| The collision between the Indian plate and the Eurasian plate caused the uplift and deformation of the Tibetan Plateau,and the crust underwent intense shortening and thickening,which also led to the lateral escape or extrusion of the blocks or materials within the plateau to its eastern margins.Therefore,the eastern margins of the Tibetan Plateau became important places to study the uplift modes of the plateau and the deep dynamic mechanisms of its eastward expansion.Currently,seismic tomography is one of the most important and effective tools for studying the internal structure of the Earth.Among them,seismic body wave tomography and surface wave tomography are the most mainstream imaging methods.They are widely used in deep structure research at home and abroad,providing important constraints on understanding regional geological structural evolution,seismogenic processes and dynamical mechanisms of strong earthquake.Obtaining high-resolution and refined velocity structures is one of the important development directions in seismology at this stage.It is also the key to promoting the understanding of geological tectonic evolution and its dynamical processes.To this end,this thesis focuses on the imaging of deep velocity structures in the eastern margins of the Tibetan Plateau region,with the following three main aspects of related research:(1)Finite-difference traveltime tomography with reciprocity principleThe finite-difference traveltime tomography method can obtain the initial traveltime field of the Earth’s medium directly from the numerical solution of the eikonal equation.Thus,it can adapt to violent changes in lateral non-uniform velocity fields.However,most finite difference algorithms do not meet the reciprocity principle of traveltime.In view of this problem,we introduced an improved traveltime calculation operator in the finite difference forward calculation and applied the improved method to the imaging of crustal and upper mantle velocity structures in the Qinling orogenic belts.The results show that the new method can basically satisfy the reciprocity principle in traveltime calculation.The new velocity model shows that crustal channel flow cannot exist underneath the entire Qinling orogenic belt.The horizontal compressive stress generated by the collision of the Indian plate and the Eurasian plate on the local low-velocity weak zone,may be the main mechanism leading to the uplift of the northeastern margin of the Tibetan Plateau.The apparent difference in seismic wave velocities beneath the eastern and western Qinling reflects that the eastward extrusion of the Tibetan Plateau may have had different effects on the structure of these two terranes.In addition,this model indicates that the structural features of the eastern Qinling and Weihe graben are relatively stable,while the lowvelocity anomalies at the uppermost mantle on the southwest and southeast margins of the Ordos Plateau suggest that the lithosphere on the southern margin of the Ordos Plateau may be undergoing destruction.(2)The weighted double-difference tomographyHigh-precision seismic wave travel times are fundamental to building reliable velocity model from the travel time tomography.However,it is difficult to quantitatively assess the quality of the manually picked seismic phase arrivals.In this paper,we used PhaseNet,a deep learning method,to automatically detect the first arrival times of the P-and S-wave of 3086 seismic events,and 87,553 high-quality arrivals were obtained.We first proposed a weighting scheme using both of the detection probability value and signal-to-noise(S/N)ratio to evaluate the arrival-time quality and utilized it in the travel-time tomography.This new weighting scheme can effectively reduce the travel-time residuals by 7%.We then adopted this weighted double-difference tomography method to invert the crustal velocity structure of the Anninghe-Xiaojiang fault zone,and the lateral resolution is up to 0.25°.Our new model shows that the velocity structures are strongly correlated with tectonic units and block boundaries,and the imaging of low-Vp,low-Vs and low Vp/Vs in the middle crust of the Ludian-Qiaojia seismic zone implies that hot and weak felsic rocks and maybe fluids exist underneath the seismogenic layer of this area,excluding the possibility of the crustal channel flow.This study demonstrates the feasibility and important scientific application of deep learning methods for travel-time tomography in dense seismic array observations,and the weighted double-difference tomography proved to be an effective way to achieve more accurate velocity structures.(3)Joint inversion of body waves and surface wavesBy comprehensively utilizing the advantages of body wave and surface wave observations and the complementary sensitivity of the two data to structures,we can obtain higher resolution three-dimensional P-and S-wave velocity structures of the crust and mantle in the study area.Based on the body-wave phase arrivals automatically picked by PhaseNet and the Rayleigh-wave phase velocity dispersion data extracted by cross-correlation methods of ambient noise,we used the joint inversion method of body waves and surface waves to obtain the crustal velocity structure in the AnningheXiaojiang Fault Zone area.The results show that the joint inversion method can fit both data sets well.Compared with separate body wave or surface wave inversion,joint inversion can better constrain P-wave and S-wave velocity structures,especially improving the imaging resolution of the middle and lower crustal velocity structures in the study area.The velocity structures obtained by the joint inversion are generally consistent with the results of separate body wave and surface wave inversion.The inversion results of these three different data types can corroborate each other.Meanwhile,it further confirms the feasibility of AI-based traveltime tomography in this study.The results of Vp,Vs and Vp/Vs show that the high-velocity block in the Shimain-Panzhihua area separates the low-velocity bodies on both sides,indicating that these low-velocity bodies are two independent anomalies.The low-velocity body on the east side presents obvious low Vp/Vs at the mid-crustal depth,further supporting that crustal channel flow may not exist in this area. |
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