Electro-Optical Measurement of Horizontal Strains Accumulating in the Swarm Earthquake Area (2)
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概要
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The authors have repeated Geodimeter surveys in the Matsushiro area since October, 1965. Their purpose is the study of accumulation of horizontal strains there at various stages of the seismic activity. The present paper will deal mainly with the strain events observed from March of this year, since the data for the earlier period was reported in the previous paper. Generally speaking, the mode of strain accumulation in the present period does not differ much from that previously observed. That is to say, extension predominated in the NS-direction whereas a considerable amount of contraction was observed in the EW-direction. As for the amount of strain accumulation, however, the present period shows an activity much higher than the previous one. Length of the Sorobeku base-line, for example, increased as much as 35cm during the period March to July inclusive, and another 43cm during the following two months. So that the total extension of this baseline after October, 1965, reached 89cm indicating the average linear strain to be 290×10-6. Strain data thus obtained for the three independent base-lines enable us to study the horizontal deformations as a plane-strain problem. Computed principal axes harmonize very well with the seismic force system there. Strain energy associated with the deformations is roughly estimated to be comparable with the energy released by the seismic activity. It is hard to believe that the earth's crust is deformed elastically to such a high strain rate as above-mentioned. We must naturally take non-elastic deformation into consideration for understanding the present event. A fracture zone that has developed remarkably since last spring is notable from this point of view. According to geologists'reports, the systematically arranged fractures seem to suggest a buried fault of strike-slip type that crosses the Sorobeku base-line obliquely. Actual conditions and characteristics of the presumed fault should be urgently studied by taking all the available data into consideration. The authors constructed supplementary networks in the Wakaho and Nakano areas in order to watch strain accumulation in localities adjacent to the present seismic area. Data from the surveys on these networks are also discussed briefly.
- 1967-03-31
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