Study Of A New Receiver Function Migration Method And Its Application To The Southeastern Tibetan Plateau

Seismic migration is a popular research topic in seismology and is widely used in the studies of seismic discontinuities in the Earth’s interior.It relocates seismic wave phases to their locations of scattering and creates accurate images of seismic discontinuities in depth.Thus,this technique is effective in imaging subsurface discontinuities with complex geological structures,such as faults,folds,and heterogeneities.And when seismic migration is joined with the receiver function(RF)technique,passive imaging based on seismic array records can be achieved in regions lacking seismic reflection surveys.In recent years,the deployment of many broadband dense seismic arrays has provided opportunities for the application of seismic passive imaging,making the development of new methods joining seismic migration and RF technique a particularly valuable topic.The southeastern(SE)Tibetan Plateau,located in the southern part of the North-South seismic zone,has formed complex geological structures,along with many seismic faults and resultant seismic activities since the collision of the Indo-China block,under the combined influence of plate extrusion,internal deformation and rotation along the Himalaya Syntaxis.Imaging the crustal structure of the SE Tibetan Plateau is important for understanding the deformation mechanism and materials migration of the Tibetan Plateau.Also,the high-density coverage of seismic arrays makes the SE Tibetan Plateau an ideal place for seismic migration experiments.Therefore,developing a new 3-D imaging method joining seismic migration and receiver function technique and applying it to the SE Tibetan Plateau is a very meaningful subject.In this thesis,we propose a three-dimensional(3-D)frequency-domain post-stack adaptive receiver function migration(ARFM)method to image subsurface discontinuities by employing normal moveout corrected and bin-stacked RFs as the data sources.In the ARFM,which is based on the concept of an exploding convertor,stacked RFs are time-reversely propagated from the surface to 3-D space,and the conversion interfaces are determined to be the energy in the migrated RF stacks at t=0.A 3-D frequency-domain screen propagator based on Pade approximant of order[1/1]is developed for backward wave propagation,and an adaptive approach is designed for normal moveout correction,bin-stacking and migration to ensure consistency between the internal discontinuities employed for moveout correction and those obtained from imaging.Numerical experiments demonstrate the validity and effectiveness of the method in retrieving various 3-D features of crustal discontinuities,including the depth,gradient transition thickness and S-velocity contrast of the discontinuities.We also discuss the capability of the ARFM for imaging in the presence of strong shallow heterogeneities,the effects of inter-receiver spacing,RF frequency and seismic noise on the ARFM imaging results,and the balance between noise suppression and lateral image resolution when selecting the appropriate bin size for stacking.In the data application,we apply the ARFM method in the imaging of subsurface discontinuities beneath the SE Tibetan Plateau.By employing 249200 high-quality RFs,we construct a 3-D image of crustal discontinuities in a region of 1000 km*1000 km in approximately east-west and north-south directions,with a theoretical lateral resolution of 10.0-21.9 km and a vertical resolution of 1.0-2.6 km.We design a simple circular searching algorithm to extract Moho depth information from ARFM images and construct a preliminary 2-D Moho depth model beneath the SE Tibetan Plateau.Based on numerical experiments of multiple median-filtered Moho models,we obtain the empirical ARFM imaging resolution and construct an optimal Moho depth model in which most features can be recovered by ARFM based on the practical RF distribution.The migrated cross-sectional images along with the optimal Moho depth model reveal various general Moho features beneath the major geological units,with eastward gradual depth decreases below the Sichuan Basin and the South China Block,both westward gentle shallowing and southward significant shallowing beneath the eastern Yunnan-Burma Block,southward rapid depth decrease from 65 km to 40 km beneath the South Sichuan-Yunnan rhombic block along with multiple step Moho features,and undulating features of deep east-west and shallow middle along with depth fluctuations of 20 km beneath the North Sichuan-Yunnan block.The imaged Moho discontinuity exhibits depth dislocations of～10 km across the Xiaojiang fault and the Red River fault,suggesting that these two major faults may extend deep into the mantle.Migration images also reveal Moho depth dislocations beneath the South SichuanYunnan block trending along the northeast-to-southwest direction,indicating the presence of a large-scale(>50 km)strong local undulating structure of Moho,which can be a left-lateral deeply penetrating blind fracture.Such structure accords with multiple results from seismic studies and geodynamic observations and can explain the mid-and lower-crustal seismicity that is not accounted for by the near-surface local faults.