Earthquake Generating Stress in Eastern Chugoku and Northern Kinki Districts, Southwest Japan

Abstract

In order to clarify earthquake generating stress in the eastern Chugoku and the northern Kinki Districts and also to examine its relations to geological structures, 65 local earthquakes of magnitude ≧ 2.5 in these districts for 5 years from 1966 to 1970 were analyzed. By the use of the method of smoothed radiation pattern, the mean orientation of the maximum pressure axis of tectonic stress prevailing in the whole region was estimated to be nearly horizontal and E-W. Next, the whole region was divided into 12 small regions, and two methods, the fault plane solution and the smoothed radiation pattern, were applied to earthquakes in respective regions. Fault plane solutions showed some features characteristic of the regions, which are interpreted to have some relations to geological structures. Also, the regional difference in directions of the maximum pressure axis was observed from smoothed radiation patterns in respective regions. These facts probably point out that earthquakes in this area are generated under a common tectonic stress of E-W compression and that geological structures act to give a preferred orientation of fracture.