Seismological Re-evaluation of Regional Stress Orientation and Fracture Angle in the Kinki District, South west Japan with Reference to Developmental Process of Conjugate Faults

Abstract

The results of studies on focal mechanism of earthquakes in the Kinki district, Southwest Japan are reviewed and re-examined in order to evaluate regional stress orientation, Φ, fracture angle, θ_0, and internal friction, μ, in the crust. Two peaks in the frequency distribution of the azimuth of the P-axis leads us to a conclusion that Φ=N85°E, 2θ_0=70°-75° and μ= 0.3-0.4. Ratio of inconsistent data given by Hashizume (1970) with the assumed axis of tectonic compression, N85°E, also suggests that 2θ_0=70°-80°, in spite of his negative conclusion. These values of 2θ_0 are in good agreement with those obtained from joint study of the Rokko Mountains. Based on the seeming nature of this situation, it is also concluded that defraction of the P-axes near the major faults suggests that fracturing occurs along the plane parallel to the active fault itself and sense of dislocation is concordant with that of the fault. Further, periodic change of the P-axes of the Matsushiro earthquake swarm leads us to a significant aspect on the developmental process of a conjugate fault system.