13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Fig.3.The effect of size of fault and the low-stress drop Substituting (19) into the left-hand side of (17) determines the critical shear stress corresponding to the infinite fault body, and marked by , the ratio is the normalized critical shear stress, the variation of which versus the characteristic size a/H of fault body describes the effect of fault body size on the fault instability, as description in Fig. 3. This result is identical with that obtained in ref. [2], and gives a quantitative description of the low-stress drop observed in earthquake source. 2.2 Fast propagation of fault After the initiation of fault growth, it may experience a stable but further unstable growth, or it may become unstable growth directly after the initiation. Before and after the unstable propagation, there is another possibility that the propagation is arrested. Since the velocity of the unstable growth may reach the order of magnitude of elastic wave velocity, it may lead to an earthquake. We here discuss the case in which earthquake rather than arrest may be caused. As pointed out just now, the state corresponding to the fast fault propagation is quite different from that corresponding to static case. It is necessary to carry out a fully dynamic analysis, which is governed by the following equations: (21) for strike slip fault; while for inverse fault (22) where and represent the speeds of longitudinal and transverse waves, respectively. The dynamics analysis reveals the low stress drop effect again. 3. Interaction between co-linear faults The principle discussed in the previous section can be used to the interaction between co-linear faults. The Ms7.2 Xingtai earthquake happened on March 8, 1966 in North China Block, immediately after the event, Prof. Li Shi-Guang, pointed out the earthquake transmission was going
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