2D Multi-scale Non-linear Seismic Velocity Imaging And The Research On The 2D Crustal Section Of A'nyemaqen Suture Zone And Its Ajacent Area

In this thesis, the development history of seismic tomography and geophysicalinversion methods is reviewed first. The advantages and defects of the globaloptimization algorithms and local linearized methods are discussed. It is pointed outthat the combination of these two types of methods will make it possible to overcomethe weakness caused by using any single method. Then, the current status of researchon the hybrid inverse methods is introduced. It is put forward that the combination ofgenetic algorithms (GA) and simplex methods will be an efficient and robust strategyof non-linear travel time inverse methods, and then a new seismic tomographymethod is developed. Besides, a brief introduction is given to the actual status ofresearch on crustal structures of Qinghai-Tibet Plateau and its northeastern margin.In Chapter 2, the general description about genetic algorithms and simplexmethods is presented. The basic principle of seismic velocity imaging based on themulti-scale hybrid inverse algorithms is expounded in detail. The forward problem issolved by use of the finite-difference method, and the hybrid method combininggenetic algorithms and simplex algorithms is applied to the inverse problem. Themulti-scale successive approximation strategy is adopted in the inverse process. Theimaging process is that the velocity field is firstly divided into different spatial scales,the velocities on the grids are taken as the inverted parameters, and then the model isparameterized by a bicubic spline function. Genetic algorithms (GA) are used firstlyto search the global optimization in a larger scale. The best individual obtained bygenetic algorithms, which experienced fully evolutions, acts as the initial model forthe simplex methods. And then the simplex methods are used to search rapidly thelocal optimization. The spatial scale will be reduced when the terminal conditions aresatisfied. The process mentioned above is repeated until the terminal conditions aresatisfied. The reason for combining genetic algorithms and simplex methods is that itcan not only reduce the calculation costs, but also avoid falling into the localoptimization area.In Chapter 3, a series of function tests and numerical simulated tests aredescribed. In the function tests, the searching abilities among genetic algorithms,simplex methods and their hybrid algorithms are compared. The advantages and defects of global, local and their hybrid algorithms are discussed in detail. Thegradient velocity, anomalous low velocity, anomalous high velocity, syncline,anticline and steep fault models are involved in the numerical simulated tests. All theresults of numerical simulated tests show that the velocity imaging method based onmulti-scale hybrid inverse algorithms proposed here is efficient. Anti-noise tests arealso done by adding 5% and 10% level random noise to the theoretical traveltimes ofthe anomalous low velocity model. It shows that the 5% level random noise makesalmost no differences to the inverse result and the 10% level random noise makes afew changes to the inverse result. The result of anti-noise tests verify that the velocityimaging method mentioned above is robust.In Chapter 4, the velocity imaging method based on the multi-scale hybridinverse algorithms is applied to research the upper crustal velocity structure inA'nyemaqen suture zone and its adjacent areas. The results obtained here arecompared with the others. The velocity imaging shows that the upper crustal velocitystructure of the eastern part of A'nyemaqen suture zone is characterized by a lowvelocity distribution, with a width of more than 20 km. The velocity structure abovethe depth of 2km is almost homogeneous laterally, but strong heterogeneities ofvelocity structures appear below the depth of 2km. A low velocity zone appearsbetween the stake numbers 260kin and 280kin. The P wave velocities increase rapidlybetween the stake numbers 280km and 290km. A smaller scale low velocity zoneappears between the stake numbers 290km and 300km, and the velocities increaserapidly again between the stake numbers 300km and 310km. The P wave velocitydistribution in the West Qinling fold zone to the north of the stake number 310km canbe divided into two parts depending on their velocity features. The P wave velocitiesare higher in the area to the south of the stake number 340km as compared with thevelocities in the area to the north. The Hoh Sai Lake-Maqin fault goes through theprofile at the stake number 283km, where the P wave velocities change severely andthe depths of the basement vary suddenly as well. The P wave velocity obviouslyvaries laterally between the stake numbers 320km and 330km, where the depth of thebasement varies in the range between 2.2km and 4.5km, and Wudu-Tewo fault goesthrough here. The velocity transition boundary between low and high velocities is located near the stake number 340km., where Zhouqu-Liangdang fault goes through.In Chapter 5, the 2D crustal section of A'nyemaqen suture zone and its adjacentarea is constructed by use of Rayinvr software package written by Zelt. Meanwhile,the skills for using the package are discussed. Rayinvr software package can deal withhead wave data, and that is its major distinguishing characteristic. The Pg waves canbe regarded as turning waves or head waves penetrating the upper crust, and thus itcan be used to reconstruct both of velocity distributions of the upper crust andtopography of the basement interfaces. The results reveal that the basement inA'nyemaqen suture zone is concave dramatically. Its maximum depth is about 5.47kmin it. As regards the both sides of A'nyemaqen suture zone, the basement in WestQinling fold zone is shallower than in Songpan-Ganzi geological block. The depth ofthe basement interface in Songpan-Ganzi block is about 3.5km. The concave featureof the basement interface also appears in Songpan-Ganzi block, and the depth of thebasement is about 4.0km between the stake numbers 170km and 250km. In thetransition zone between A'nyemaqen suture zone and West Qinling fold zone, thebasement shallows rapidly up to a depth of 1.8km, then deepens northwards up to4.7km, and finally becomes flat afterwards. Different inverse schemes and parametersare tested in order to compare their effects, when Rayinvr software package is used. Itis found that the simultaneous inversion of velocities and depths with suitableparameters is better than their individual inversion.The following are some major results about the 2D crustal section ofA'nyemaqen suture zone and its adjacent area. There are low velocity distributions inthe middle and lower crust in the depth range from 20km to 45km in the easternsegment of A'nyemaqen suture zone. The decrease of velocity values is about0.2～0.3km/s in A'nyemaqen suture zone and the scale of the low velocity distributionsreduces with depths. Obvious discrepancies of velocity structures exist between thesouthem and northem parts of A'nyemaqen suture zone: a more complicated structureis found in the south as compared to the north. Thickness of the crust varies very littlealong the profile, with an approximate value of about 48～51km, though a slightincrease appears in A'nyemaqen suture zone. Imaging of the crustal section shows thatthe lower crust of Songpan-Ganzi block dives under West QinLing fold zone. Geodynamic implications revealed by the results can be summed up as follows: thatstrike-slip and horizontal dislocating tectonic is the major tectonic background in thisarea and substance of lower crest tends to flow laterally. Finally, the results aresummarized in Chapter 6.