13th International Conference on Fracture June 16–21, 2013, Beijing, China -10- Substituting the earthquake fault geometry parameters and , km/s into (39) can determine the critic stress which can induce the Luanxian earthquake is bar. Because bar bar, this means the Tangshan earthquake can induce the Luanxian fault to occur earthquake. In fact, the Ms7.1 Luanxian earthquake happened on July 29, 1976 (only 15 hours after the main shock of Tangshan earthquake). 4. Interaction between parallel faults The zone of 2008 Ms8.0 Wenchuan earthquake and the simplified mathematical model are shown in Fig.5.The zone is located in the Longmenshan earthquake region that is the boundary between Tibet Block and South China Block. The line AB in Fig 5 represents Maoxian fault, which does not appear earthquake during the Wenchuan earthquake event 0f 2008, and can be treated as a free boundary. The line A B represents the main shock fault, the Yingxiu-Beichuan fault, the epicenter is the Yingxiu Town, and line A B the Pengxian-Guangxian fault, where the strong earthquake happened induced by the main shock, and line CD the boundary of South China Block, which can be seen as a fixed boundary. The main shock and induced shock are mainly the inverse motion rather than strike slip motion. This complicated configuration is not available for analytic solving. We have done the numerical analysis by finite element for initiation of fault growth only; the fast fault propagation has not been completed yet. At meantime, for the main shock fault (fault A B ) an approximate solution is given as below (40) The analytic solution for fault A B in general cannot be obtained, we introduce an approximate formula [11] (41) For the dynamic analytic solution cannot be obtained even if for approximate study. The comparison between numerical and approximate analytic solutions, we find that K !!! f static yingxiu K !!! f static pengxian !3.0!8.0 (42) This means the accuracy of the approximate analytic solution and numerical solution is in the same order of magnitude. This also shows the main shock induces the shock of Pengxian-Guanxian. The shock is propagates in elastic wave speed, after happening of the main shock, the induced shock occurred immediately, because the distance between the two faults is only 10 to 20 kilometers. 5. Conclusion and discussion The block hypothesis on the strong earthquake in plate in China land is explained with fracture theory, the single fault analysis explained the low stress drop phenomena, in the co-linear faults analysis the theoretical prediction is in good agreement to the observation results for Xingtai-Hejianfault interaction and Tangshan-Luanxia fault interaction. However the analysis on parallel faults is as just a preliminary work, which must be down the further analysis.
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