地震と地質構造の形成をめぐる若干の問題

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Attempts are made here in order to obtain some informations on palaeo-seismicity from the knowledges about mechanisms and processes of the structure formation in nature and/or in laboratory experiments. The first porblem is to clarify the genetical relation between the earthquakes and the geologic structures, such as fault and fold. Recent development of study on the earthquake mechanism states that "earthquake is faulting itself". While, as shwon in Fig. 1, the energy transfered by seismic shock wave generated at faulting must result in rock failure, which leads to the secondary faulting and folding, which gives rise to the sccondary earthquake,…… This cyclic chain is cut where the deviatoric stress due to seismic energy is lowered below the rupture strength or the yield value of surrounding rocks. It is noteworthy that the most of the so-called "palaeo-stress field" restored by the analysis of minor fault pattern may represent the secondary stress field which is different from the primary one directly related to the tectogenetic cause (Fig. 2). The second problem is to find some clues to palaeo-seismicity from the fault and faulting. There are two kinds of fault plane markings available to this purpose, namely, wear grooves and striations on slickensides. A length of wear groove reveals one stick-slip movement of seismic fault, and it corresponds to one time stress drop. Therefore, by making the precise survey on both the natural and the experimental markings, we can obtain various informations on the earthquakes in the past. Striations on slicken sides give us the knowledges about the slip direction in faulting. Step structures developed on the slicken sides in the cross direction to the striations may be a directorin direct result of stick-slip. However, its genetical relation to wear grooves is not detailed yet. Another important indications are curved striations and superposed slicken sides with differently oriented striations (Fig. 3). The former suggests that the relative movement on both sides of fault plane was not of simple translation, but was accompanied with some rotation. The latter striations display the successive changes of slip direction at sometimes movements of the same fault. It is very important to determine the types and directions of slip in faulting, by which the nature of seismicity is greatly affected. The next approach to the subject can be made by examining the growth rate of the geologic structure and its relation to the frequency of earthuqake. The essential of this problem is lying in the phenomenon of tectonic flow. The rheology equation of the simplified tectonic flow of rocks is deseribed as, η=σ_<tect>/3ε, where, η is equivalent viscosity, σ_<tect> is tectonic stress, and ε is strain rate. Since σ_<tect> in the past can not be directly measured, if η at various differential stresses (=σ_<tect>) are measured, it enables us to discuss quantitatively the relationship among η, σ_<tect> and ε. An example is presented in Table 1. Now, provided that the tectonic stress is 100 b, which is considered to be adequate from various studies hitherto made, the strain reaches 30% in the short period of 1 y (Table 2). Such a catastrophic event clearly contradicts the experience in geology. Two solutions are possible in this case. One is to take very small value of tectonic stress. For instance, if σ_<tect> is 2b, the time required for 30% strain is 1×10^5 y. Another solution is given by assuming that the tectonic stress over than 100b acts only a few days during 1000 y (Table 2). Such a condition might be satisfied before the great earthquake. This solution gives the significant due to clarify the changes of seismicity with time.
1976-03-25

地震と地質構造の形成をめぐる若干の問題

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