房総半島大多喜付近の小断層系

(構造地質学ノート4)

抄録

In the studied area, a large number of minor faults ranging from several millimeters to a few meters in displacements are developed. They are classified into four or five fault systems on the basis of parallelism of strikes, mode of occurrence and successive relationship. Among them, the most predominant one-the main minor fault system -is that of the normal faults with the strikes of N-S to NNE-SSW and dips toward E or W. From the analytical study of these faults, it is considered that they were formed during middle to late Pleistocene under the stress field where σ_1, the axis of maximum (tensional) principal stress, lies nearly horizontal and in east and west direction. On the other hand, the major faults of normal sence, whose range in displacement is from several meters to a handred meter, are also developed well in the present area and have been represented in detailed geological survey. They are almost in paralled to the above main minor fault system, and therefore, both are considered to be formed under the same stress field. Of these normal, major- and minor-faults, the ones which dip to east tend to be more frequently developed than those dipping to west, and the greater the fault displacement, the more marked the tendency. The frequency of occurrence of minor faults seems to become high in inverse proportion to the magnitude of displacement. These phenomena may suggest such mechanism that the minor faults were formed originally as a conjugate set, and then rather one of the conjugate pair dipping to east grew to become a large fault, and thus the present structure stepping down to east has been constituted.