岐阜県中部地震に関連した断層

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A fault related to the Gifuken-chubu earthquake of September 9, 1969, was discovered by the field survey. The precise hypocentral distribution of aftershocks observed by WATANABE and KURISO (1970) was much utilized for tracing the fault. The fault named Hatasa fault is a left-lateral strike-slip fault with a trend of N25°W and a length of 17 km (Fig. 1). The evidences of old activities in thc geologic time as well as recent activities of the fault movement were revealed by the study on the structures of deformed rocks. The old activities of the fault seem to be related to those of the "Nohi western boundary crush zone" which exists on the southern extension of the Hatasa fault. The recent geologic structure in the area around the Hatasa fault is characterized by the block structure consisting of the NW trending left-lateral and the NE trending right-lateral strike-slip faults which have been active in the Quaternary time (Fig. 2). These blocks are 15-25 km in dimension, and the size of them is considered to mean the minimum unit of blocks which characterizes the block structure of the region. The block structure around the Hatasa fault is comprised in the block structure of higher order which is composed of the Yanagase, the Neo-valley, the Atera and the Atotsugawa strike-slip faults. The occurrence of large earthquakes as the 1891 Nobi (M=8.4) corresponds to the latter large block structure, whereas earthquakes of middle magnitude as the present Gifuken-chubu (M=6.6) and the 1934 Hachiman (M=6.2) occurred in the former one. The geological and physiographical evidences show that the Oppara fault truncating the northern extremity of the Hatasa fault has a right-lateral strike-slip displacement of 3-4 km (Fig. 3). By the restoration of the displacement, the Miogo fault is joined to the Hatasa fault. The Gifuken-chubu earthquake attacked most strongly the Hatasa area on the northern part of the Hatasa fault. In this area residents noticed that the ground water had become slightly muddy twice during about a week before the occurrence of the main shock. The aftershocks were clustered in the southern part of the fault, as shown in Fig. 1. These phenomena would be explained by the following interpretation. As the northern part of the Hatasa fault corresponds to the central part of the original Hatasa-Miogo fault set, the fault plane in that portion is presumed to have been already matured and relatively fully smoothened. Therefore, fault creep began prior to rapid slip in this part, and disturbed the ground water system. At the time of the main shock, fault slippage occurred almost thoroughly and resulted in the strong tremor: most of stored strain energy was released in the northern part of the Hatasa fault. On the other hand, many aftershocks continued to occur in the southern part where elastic strain was not released adequately. The theoretical vertical displacement computed from a fault model by MIKUMO (1973) shows a discontinuous boundry which is conjugate to the Hatasa fault (Fig. 4). The Oppara fault exists along the boundary. It is presumed that the fault also participated in the crustal deformation associated with the present earthquake.
1976-03-25

岐阜県中部地震に関連した断層

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