Local and global optimization methods are widely used in geophysical inversion but each has its own advantages and disadvantages. The combination of the two methods will make it possible to overcome their weaknesses. Based on the simulated annealing genetic algorithm （SAGA） and the simplex algorithm, an efficient and robust 2-D nonlinear method for seismic travel-time inversion is presented in this paper. First we do a global search over a large range by SAGA and then do a rapid local search using the simplex method. A multi-scale tomography method is adopted in order to reduce non-uniqueness. The velocity field is divided into different spatial scales and velocities at the grid nodes are taken as unknown parameters. The model is parameterized by a bi-cubic spline function. The finite-difference method is used to solve the forward problem while the hybrid method combining multi-scale SAGA and simplex algorithms is applied to the inverse problem. The algorithm has been applied to a numerical test and a travel-time perturbation test using an anomalous low-velocity body. For a practical example, it is used in the study of upper crustal velocity structure of the A＇nyemaqen suture zone at the north-east edge of the Qinghai-Tibet Plateau. The model test and practical application both prove that the method is effective and robust.
In 2010,a 500-km-long wide-angle reflection/refraction seismic profile was completed,running northwest from the central Sichuan Basin.This profile orthogonally crosses the meizoseismal area of great Wenchuan earthquake of 12 May 2008,which occurred in the central part of the Longmenshan.The profile also passes through the northwestern Sichuan Plateau,along which a new deep seismic sounding observation system was set up that was much improved over previous datasets and enabled abundant observations to be recorded.Seismic wave phase records that reflect the structural characteristics of different tectonic blocks,especially the complicated phase features associated with the Wenchuan earthquake,were calculated and analyzed in detail.A 2D crustal P-wave velocity model for the orogenic belt in the central Longmenshan and its margins was determined,and crustal structure differences between the stable Sichuan Basin and the thickened northwestern Sichuan Plateau were characterized.Lithological variations within the upper and lower crust in the interior of the plateau,especially a great velocity decrease and plastic rheological properties associated with strong lithologic weakening in lower crust,were detected.From west to east in the lower crust beneath the orogenic belt lying between the Sichuan Basin and the northwestern Sichuan Plateau,a giant shovel-like upwelling is observed that dips gently in the lower part and at higher angles in the upper part;this is inferred to be related to the fault systems in the central Longmenshan.An upwelling in the upper-middle crust along the eastern margin of the orogenic belt is associated with steeply dipping thrusts that strongly uplift the upper crust and crystalline basement beneath a central fault system in the Longmenshan.The data,combined with an understanding of the regional tectonic stress field and previous geological results,enable a discussion of basin-and-range coupling,orogenic tectonics,the crustal fault system,and the seismogenic tectonic environment of the central Lon
This paper uses deep seismic sounding （DSS） data to contrast and analyze the crustal structures of three plateau basins （Songpan-Garze, Qaidam, Longzhong） in the northeastern margin of the Qinghai-Xizang （Tibetan） plateau, as well as two stable cratonic basins （Ordos, Sichuan） in its peripheral areas. Plateau basin crustal structures, lithological variations and crustal thickening mechanisms were investigated. The results show that, compared to the peripheral stable cratonic basins, the crystalline crusts of plateau basins in the northeastern margin are up to 10 15 km thicker, and the relative medium velocity difference is about 5% less. The medium velocity change in crustal layers of plateau basin indicates that the upper crust undergoes brittle deformation, whereas the lower crust deforms plastically with low velocity. The middle crust shows a brittle-to-plastic transition zone in this region. Thickening in the lower crust （about 5 10 km）, and rheological characteristics that show low- medium velocity （relatively reduced by 7%）, suggest that crustal thickening mainly takes place in lower crust in the northeastern margin of the Tibetan plateau. The crust along the northeastern margin shows evidence of wholesale block movement, and crustal shortening and thickening seem to be the main deformation features of this region. The GPS data show that the block motion modes and crustal thickening in the Tibetan plateau is closely related to the peripheral tectonic stress field and motion direction of the Indian plate. The Mani-Yushu- Xianshuihe fold belt along the boundary between the Qiangtang block and the Bayan Har block divides the different plateau thickening tectonic environments into the middle-western plateau, the northeastern margin and the southeastern plateau.
