Distribution and feature of fracture zone in fault steps; An Eexample of the western part of the Atotsugawa Fault

Niwa, Masakazu   ; Shimada, Koji   ; Nohara, Tsuyoshi   ; Miwa, Atsushi*; Kurosawa, Hideki*

Detailed topographical and geological researches for understanding fracture zone structures in fault steps are necessary to estimate the extent of the mechanical damage to basement rocks due to strike-slip fault activities. Our research of the western part of the Atotsugawa Fault in the Hida City, Gifu Prefecture shows that the faults high-angle oblique to the principal direction of the Atotsugawa Fault characteristically occur in fault steps, which effect that the fracture zone increases in width. The Atotsugawa fault, which is one of the Class A active faults, trends ENE-WSW and shows dextral strike-slip displacement. We firstly checked the occurrence of the fault step in the Miyagawa-cho area of the western part of the Atotsugawa Fault by way of aerial photograph interpretation. Then we researched the distribution and feature of fracture zone in the Kawai-cho area where the fault continues lineally and in the Miyagawa-cho area where the fault steps. In the Kawai-cho area, Fault planes mostly strike ENE-WSW to E-W and dip steeply N or S, subparallel to the principal direction of the Atotsugawa Fault. Width of the fracture zone is several tens meters. In the Miyagawa-cho area, NW-SE and N-S strike faults dominantly occur as well as ENE-WSW to E-W strike faults. Width of the fracture zone is more than a hundred meter. Remarkable existence of NW-SE and N-S strike faults distinguishes the fracture zone in the Miyagawa-cho area as that in the Kawai-cho area.