Assessing The Seismic Hazards Of Active Faults Using Coulomb Failure Criterion

The seismic hazard assessments to the active faults are important tasks to ensure the safety of people lives and property and promote social stability and sustainable economic development. Additionally it is an academic challenge, which has not been solved, in the earth science. The primary work is to evaluate the possibilities and hazards of destructive earthquakes on active faults in a time span. The results of assessments are valuable to determine the seismic fortification levels of structures and buildings, formulate the urban planning and mitigate the earthquake disasters.In this thesis, a new method using the rock failure criteria is proposed to assess the seismic hazards of active faults. Based on the physical mechanisms of earthquake preparation and occurrence, it is supposed that the stress accumulation on the active faults is mainly from the plate tectonic motion and the geological activities such as earthquakes and volcanoes during the seismogenic process. Consequently, the main contents of this new method can be divided into:(1) calculating the tectonic stressing rates of active faults,(2) calculating the stress disturbance from the historical geological activities to the active faults,(3) assessing the stress disturbance from the potential geological activities to the active faults. According to this method, a new Active Fault Earthquake Risk Evaluating System (AFERES) is developed.With the AFERES, the tectonic stressing rates of the main faults in the Tibetan plateau are inverted using GPS data. Two end-member crust motion and deformation hypotheses, continuum and micro-plate, of the Tibetan plateau are adopted. The least-square collocation method and the elastic block model are used respectively for these two models. The results show the tectonic stressing rates of the faults in the eastern Tibetan plateau than in the western. The stressing rates of the Xianshuihe fault system (including the Ganzi-Yushu fault, Xianshuihe fault, Anninghe fault and Xiaojiang fault) are higher than the other faults. on the majority of active faults is～4kPa/yr. The average level is5-9kPa/yroThe stress disturbances from2008Wenchuan earthquake and2010Yushu earthquake to their adjacent active faults are obtained. For the Yushu event, a refined rupture trace of the causative fault is extracted from two InSAR coseismic interferograms and the field investigation results. A new method to discretize the fault geometry using Triangular Dislocation Elements (TDEs), which can keep a good consistency with the fault geometry modelled with Rectangular Dislocation Elements (RDEs) and avoid the dislocation gaps and overlaps, is presented. Consequently the comprehensive comparisons between RDE and TDE models can be implemented. Due to the classic Laplacian operator minimizes the slip on the boundary RDEs of the fault, a modification to the classic Laplacian operator is accomplished to get more reasonable RDE models. The inversion and comparison results show that two larger slip concentration patches occurred in the Yushu earthquake. The largest locates southeast of the hypocentre near Luorongda country with the maximum slip of～1.6m at the surface. The second largest slip patches are in the middle of the causative fault near the hypocentre. The magnitude of this event is Mw6.76-6.81. Furthermore, the the analytic expressions for the strains associated with angular dislocations in an elastic half-space are re-derived. The computing efficiency increases by～13times than the code presented by Meade, which is mainly attributed to that the terms frequently used in the formulae are extracted.The Xianshuihe fault is selected as an object to be assessed by the method presented in this paper. The magnitudes and recurrence intervals of the characteristic earthquakes on five segments of the Xianshuihe fault have been analysed. The results show the magnitude is7.0～7.5. The recurrence intervals of the Kangding-Shimian and Qianning-Kangding segments are both over200years. The recurrence interval of the Zhuwo-Luhuo segment is～150years. The recurrence intervals of the Luhuo-Daofu and Daofu-Qianning segments are both～100years. The calculated tectonic stress accumulation amount show the value of the Qianning-Kangding segment is the highest, about7.46bar for the time span from the last characteristic earthquake to the next50years and10.95bar for the time span from the last characteristic earthquake to the next100years。 The second highest is the Zhuwo-Luhuo segment, whose maximum values is4.82bar and7.53bar respectively for the above two time span. The stress accumulation of the Kangding-Shimian segment is the lowest, about equal to half the value of the Qianning-Kangding segment.The2008Wenchuan earthquake has an acceleration effect on the potential earthquakes of the Qianning-Kangding and Daofu-Qianning segment of the Xianshuihe fault. The maximum acceleration approximates4years. On the Kangding-Shimian segment, there is an outstanding delay effect. The maximum amount reaches7years. However, for the recurrence interval long for several hundred years, these influences are not obvious. The effect of the2010Yushu earthquake on the Xianshuihe fault can be omitted. In the next100years, the potential earthquakes on the Ganzi-Yushu fault, Longmenshan fault and Daliangshan fault have little effect on the recurrences of the characteristic earthquakes of the Xianshuihe fault. The potential earthquakes on the Anninghe fault have great effect on the recurrences of the characteristic earthquakes of the Xianshuihe fault. According to the stress drop of1MPa, the recurrence times of the potential earthquakes on the Zhuwo-Luhuo, Luhuo-Daofu, Daofu-Qianning, Qianning-Kangding and Kangding-Shimian segments are respectively in AD2158,2194,2143,2096and2243.Finally, the aftershock regional seismic hazard assessments to the Wenchuan earthquake and Yushu earthquake are carried out. The results show the background seismic activity rate in the Yushu region is higher than the Longmenshan region. The aftershock duration time of the Wenchuan earthquake is～230years longer than the Yushu earthquake. The regions of the seismic activity increased mainly locate in～100km wide zone along the causative fault of the Wenchuan earthquake. The width for the Yushu earthquake is～10km.