Study On Seismic Wave Travel Time Calculation And Earthquake Location Method For Three-Dimensional Anisotropic Media

Anisotropy is common within the Earth.In recent decades,with the massive increase of data from stations and the construction of mobile stations,the improvement of seismological methods and the development of computer technology,scientists have studied the internal structure of the Earth in a more detailed manner.The study of the earth's anisotropy is a vital part.Previous observations of earthquakes' traveltime,ray paths,and waveforms were mostly based on isotropic assumptions,although previous laboratory results have shown that most of the Earth's rocks are weakly anisotropic.However,the superposition effect of the seismic wave along a long path will enhance the influence of anisotropy.In this case,ignoring the influence of anisotropy will cause a certain degree of error in the study of the internal structure of the Earth and the earthquake location.This paper first introduces the significance of the efficient traveltime calculation method and earthquake location method in anisotropic media,and then summarizes the common methods and research progress of traveltime calculation and earthquake location of anisotropic media around the world.As for the seismic wave velocity for a given ray angle of a transversely isotropic medium,we no longer use the group velocity approximation formula based on the weak anisotropy hypothesis,but accurately determine the seismic wave group velocity of a given ray angle by the dichotomy,then precisely calculate traveltime and ray paths.Then we focus on the algorithm principle of the minimum travel time tree method,and because of its robustness,it is applied to the travel time calculation in three-dimensional anisotropic media.In order to quickly calculate the seismic wave travel time without reducing the accuracy,we improved the minimum travel time ray tracing method for the wavelet propagation region to dynamically adjust with the seismic wave propagation,and as for the most time-consuming wavelet propagation travel time calculation part of the algorithm,using multi-core multi-thread parallelization processing.In addition,the wavefront points are managed using a more efficient minimum heap sorting to further reduce the amount of computation.The uniform and complex anisotropic models are used to test respectively.The examples show that the method can quickly and accurately calculate the seismic wave travel time and ray path in the three-dimensional complex transversely isotropic medium with arbitrary tilt of the symmetry axis.Compared with 4 cores and 8 threads,the calculation speed can be increased by 6 times.Then we apply the improved minimum travel time travel time calculation method to the seismic location of anisotropic media,select the more robust intersection seismic location method,and perform seismic localization in the model of complex anisotropic media.The results show that the seismic location results are accurate when the velocity model and arrival time are accurate;when there is disturbance at the traveltime,the positioning results are biased.At the same time,the numerical model localization experiments show that the source trajectory of P wave to time difference constraint of different stations is not very constrained to the source location,especially when the earthquake is outside the station network;the time difference constrained source with different types of waves of the same station The trajectory can significantly enhance the constraints on the source location,especially the source depth.