Displacement Distribution Along the Kaminirogawa Fault and a Dislocation Model

Lateral change of left-lateral strike-slip displacement along the Kaminirogawa fault was clarified by using a displacement-distance method. The Kaminirogawa fault is one of the major strike-slip faults in the Outer Zone of Southwest Japan, and measures 90 km in length on land-area from west of Tokushima to east of Kochi. The fault cuts and displaces the ENE-trending Sambagawa, Chichibu, Kurosegawa, Sambosan, and Shimanto Terrains. Eight values of left-lateral displacement along the fault were obtained by correlating geological markers on both sides of the fault, such as fold axial surface, faults, and bedding surface. They were plotted in the displacement-distance graph. The distance is measured along the fault from the arbitrally located reference point to the middle point between displaced geological markers on both sides of the fault. The graph shows that the displacement distribution is a triangle-type. The displacement is maximum, 12.9 km, at the center of the fault, and it decreases toward both northeast and southwest at a constant rate. The rate of displacement change is 0.206, which means about 2 km displacement increase along 10 km fault trace. Northeast and southwest fault tips of the fault are inferred to be situated at west of Tokushima, to the south of the MTL, and at 30 km south of Kochi in Tosa Bay respectively. Thus, the fault measures 125.2 km in total length. The Kaminirogawa fault is not a Riedel-shear type fault associated with the MTL. The displacement distribution of the triangle-type is well explained by a dislocation model of fault propagation and displacement increase. The model requires linear positive correlation between length and maximum displacement of strike-slip faults. Formerly clarified their correlation (RANALLI, 1977) is nonlinear positive, and is explained by the changes in the growth rates of displacement and length. Transform, transfer, and trench-linked transcurrent faults should be excluded in considering correlation between length and maximum displacement of strike-slip faults, because they abruptly decrease displacement at their fault “tips”