| The Lushan Ms 7.0 earthquake occurred at the southern section of Longmen fault zone on April 20, 2013. Because of the large magnitude and the complex geological conditions, and the reason that it is similar to the Wenchuan earthquake located in the northern part of the Longmen Fault Zone in time and space, it is necessary to analyze the crustal deformation before the Lushan earthquake and to compare strain with the stress after the Wenchuan earthquake. On the other hand, because the seismogenic fault of Lushan earthquake did not rupture to the surface, and different researchers have inconsistent opinion, it needs more researches to analyze the slip distribution of Lushan earthquake and find the inner reason of it. This research uses GPS velocity field data in different periods and coseismic displacement data of the earthquake to analyze the strain, stress before Lushan earthquake and coseismic dislocation. The main work of the paper as followed:1. Based on the 1999~2007 and 2009~2013 GPS velocity field data, the strain and its relationship with the stress in the vicinity of the Longmen Fault Zone before the Lushan earthquake are discussed.The plane strain rate and principal strain in the vicinity of the Longmen fault zone were calculated by the least square collocation method. The results show that, before the Wenchuan earthquake, the strain accumulation rate in the north of the Longmen Fault Zone is close to zero. It suggest that the strain accumulation reach the limit value in this area. When the Bayan Har block continues to push, the northern section is more likely to rupture than the southern section; after the Wenchuan earthquake, the southern section of Longmen Fault Zone was still in the locking state, while the strain accumulation rate increased, which increased the earthquake risk in the region; the main strain mainly concentrated in the Xianshuihe Fault Zone before the Wenchuan earthquake and showed sinistral deformation background. Stress changes caused by Afterslip and stick slip were calculated by half-space dislocation program and were compared with the main strain. Results showed that the main strain decreases on the south side of Bayan Har block and the increases on the north side mainly due to the influence of Wenchuan earthquake. Changes of main strain in the focal region of Lushan earthquake are mainly affected by the impact of regional tectonic. The affection of Wenchuan earthquake in the Sichuan-Yunnan block is not significant.2. Taking the seismic displacement data of Lushan as a constraint, the inversion of Lushan earthquake co-seismic slip distribution is calculated, and the reasons of the sinistral slip of Lushan earthquake are analyzed.On the basis of previous studies, the fault model is set to be a curved surface, and the dip angle varies with depth. The fault can be simulated by exponential function. Considering that the fault did not rupture to the surface and the earth’s stratification, the co-seismic slip distribution of Lushan earthquake is obtained by using co-seismic displacement and the SBIF inverse method. Changes of main compressive strain direction in the focal region of Lushan earthquake before and after Wenchuan earthquake is analyzed by the least square collocation method to find the reason of sinistral slip. The result showed that the rupture is dominated by thrust faulting, with a slight but still statistically significant sinistral component; the sinistral slip characteristic is more obvious in the south of the fault and slip angle decreases as the fault depth increases; the co-seismic displacement of the model values and GPS observations fits well. The large vertical error of the LS06 point in the setting of the planar fault is corrected. Because the north section of Longmen Fault was unlock after Wenchuan earthquake, the main compressive main strain direction in the Lushan earthquake focal region rotated clockwise, suggesting that the long-term strain accumulation is the reason why Lushan earthquake has a slight sinistral characteristic. |
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