Study Of The Focal Mechanism Complexity Of Large Earthquake Under The Constraint Of High-frequency Source

The complexity of seismic source is reflected in many aspects,such as the complexity of fault geometry,the complexity of fault surface slip distribution,the complexity of seismic energy changes with time and the complexity of fault dislocation mode,and so on.According to the displacement representation theorem described by the moment tensor,the complexity of seismic source can be described by the moment tensor distribution of discrete point sources inside the source body,and the moment tensor of each point source can also have independent source time function.However,the limited observational data do not allow us to invert all the parameters of such a source simultaneously.Therefore,the inversion of finite fault is limited by the inversion of slip size and slip angle on one or several plane faults,and the inversion of focal mechanism with time-varying space can only be carried out under certain constraint conditions.The generalized array technique can obtain the track of high frequency source through the backprojection of the far-field array seismic record without knowing the fault,which is undoubtedly a good constraint to the spatial distribution of the fault.The moment centroid tensor inversion technology can determine the moment centroid and source mechanism information with the help of seismic records of far-field network.It can be seen that in the spatiotemporal complexity inversion of source mechanism,the trajectory of high frequency source is taken as a constraint,which substantially increases the constraint from observation and reduces the artificial hypothesis,and the inversion results are bound to be closer to the reality.Based on the above considerations,this study includes three key points: first,the improvement and expansion of generalized array technology;second,the centroid moment tensor inversion by multi-seismic phase;third,the moment tensor inversion method and its practical application for complex seismic sources with the high-frequency source trajectory obtained by array technology as constraints.The "warm-up" and promotion of the first two points are the foundation and guarantee to achieve the third goal.In the improvement and extension of generalized array technology,gaussian function is used to optimize the normally elliptic array response into a circle first,so that the array resolution in different directions can be balanced,and the distortion of projection results can be avoided to a large extent.Then,the array technology is extended to the use of S-wave and PKIKP wave,which expands the perspective of high frequency source and greatly increases the "perspective" distance of the array technology.At the same time,the corresponding software package was completed and instructions for use were written(Appendix A).Finally,the software package was used to analyze the 2018 Alaskan Mw7.9 earthquake,test the weighted effect,analyze the 2013 Mw8.3 deep earthquake in the Sea of Okhotsk,test the S-wave imaging function,analyze the 2012 Sumatra Offshore Mw8.6 earthquake,and test the imaging function of PKIKP wave.In the part of centroid moment tensor inversion.first of all,the technical details are sorted out.Then,the corresponding software package and application instructions(Appendix B)were prepared,taking full account of the different processing technology characteristics of body wave,mantle wave and W-phase.Finally,the Mw8.8 earthquake in Chile in 2010,Mw9.0 earthquake in Japan in 2011,Mw7.9earthquake in the Gulf of Alaska in 2018,Mw8.2 earthquake in Fiji in 2018 and Mw7.7 earthquake in Cuba in 2020 were analyzed by software package inversion,and the methods and software were verified.In the moment tensor inversion method of complex seismic source constrained by high frequency source locus and its application research,the general inversion method and technical route are firstly constructed.This method is suitable not only for continuous source but also for discontinuous source.Then,the inversion analysis function of continuous source was tested by the inversion study of 2001 Kunlun Mw7.8 earthquake and 2010 Chile Mw8.8 earthquake,and the analysis function of discontinuous source was verified by the 2013 South Scotia Ridge Mw7.8 earthquake.To sum up,this study used previous study as the foundation,combed the complexity of earthquake source analysis theory and method,in view of the existing practical problems faced by the generalized array technology improvement and development,and the high frequency source array of imaging technology and thecentroid moment tensor inversion of network technology in combination with a large earthquake focal mechanism is a new method of inversion analysis,and through the actual cases of the inspection.