Seismic P-wave Travel-time Tomography In Heterogeneous And Anisotropic Media

Continental dynamics has become one of the frontier fields in present earth sciences. Continental dynamics involved in a wide range of issues, while the key problems of continental dynamics are the tectonic deformation and its mechanisms. Seismic anisotropy is a useful indicator of geodynamic processes. Results of seismic anisotropy can be used to infer the mode of the upper mantle convection, and help us understand the stress regime and evolution of the Earth.Seismic tomography is the major method for studying the three-dimensional (3-D) velocity structure of the Earth, especially the structure of the crust and upper mantle. With the progress of seismic observational techniques, data accumulation as well as the development of computing technology, it is possible to invert anisotropic parameters using travel time data. It has great significance using seismic tomography methods to simultaneously invert for both isotropic and anisotropic structures for the understanding of Earth's structure and dynamic processes.In this work, in light of some problems in the theoretical and applied investigation of anisotropic tomography method, the approximation expressions of qP velocity perturbation under the condition of weak anisotropy have been discussed systematically, and further the anisotropic P-wave travel-time tomography algorithm has been developed. The method was numerically tested by many different theoretical models, and the application conditions of the method were discussed. Finally, the method was successfully applied to study the Chinese Tianshan orogenic belt and Longmenshan area.This dissertation consists of the following four parts:ⅠTheory:Backus (1970) has proved that any fourth-order tensor can be uniquely represented as a linear combination of 21 canonical harmonic tensors. Based on Backus's theory, I have reproduced the expression of Cartesian components of the canonical harmonic tensors and the coefficients of qP velocity perturbation expansion, and corrected the misprints in the literature of Smith & Dahlen (1973) and Bokelmann (2002). According to qP velocity of the harmonic perturbation expansion formula, the reduction formula for different observation systems and different anisotropic media is derived. Based on the Bond rotation theory, the change rule of elements of elastic tensor is proved. It proves that the azimuthal anisotropy of orthogonal symmetry with vertical axis can be described by only 4 parameters. For any anisotropy case, only six parameters are sufficient to describe the three-dimensional anisotropy.ⅡComputational algorithmBased on the theory of harmonic decomposition of elastic tensor under the conditions of weak anisotropy and the seismic tomography method, the anisotropic seismic-tomography algorithm is developed. The algorithm uses a uniform grid nodes and cubic B-spline interpolation function to describe the isotropic velocity field, and uses blocks to describe the anisotropic velocity perturbation field. This kind of velocity parameterization method has several advantages: a) The velocity model is smooth, without adding an additional smooth term in the inversion; b) The resolutions of the isotropic and anisotropic structures can be considered independently, which can reduce the coupling between effects of heterogeneity and anisotropy. By comparison of the various methods of seismic travel time calculation, I construct a fast marching method in isotropic media and anisotropic fast marching method for a weakly anisotropic media. The travel-time calculation algorithm adopts the reciprocity theorem to reduce the cost of forward calculation. First of all, the travel-time field was calculated by taking the locations of seismic stations as fictitious sources, and then by calculating the ray path between stations and local earthquakes or intersection between the teleseismic ray path and model boundary. Such a calculation procedure can significantly reduce the cost of forward calculation, especially for the observation system of multi-sources and multi-receivers. In inversion, the Tarantola (1982, 1987) non-linear inversion method was adopted, and the damping factor for heterogeneous and anisotropic model parameters can be set up separately.ⅢNumerical testsOur numerical tests for different theoretical models show the following results. a) The calculation accuracy of the isotropic and weak anisotropic fast marching algorithm proposed in this work meets the requirements of the seismic travel-time tomography (the calculation error is less than the data picking error). b) The magnitudes of anisotropy and heterogeneity are coupled. c) The direction of anisotropy and the heterogeneous structure can be decoupled. d) The P-wave travel time method can invert for multi-layered anisotropy when the travel-time data have good azimuth coverage. e) For the teleseismic inversion, the anisotropy parameters can be well inverted with azimuth coverage more then 180°. f) The inversion for the direction of anisotropy in the three-dimensional media needs an epicenter distance coverage great than 50°.ⅣApplications1. Using the P-wave travel time data recorded by the passive seismic array across the Chinese Tianshan and regional seismic network, P-wave velocity structure of the crust and upper mantle down to 400 km depth along the Kuqa-Kuytun profile is determined by using the seismic tomography technique of Zhao et al. (1992, 1994). Based on the obtained isotropic structure, we further explain the remaining travel time residuals and obtain the fast P-wave direction along the profile using the P-wave velocity perturbation formula in weak anisotropic media and the linear inversion method. The results show the following features.a) Along the Kuqa-Kuytun profile, the crust of the Chinese Tianshan has an obviously blocked structure, and the crust beneath the south and north Tianshan is subjected to an obvious lateral distortion. This manifests that the Tarim basin exerts the strong lateral compression on the Tianshan crust. There are high-velocity anomalies with thickness of 60-90 km on the top of the upper-mantle beneath the Tarim and Junggar basin. The high-velocity anomaly beneath the Tarim and south Tianshan is obviously distorted. The high-velocity anomaly beneath the Junggar and North Tianshan has thrusted down to the depth of 300 km underneath the south side of the central Tianshan. Both of them form an asymmetric bilateral thrust. In the depth range of 150-400 km beneath the Tarim and Junggar basin, there exist low-velocity anomalies. Among them, the low-velocity anomaly