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.
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