3D S-wave Velocity Sturcture Imaging Of The European Continent Using Multimodal Surface Waves Extracted From Ambient Noise

Surface waves propagate along the interface of different media and have broad applications in various fields,such as semiconductor,civil engineering,new energy field and geophysics.In seismology,by studying the dispersion characteristics of surface waves,we can obtain rich information about the internal structure of the earth.The classical way of obtaining surface wave dispersion curves based on seismic events is often limited by the number of events and stations,and it is difficult to obtain short-period dispersion curves.Benefiting from the development of the ambient noise method,the utilization rate of station data and the frequency band range of dispersion curves have been significantly improved.However,it is difficult to extract high-quality higher-mode surface wave dispersion curves from ambient noise data by classical methods.The newly developed frequency-Bessel transform(F-J)method can extract high-quality multimodal surface wave dispersion curves from ambient noise data.To improve the imaging effect and efficiency,we have summarized and proposed "The identification and inversion workflow based on the fundamental-mode constraint",which can effectively identify dispersion curves and improve inversion efficiency by using fundamental-mode information.By conducting the one-dimensional shear wave velocity structure inversion on two arrays in the northwestern bohemian massif,we found that the addition of higher-mode dispersion curves not only directly and significantly improved the inversion results in the depth range of the sensitivity function but also indirectly improved the results of the entire inversion depth.Furthermore,the previous shear wave velocity models in this area are mainly obtained from the fundamental-mode dispersion curves.By analyzing the existing velocity models in this area,it is found that the velocity models based on phase velocity dispersion curves perform better fitting with the observed fundamentalmode dispersion curves than those using group velocity dispersion curves.However,the theoretical dispersion curves of these models still perform bad fitting at higher modes.With more than 2000 seismic stations distributed in the European continent,we utilized the F-J method and moving array technique to extract multimodal Rayleigh wave dispersion curves from each subarray and inverted them to obtain the one-dimensional shear wave velocity structure beneath the subarray.Subsequently,we constructed a three-dimensional shear wave velocity model of the European continent with the Kriging interpolation method and validated the model using full waveform simulation,which shows good performance in terms of travel time delays and waveform fitting.Based on the three-dimensional shear wave velocity model of the European continent inverted by the multimodal surface wave dispersion curves,we found that there exists a widespread mid-crustal low-velocity zone(LVZ)beneath the tectonic active European continent,with a depth range of about 10-20km,and the detachment of midcrust LVZ was observed in some regions.The causes of the mid-crustal LVZ may be related to fluid activity in the crust,partial melting of the mid-crustal material under temperature and pressure conditions,or the remnants of ancient structures.There are two different views on the formation of the Eifel volcanic fields.Some researchers believe that it is caused by the mantle plume beneath the area,while others think the Eifel mantle plume hypothesis is still flawed,for example,there is a lack of support from evidence such as mantle plume head and the temporal pattern of volcanic activity,it is caused by the factors such as lithospheric extension and orogeny.Based on the model,we observed the mantle plume-like(or hotspot-like)structures beneath the Eifel volcanic fields and its adjacent European Cenozoic Rift System,and the two are interconnected.In addition,we also observed a phenomenon similar to the spreading of plume heads in the upper mantle beneath the region.We speculate that a large amount of hot mantle material carried by the mantle plume(or hotspot)beneath the Eifel volcanic fields and the European Cenozoic Rift System was transported around the Alps by the counter-clockwise rotation of Italy,which may be an important reason for the large-scale mantle LVZ around the Alps.In the study of lithospheric structure imaging of the European continent,we used the frequency-Bessel transform method to extract a large number of high-quality highermode surface wave dispersion curves from the ambient noise data of the European continent for the first time.On the basis of fundamental mode dispersion curves,adding higher-mode dispersion curves significantly enhances the constraints on imaging the lithospheric structure of the European continent.Our model has a higher vertical resolution than that using the traditional surface wave method.The three-dimensional shear wave velocity model of the European continent and the related findings can provide new insights into the lithospheric tectonics of the European continental,continental dynamics,and plate motion.