Abstract
The preferred orientations of dike swarms of 12 areas that formed in the Early and Middle Miocene are investigated in order to know the azimuths of horizontal maximum compressional principal tectonic stress (σHmax) in the Southwest Japan. The most concentrated trends of dikes are parallel to the Southwest Japan Arc in each region to the north of the Butsuzo tectonic line and are perpendicular to that in each region to the south. Therefore, the directions of σHmax obtained from the preferred orientation of dike swarms are parallel to the Southwest Japan Arc on the northern side of the Butsuzo tectonic line and nearly perpendicular to it on the southern side, respectively. This results coincide with the directions obtained from the strike and the sense of motion of faults and folds formed in the same age.
