Teleseismic Full Waveform Inversion Method And Its Application In Southwestern Fujian Province

The southwestern Fujian Province is rich in mineral resources and diverse geological tectonic phenomena.It has always been a hot spot in geoscience research.Over the years,a large number of geologists have conducted in-depth research on the study area,but there is still insufficient understanding of the deep underground structure of the study area.Considering the actual observation conditions in the study area,this study used the teleseismic full waveform inversion method to invert the fine crustal velocity structure in the southwestern Fujian Province.In this study,the SEM-FK hybrid method is used to simulate the teleseismic waveforms which are simplified as plane-waves in a complicated model.Since the recorded seismograms are affected by both the source(or the source-time function)and the structural variations,we have modified the method to make it suitable for the real data more conveniently.The validity and reliability of the method are verified by model tests,which lay the foundation for the application of the full waveform inversion method.Then,29 broadband seismic stations are arranged in the study area to collect the teleseismic waveform data.According to the actual waveform situation and the model test results,an appropriate inversion strategy is selected.The frequency of the teleseismic waveform signal used in the inversion is as high as 1 HZ,and the misfit function used is the relative traveltime residual of teleseismic direct P.We used both the 2D and 3D full waveform inversion method to obtain the crustal velocity structure in the southwestern Fujian Province.Finally,by analyzing the misfit function and the difference between the theoretical waveform and the actual waveform,comparing the 2D and 3D inversion models and combining with the existing research results,the rationality of the inversion model is demonstrated.In the inversion model,there exists an obvious lower velocity zone(LVZ)in the crust beneath the east side of the ZDF.Combing with the seismicity,we proposed that the ZDF might extend downward into the lower crust and is still active at present,and more fluids(water)might exist in the crust on the east side of the ZDF than that on the west side.According to previous research results,we propose that there was magmatism during the late Jurassic and early Cretaceous in the study area,which is generated in an extensional environment responding to the slab rollback and concomitant retreating arc system of the paleo-Pacific plate.The large-scale late Jurassic and early Cretaceous magmatism formatted magma chambers and channels beneath the ZDF and is considered as one of the reasons for the abundant metal mineral resources in the study area.In the Cenozoic,active subductions of the Pacific and Philippine plates took place in East Asia.The subducted plates have driven the lower mantle material to arise and reheat the Mesozoic magma chambers and channels.