Fault-forming Process of the Komyo Fault in Central Japan.

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In the fracture zone surrounding the Komyo fault, a large left-lateral strike-slip fault, there are many minor faults with distinct striations on their fault surfaces. The minor faults can be divided into two groups by their attitudes and by their senses of slip ; one group consists of left-lateral faults trending about N15°W, and the other of right-lateral faults trending about N75°W. The mode of occurrence of the minor faults is not dependent on lithology except for ductile, weak rocks such as mudstone, but changes systematically with distance from the main (Komyo) fault. The minor faults occur mostly within a zone 2000m wide adjacent to the main fault, and their frequency decreases with distance from the main fault. The frequency of left-lateral minor faults becomes considerably larger toward the main fault as compared with the frequency of right-lateral minor faults. The strikes of the left-lateral minor faults are, however, not necessarily exactly parallel to the main fault, but rotate counterclockwise up to 20° as they approach it. The process of development of the Komyo fault comprises the following four stages ; (I) flexuring stage, (II) minor fault-forming stage, (III) stage of minor fault proliferation and of appearance of the main fault, and (IV) stage of increasing displacements along the main fault and concurrent flexuring in the adjacent region. Thus it is inferred that minor faults were formed mostly at the earlier stages of the whole process of the Komyo fault development and then were rotated by the drag of strata due to the succeeding displacements along the main fault. Three types of earthquakes observed with high accuracy are considered as examples of faulting at the present time. Earthquake faults which have been produced in association with the three earthquake types are compared with the four stages of the Komyo fault development. As the result, the Matsushiro earthquake, the Gifuken-chubu earthquake, and the Parkfield-Cholame earthquake are considered to represent, respectively, stage II to III, stage III, and stage IV of fault development.
1976-03-31

Fault-forming Process of the Komyo Fault in Central Japan.

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