1998 Volume 9 Pages 33-56
The Rokko Mts. and Awaji Island have been formed under an east-west compressive stress field accompanied by the right-lateral and vertical components of faulting along the Rokko-Awaji fault system. At the 1995 Hyogoken-Nanbu Earthquake, the Nojima earthquake fault appeared almost along the pre-existing Nojima fault within this fault system, which was previously known to be active. Many precise surveys have been performed after this earthquake to describe the various features of the earthquake fault, such as the detailed locality and behaviour of faulting, amount of displacement, and other geomorphic and geologic characteristics. In addition, the following subjects are discussed based on these features: the segmentation of the faulting, its total length, mode and amount of displacement at both ends, and the relationship between the Nojima fault and the other faults on the east side of the island. From the measurement of three dimensional crustal deformation around the earthquake fault using air-photographs, it was revealed that the island in the northwestern side of the fault moved eastward up to 1.7m and dislocated vertically from ―1.2m to + 1.7m, whereas the southeastern side was displaced southward up to 2.5m and uplifted up to 1.7m. These acute crustal movements are not parallel to the fault trace. The three dimensional crustal deformations were at least in the order of lm, even in sites more than 1 km apart from the fault trace. Many kinds of surveys also revealed the characteristics of the Nojima fault including the fault topography, recurrence time of faulting, total displacements, and the subsurface geology. Especially, seismic reflection surveys and three deep-core drillings across the fault clarified the subsurface structure around the fault, and the characteristics and inclination of its shattered zones. The Nojima fault accompanied with a present fault scarp higher than 250m had started remarkable vertical displacement after the deposition of the Osaka Group. It has repeatedly moved at a recurrence interval of ca. 2,000 years during the Late Quaternary. Total right-lateral displacement attains at least 300m, being almost equal to or less than its total vertical dislocation. The studies on the fault planes and shattered zones revealed dips of the planes, mode of faulting, and the relationship between the amount of displacement and the width and structure of the shattered zones. Furthermore, detailed observation and analysis of the drilling cores now in progress indicate the structure and dip of the fault at relatively deeper levels, as well as the characteristi cosf shattered rocks accompanying the fault. From the ample evidence mentioned above, the Nojima fault is considered to be different in faulting history between the parts north and south of Hikinoura Village in Hokudan Town. Right-lateral faulting and east-side uplifting have continued until now in the northern part. In contrast, in the southern part of the fault, the bifurcated fault corresponding to the geologic boundary between the granitic rocks and the Osaka group has acted as an active fault, although its activity is weak at present. Along the Nojima earthquake fault, a wide flexure zone had been formed along the Mizukoshi flexure, but it changed into an active fault with a predominantly right-latera lcomponent probably during the Late Quaternary.