Reappraisal of Late Quaternary Activities along Active Faults in the Northern Part of Awaji Island, Southwestern Japan

Abstract

Active faults are densely distributed in the northern part of Awaji Island, southwestern Japan, and generally define the major geomorphic boundaries. The aims of this paper are as follows: 1) to precisely describe the deformed landforms along the seven known active faults in the area (Kusumoto, Higashiura, Nodao, Nojima, Asano, Ikuha, and Shizuki) and two newly identified active faults (Kamaguchi and Hikinoura), based on the interpretation of large-scale airphotos; 2) to estimate the degree of fault activity, by analyzing the presence and location of recent fault traces and the relationship between the amount of horizontal offset and the length of valleys for offset streams; and 3) to discuss the implications of different fault systems. The active faults can be divided into three groups. Group A) are the western marginal faults (Nojima, Asano, and 1995 Nojima earthquake fault); Group B) are the eastern marginal fault group (Kusumoto, Higashiura, Nodao, and Kamaguchi). These two fault groups trend NNE-SSW and are characterized by dextral-slip, as demonstrated by offset of streams and ridges, accompanied by uplift of the mountain side. In contrast, a third fault group, C) are transverse faults striking in the NNW-SSE direction, and are mostly characterized by uplift of the mountain side, without horizontal displacement. Judging from the value of a in formula 1 (D=aL, where D=the amount of offset on the stream and L=the stream length from the fault trace to the valley-head) (Matsuda, 1975), faults in groups A and B belong to class A in terms of degree of fault activity. Mountain growth on northern Awaji Island has resulted from the activity of these fault groups, which are oblique to the principal stress field. Fault activity has continued to the present; faults of group C are normal to groups A and B in their strikes, and may represent the secondary consequence of mountain growth.