FAULT TOPOGRAPHY ALONG THE YAMASAKI FAULT SYSTEM, WESTERN KINKI DISTRICT, JAPAN

Abstract

The Yamasaki fault system, a major active fault zone in Kinki district, extends about 80 km from WNW to ESE along the southern foot of eastern Chugoku Mountains, and has left lateral displacements with a relative uplift of the NE block. The displacements by these fault activities are clearly expressed on the surface topographies. Especially, most valleys which flow crossing the surface trace of the fault show typical sinistral dislocations (offset streams). In addition, this fault system is famous for its high seismicity. Recently, many geophysical and geochemical studies on the Yamasaki fault system are carried out. But fundamental topographic or geologic investigations are scarce. Therefore, the writer mainly describes and analyzes the fault topography along this fault system in this paper. The findings of this study can be summarized as follows. 1. Faults distributed in the study area compose the active fault system (namely, the Yamasaki fault system), which consists of four relatively large faults (viz. Sayo, Hijima, Yasutomi and Goji faults) and some small faults (viz. Toyonari, Kuwano, Kami-Mikawa, Naka-Mikawa, Kawato and Ohkurayama faults). 2. It is considered that the Sayo, Hijima and Yasutomi faults are the main traces of this fault system judging from the surface expressions of these fault displacements. On the other hand, it seems that the Goji and Kuwano faults are inactive. Others are subsidiary faults located at the terminals of each large fault. 3. The relation D =a L (where D is an amount of stream offset, L is a length of the river upstream from the fault and a is certain coefficient) is acceptable only for streams along the Sayo fault segment, where coefficient a is about 0. 3. 4. Surface topographies observed along the Yamasaki fault system reveal that active strike-slip faulting has played an important role on the topographic evolution of the surrounding mountains. 5. Relatively large rivers crossing the fault in this area have little offset. The reason of this phenomenon is ascribable to its modes of fault dislocations and river erosion. It would be seen that the offsets of stream originated by faulting were obscured by the lateral erosion of these rivers.