| The central region of Mongolia is located in the fringe of the Siberia plate and adjacent to the Sino Korean plate. In current, it is stable in the geologic period and the earthquake is relatively sparse, but in the geologic history it’s the suture zone of the Paleoasian Ocean which has occurred the intense tectonic movement during the closing process. The mantle of the evolution history of the dynamic process in the region whose north neighbor is Baikal rift plays an irreplaceable role. The study on the deep-seated structure is of great practical significant in understanding the geological background comprehensively.We selected the seismic events with the magnitude greater than5and the distance range between85°and135°from the center of the array, recorded by60wide-band seismic monitors employed in the central Mongolia According to SKS seismic phase frequency, we chose0.02Hz-1Hz band pass filter, respectively through three methods (the rotation related (RC), the minimum energy (SC) and the smallest characteristic value (EV)) of calculating the SKS SKKS phase to obtain the fast wave polarization direction (φ) and torsion wave latency (6t) in each statin below. Aligning the three methods, the effective events are screened. Relative to a single method, the credibility of the splitting results are improved, but the results are reducing.In the screening process, we found that the splitting parameters of the three methods has a good consistency except the northwest area, in which the results of SC and EV methods fit better, but the fast wave delay results of RC method are relatively low. The result of the differences caused by different methods can be forward to quantitative analysis. So, the results of SC and EV methods are researched. The time delay of the splitting is between0.8s and2s in the survey region with the effective results. According to the results of surface wave imaging, the lithospheric thickness in the central Mongolia is less than100Km suggest that the anisotropies are originated from the combined effect of both lithosphere and asthenosphere. The polarization directions of the fast shear-waves at stations located at the northwest region which is adjacent to Khangai Mountains and Hentiyn Mountains the structure directs roughly ENE-WSW, are about40°north west, approximate vertical with regional tectonic movements, which is clockwise around15°Angle with the abosulte plate motion direction by caculating the NNR-1A plate model. It suggests that the mantle flow direction and the absolute plate motion direction are almost consistent, but the crumpling effect between the plates makes the plate movements maybe produce certain deviation. The splitting parameters at the southeast region which is the Gobi Desert are similar with the northwest region, while at the southeast margin some stations show about E-W in the direction of fast wave splitting. These indicate that the mantle flow direction is changing in this region, or the area has more complicated anisotropic structure and distribution. Many efforts have been made in the past across baikalian fracture to obtain the same splitting results. Due to the less of effective data, results and analysis still needs more testing data.Combined with the body wave imaging and surface wave imaging results, we can more accurately determine the anisotropies in the area according to the splitting parameters, which provides more comprehensivw and accurate anisotropic restraint condition to research the dynamic problems of central Mongolia. Also the research of mantle flow is important for predicting the crustal stress distribution, earthquake cause expansion and regional tectonic movement trend in the future. |
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