Surface Wave Tomography From Earthquakes In The South-central Mongolia And Northeastern Tibetan Plateau

The northeastern Tibet and Mongolia Plateau are effected by collision of Eurasian and Indian Plates in the near field and remote field respectively.It is essential to further research the two areas for better understanding the deformation,tectonic movement and the charging boundary of the collision. This thesis is devoted to surface wave tomography in the south-central Mongolia and NE Tibet based on observations of seismic arrays.1. South-central MongoliaMongolia plateau is the interior of Central Asian Orogenic Belt(CAOB) and one of the key regions to study the deformation of continental lithosphere. It is poorly understood because of inadequacy of seismic data. In this study, we collected seismic waveform from69broadband seismic stations deployed in the south-central Mongolia. The fundamental mode Rayleigh-wave and Love-wave phase velocity dispersion along7181and901two-station paths, respectively, by the wavelet transformation method. These dispersion data were utilized to construct the first2-D phase velocity maps with azimuthal anisotropy for the period range10-80s by using the Continuous Regionalization method. In comparison with other results, our maps have high resolution. The Rayleigh wave phase velocity dispersions at cell were inverted to determine1-D shear wave velocity structure by applying the nonlinear least square algorithm and then assembled into3-D model by simple linear method. Out results show:(1) The Middle Gobi Belt is charactered by low velocity in the entire crust and upper mantle with70kn depth. The azimuthal anisotropy is weak and relatively disorder. Based on the distributions of Cenozoic volcanic rock in the Middle Gobi, it refers that the low velocity anomaly is related to the Cenozoic volcanism and the vertical mantle flow maybe exist in this region. (2) The fast-wave directions and S-wave velocities show a big difference on two sides of the Main Mongolian Lineament(MML). It confirms that the default exist indeed and the depth maybe extends to the entire litho sphere.(3) The Southern Gobi, in the south of MML, the azimuthal anisotropy mainly shows NNE-SSW. We propose it is related to the remote influence of India-Eurasian collision. It is interesting that the direction changes to be NWW-SEE, similar to the direction of Absolute Plate Motion(APM). It indicates the azimuthal anisotropy in the upper mantle is controlled by mantle flow. Based on the different fast-wave direction in the crust and upper mantle, it suggests that the deformation is decoupled.(4) The azimuthal anisotropy in Hangay-Hentiy Basin with adjacent region mainly keeps NNW-SSE, with the feature of vertically coherent deformation in our period. The direciont is normal to the Baikal Rift and parallel to the open of the rift. We propose that the stress in this region is controlled by Baikal Rift.2. NE TibetThe teleseismic waveforms recorded by permanent and temporary networks located in NE Tibet, occurring from Aug.2007to Jan.2012, have been used to obtain the fundamental mode Rayleigh-wave dispersion data. The Rayleigh wave phase and group velocities along1216and653paths respectively, were estimated by the wavelet transformation method. These dispersion data were utilized to construct2-D phase and group velocity maps for the period range15-140s. The tests show that the horizontal resolution across the region is about0.5°for the periods used in this study. The Rayleigh wave phase and group velocity dispersions at each0.5°cell were jointly inverted to determine1-D shear wave velocity structure and then assembled into3-D model by simple linear method. The results show:(1) The high velocity anomaly extends to180km beneath Bangong-Nujiang Suture zone. We interpret the anomaly as Indian Plate. It refers that the Indian Plate is preserved till it gets subducted beneath Bangong-Nujiang Suture zone.(2) Our results shows the high velocity anomaly with100-250k depth beneath north of Qaidam basin and Qilian mountain. Based on the other results, we infer that the Asian plate is imaged beneath north of Qaidam basin and Qilian mountain, and does not show obvious sign of southward underthrusting.(3) Between the two plates, a low velocity and low resistance anomaly extends from the top of upper mantle down to250km, which may be caused by the mantle delamination.