of Awa-Ikeda, Northeastern Shikoku

Abstract

The Median Dislocation (or Tectonic) Line extends nearly 900 kilometers from Central Honshu to Kyushu through Kii and northern Shikoku, trending from ENE to WSW and dividing the southwest Japan into the Inner and Outer Zones.
Many terraces and alluvial fans displaced both vertically and horizontally are found along this great fault line in the lower drainage basin of the Yoshino River, northeastern Shikoku.
This paper describes mainly landforms by the recent faulting in the surroundings of Awa-Ikeda. It is generally recognized in this area that horizontal component of the displacement is larger than vertical one. It is considered, therefore, that the Median Dislocation Line is a great right-lateral fault, at least, active in the Quaternary, as Kaneko (1965) has pointed out from a largely photogeological study. The displacement along the Ikeda fault (the Median Dislocation Line) is estimated as follows:
At Awa-Ikeda (Fig. 2·4, Photo. 1·2), horizontal and vertical displacements amount to about 200 meters (or more) and 50 meters, respectively, in the recent 30, 000 years, judging from the offset of back scarps of fluvial terraces and the difference of heights in bases of the terrace deposits, which have a clay bed including the buried woods caluculated at 27, 700 and 23, 600 years B. P. At Higashi-Suzu (Fig. 5-B: central part), horizontal and vertical displacements are about 45 meters and 5-8 meters, respectively, probably in the newer age than 10, 000 years. At the lower reach of the Nishitani Creek (Fig. 5-B: east part), about 50 meters and 7-9 meters, and at Aziro (Fig. 7: west part), about 70 meters and 15 meters, respectively, are measured.