beneath the Tarim block upwells below the south Tianshan. At the depth of 200-300 km beneath the Tarim and South Tianshan, there exists a high-velocity anomaly, which could be the lithospheric detachment of the Tarim caused by the upwelling of the upper-mantle's hot material.b) The subduction of the Tarim block could involve the whole lithosphere, but its front is limited merely to the northern border of the South Tianshan. The far-field effect caused by the Tibet uplift could not only drive the subduction of the Tarim block, but also cause the upwelling of the upper mantle beneath the southern side of the Tianshan. The widespread low-velocity anomalies within the Tianshan upper mantle should facilitate the mantle deformation. The strong heterogeneity in the upper-mantle should be in favor of the small-scale mantle convection. In view of the velocity structure of the crust and upper mantle beneath the Tianshan, it could be inferred that the rapid uplift of the Tianshan since the Neogene is caused mainly by the far-field effect due to the uplift of the Tibetan plateau, and the relatively weak upper mantle beneath the Tianshan provides an essential condition for prompting the uplift and deformation of the Tianshan orogen.c) The fast P-wave direction is approximately in north-south in the Tarim Basin and South Tianshan, which accords with the Tarim block extrusion to the Tianshan. In the central Tianshan fold belt and North Tianshan, the fast P-wave direction turns gradually to NW-SE, and it is nearly in east-west in the North Tianshan fold belt and the front edge of the Junggar Basin. On the whole, the P-wave fast axis is parallel to the strike of the Tianshan orogenic belt, and perpendicular to the direction of relative movement of the plate. The crust and upper mantle have vertically coherence deformation in the Tarim Basin and South Tianshan, and decoupling in the North Tianshan and Junggar Basin because of the small-scale convection. These results suggest that the Tianshan Mountains was extruded by the two basins, the upper mantle materials have migrated along the direction parallel to the mountain, which can be described by a dynamic model something like the overpass. Based on the crust and upper mantle velocity structure and anisotropy directions, we infer that the upper mantle convection plays an important role in the process of the deformation and uplift in the Tianshan orogenic belt.2. The 3-D P-wave velocity structure of the crust and upper mantle down to 400 km depth and fast P-wave directions in the upper mantle are determined by applying our anisotropic tomography inversion technique to teleseismic travel time data recorded by the West Sichuan Seismic Array deployed in the Longmenshan area. The results show the following features.a) The crustal structure of the study area correlates with the surface geological features. The Sichuan Basin is imaged as a high-velocity feature, while the Songpan-Ganzi and Chuandian blocks are imaged as low-velocity features. The lithospheric thickness of the Sichuan basin with high-velocity has lateral variations from 250 km in south to 100 km in north. This high-velocity anomaly may represent the Sichuan Basin's lithosphere. Relative to the Sichuan Basin, the crust and upper-mantle of the Songpan-Ganze block are relatively weak, but the velocity structure shows no evidence for the subduction of the Sichuan basin downward and channel flow. The uplift of the Songpan-Ganze block may be related to the mantle upwelling. The Sichuan basin vertically contacts with the Chuandian block, but the Sichuan basin fronts in the Longmenshan area become thinner from east to west with the feature of the Songpan-Ganzi block incursion into Sichuan basin in the upper-mantle. This feature shows different dynamic mechanisms between the Sichuan basin with the Chuandian block and the Sichuan basin with the Songpan-Ganzi block. The Xianshuihe and Longmenshan faults all cut through the whole crust. The Xianshuihe fault belt exhibits as a broad low-velocity zone.b) The crust of the Longmenshan fault belt and the Sichuan Basin shows high-velocity characteristics, and the boundary between the Sichuan Basin and Songpan-Ganze block may be the Wenchuan-Maoxian fault. The Longmen Shan fault belt was divided into two parts by the extrusion of low-velocity from the Songpan-Ganzi block bounded by Wenchuan: the southern part and the northern part. The two parts are all characterized by high-velocity. The great Wenchuan earthquake (Ms 8.0) and its aftershocks are all distributed in the high-velocity zone of the northern part of the Longmen Shan fault belt. This feature of deep structure may have controlled the occurrence of the Wenchuan mainshock and the distribution of its aftershocks. Based on the crustal structure of the Songpan-Ganzi block, we can infer that the uplift of the Songpan-Ganzi block and thrust nappe of the Longmenshan are a result of the eastward extrusion of the Tibetan Plateau and upper-mantle upwelling. The strength of rock between the two parts of Longmenshan is weak. During the process of long-term deformation, it is possible to accumulate high stress in the rigid crust of the northern part of Lomgmenshan. Wenchuan is located at the southern end of the northern part of Longmenshan, where large stress is easy to build up. These factors may be the reason why the rupture of the Ms 8.0 earthquake initiated there.c) P-wave anisotropy results show that the directions of upper-mantle flow in the Sichuan-Yunnan block are consistent with the GPS results, which means that the crust and upper mantle deformation of the Sichuan-Yunnan block is coupled. The flow direction of the Songpan-Ganze block's upper mantle is in north-east direction, and the GPS results show a nearly east-west direction, suggesting that the deformation of the crust and upper-mantle in the Songpan-Ganze block is decoupled. The fast P-wave direction in the northern Sichuan Basin is accordance with the GPS results, but it is parallel to the tectonic line and perpendicular with SKS results in the southern part of the Suchuan Basin. Therefore there are different deformation modes of the crust and upper-mantle in the northern and southern parts of the Sichuan Basin, which are consistent with the velocity structure we determined.d) The results of this work do not support the dynamic model of subduction of the Sichuan Basin beneath the Songpan-Ganze block. In contrast, the flow of upper-mantle plays an important role in the evolution process of this area. In the upper-mantle, the material of the Songpan-Ganzi block may have invaded into the upper-mantle of the Sichuan Basin.