Abstract
Geophysical surveys were carried out to trace the fracture zones in the Awakura and the Ute areas, the northeastern part of Okayama Prefecture, which borders on Tottori and Hyogo Prefectures. The Yamasaki fault terminates in these areas. The methods employed were ELF-magnetotelluric sounding and γ-ray survey. The trends of low resistivity zones were deduced from the principal axis directions of impedance tensor. The peak positions of γ-ray count rate suggest the location of fault lines at the surface layer. The traces of faults were estimated from the results of the surveys combining with the topographic lineaments. The estimated traces show that the Yamasaki fault sprays westward into several branches. A north-south striking fracture, which is tentatively named the Ute fault, is also estimated along the west side of the Kajinami River in Katsuta-cho. The northern extension of the Ute fault is considered to be linked with the linear array of epicenters between Tottori and Chizu. The Ute fault is a candidate for the boundary of tectonic blocks dividing the Chugoku and the Kinki districts.
