Rupture Characteristics Of The Listric Thrust Fault And The Parallel Thrust Faults

There distributed three large thrust faults in near parallel at the middle part of the Longmenshan Fault Zone (LFZ). All three faults are classic listric intra-plate thrust faults. On12May2008, the Wenchuan Ms8.0earthquake occurred on the LFZ, ruptured the Beichuan-Yingxiu fault and the Guanxian-Jiangyou fault. The Wenchuan earthquake is the first occurrence of an Ms8.0thrust earthquake ever recorded in the interior of the continent with high dip angle. The dips of the surface ruptures along the Beichuan-Yingxiu fault during the Wenchuan earthquake are up to70°-80°.The Wenchuan earthquake is a new type of earthquake event that deserves further studies. There are questions need to be answered, such as what may cause the occurrence of the strong shock while the fault slip rate had been very low, or why the slip distribution with the fault depth in the Wenchuan earthquake is different from usual large crustal earthquakes, or why the range front fault suffered synchronous rupture but the back range fault did not, and how much effect the rupture of the range front fault had have on the rupture of the central fault. Research on these topics is important for the understanding of the mechanism of intra-plate earthquakes, and is helpful for the research of earthquake prediction.In order to study these problems, this thesis focuses on the following two aspects of research.1. The rupture characteristics of the listric thrust fault with high dip angles near the crustal surfaceBased on the wide-accepted knowledge of the shallow crustal structure and dynamic boundary conditions at the middle part of the LFZ, the optimal2D fault occurrences of the Beichuan-Yingxiu fault and the Guanxian-Jiangyou fault are discussed with a FEA model. By finite element experiments, the rupture phenomena of the listric thrust faults with high dip angles near the crustal surface are compared with the rupture phenomena of the linear faults with different dip angles, and with other listric thrust faults of different shapes. Results show that:1). the73°natural spline designed in this thesis is suitable for the simulation of the2D fault occurrence of the Beichuan-Yingxiu fault in the Wenchuan earthquake. The occurrence curve based on the results of WFSD is suitable for the simulation of the2D fault occurrence of the Guanxian-Jiangyou fault.2). Under strong lateral push, the shallow near-surface portion of the listric thrust faults with high dip angles near the crustal surface become barriers that prevent or reduce thrust rupture to some extent. Detailed features are as follows: (1). When creep happens, the amount of creep slip near the crustal surface of the faults is much smaller than in central part of the faults, thus it’s difficult for the faults to release internal shear deformation by creep. As a result, the most likely place to occur initial thrust rupture is located at the deep part of the faults.(2). When thrust rupture occurs, the maximum amount of coseismic slip will not necessarily be located near the crustal surface. If a thrust rupture breaks the crustal surface seriously and the maximum amount of coseismic slip is located near the crustal surface, then the rupture should extend very deep into the crust, and a slip distribution platform with relatively high amount of slip will appear at the central part of the fault.(3). The dip angles of listric faults near the crustal surface are greater, and the portions with high dip angles are deeper, then the barriers become more effective. In this thesis, the dip angles of the listric faults near the crustal surface should be at least>65°, then the faults could be called listric thrust faults with high dip angles near the crustal surface.3). The listric fault geometry with high dip angles near the crustal surface and the strong push from Bayan Har block are probably the most important reasons for the occurrence of the rare slip distribution with the fault depth in the Wenchuan earthquake. Previous thrust ruptures whose slip were accumulated in the middle of the fault are not indispensable condition for that.4). Results from simulations of the pushing strength of Bayan Har block on the LFZ show that:within the200km range following the strike of Beichuan-Yingxiu fault from Yingxiu to Dengjiaba, the pushing strength of Bayan Har block on the LFZ may not be the same. Overall, from southwest to northeast, the pushing strength decreases. The pushing strengths around Hongkou and Beichuan are significantly higher than the other segments. Thus it’s concluded that, during the seismo genic time of the Wenchuan earthquake, the shallow crust or some crust layer of Bayan Har block may move faster towards southeast at these two places.2. The interaction of ruptures in the parallel thrust faultsBased on the wide-accepted knowledge of the shallow crustal structure and dynamic boundary conditions at the middle part of LFZ, a number of FEA models of parallel thrust faults were constructed using the suitable occurrences of the Beichuan-Yingxiu fault and the Guanxian-Jiangyou fault.Results from coseismic rupture experiments of the range front fault and the central fault show that:1). There are alternatives between the thrust slips of the range front fault and the central fault when the two faults occur synchronous rupture. This may be the reasonable explanation for the crest on trough phenomenon in the thrust slip of the range front fault and the central fault in the Wenchuan earthquake.2). Because the synchronous rupture of the range front fault in the Wenchuan earthquake, the central fault may have suffered a reduction of approximately10%in total thrust rupture release at the segment where synchronous rupture happened. The reduction of the scalar seismic moment is about9.54*1018N·m.Results from long-term rupture experiments of three parallel thrust faults at different distances show that:1). The finite element method constructed in this thesis can effectively eliminate the interference of the cumulative stress during the long-term rupture evolution of the models, achieve the characterized seismic moments and recurrence intervals of the simulated characteristic earthquakes on the stick-slip thrust faults. When applied to parallel thrust faults, it can reveal the interaction between the faults, and determine whether characteristic earthquake model can be applied to a fault in the parallel thrust faults from the perspective of mutual influence of fault activities.2). The distance between the parallel thrust faults is the major determinant of the interaction between fault activities. The shorter the distance between the parallel faults, the greater the interaction between fault activities, the greater interference the rupture regularity of a fault suffers, the higher the risk of multi-fault synchronous rupture.3). Under similar circumstances to the middle part of the LFZ, when the distance between the parallel faults is less than20km, the ruptures of two parallel thrust faults will appear master-slave relationship. The fault located forward in the direction of lateral push becomes major rupture release fault, and the fault located afterward becomes subordinate rupture release fault. The shorter the distance between the parallel faults, the more notable the master-slave relationship.4). The risk of two parallel thrust faults occurring synchronous thrust rupture is very low, and has little to do with the distance between the parallel faults. The risk of two parallel thrust faults occurring synchronous thrust rupture around crustal surface becomes significantly lower, and shares a special relationship with the distance between the parallel faults. Under similar circumstances to the middle part of the LFZ, when the distance between the parallel faults is between10km-20km, two parallel thrust faults are exposed to highest risks of occurring synchronous thrust rupture around crustal surface.Results from long-term rupture experiments of three parallel thrust faults at the same distances as the ground distances of the three main faults at the middle part of the LFZ show that:1). Seismic activity of the back range fault may be relatively independent. 2). Because of the distance of12km, the seismic activity of the range front fault is affected by the ruptures of the central fault, while maintaining a certain degree of independence. The distance of12km also makes the risk of two faults occurring synchronous thrust rupture around crustal surface at highest level. That may be an important cause of the coseismic surface thrust rupture of the range front fault and the central fault in the Wenchuan earthquake.In a summary, this thesis systematically revel the rupture characteristics of the listric thrust fault with high dip angles near the crustal surface, discussed the influence of the mutual distances on the seismic activity of the parallel thrust faults.Some of the problems are simplified in the research. For example, only one class of comparable listric faults was compared when discussing the different shapes of listric faults. In the study of the interaction between parallel thrust faults, this theis focuses on the influence of the mutual distances, the variations of the fault geometry and bottom connection modes had not yet been considered. All these require further research.