| The northeastern margin of Tibet plateau is enclosed by the Alxa block, Tibetblock, Ordos block and Yangtze block, which is long-term influenced by the collisionand extrusion from Indian plate and Eurasian plate. The region of topography,geological and the subsurface structure are very complicated. To study the crustalstructure in this area is helpful for understanding the interactions of these blocks.Taking this area as research objects, this paper has finished the following four partswith receiver functions to do:(1) The analysis of the Vp influence to receiver function H-K stacking method.From three aspects of formula derivation, different model numerical tests andobservational data processing, we studied systematically the impact of the P wavevelocity (Vp) to receiver function H(the earth’s crust thickness)-k(wave velocity ratio)stacking method. The results show, the Vp change is positively related to the H, andnegatively related to the k; and the more complex the crustal model, the greater theinfluence of the Vp. Especially when the Vp deviate more than5%, crust’s variablethickness H caused by Vp will be more than5km, and the change of k may alsoexceed0.01, which will lead to unreliable results, even errors.(2) H-k stacking method within crust and its application. Based on principle of H-kstacking method and the numerical test, H-k stacking method is used to study thediscontinuity in the crust. The results shows when one converted wave and at leastone more reflection wave are clear, we can apply H-k stacking method to the shelldiscontinuity by adjusting the weighting factor of each phase, and find accurately thevelocity discontinuity within the crust.(3) By using the tele-seismic waveforms from2008to2009of Minxian stationwhich are nearest to the source of2013Minxian-Zhangxian earthquake, andcalculating the receiver function, the earth’s crust thickness and wave velocity ratio ofthe seismic source area and its adjacent area were identified. The results show, belowthe Minxian station, the earth’s crust velocity structure appears stronger lateralheterogeneity, each position of the receiver function is quite different, especially in the source area and adjacent area exists obvious difference. The middle and lowercrust in adjacent area exists obvious low-velocity layer, while the source area existsobvious high-velocity area in middle and lower crust. In addition, the crustal averagewave velocity ratio of focal region is1.76, while wave velocity ratio on the uppercrust is only1.62. The results indicate that the hard upper crust covers on the softmiddle and lower crust in the focal region, meanwhile, Minxian earthquake rupture islikely to be that lower crust flow activities lead to rupture in the upper crust.(4) Collecting tele-seismic waveforms with the epicentral distance of30°~90°and Ms>5.5during January2007to December2011from33permanent stations in thenortheastern margin of Tibet. In this study, Receiver functions of permanent seismicstations of the northeastern margin of the Tibet were used to determine thestratification of crust with H-k stacking method. We got the exact depth and velocityratio of each layer. The results show that there is20km topography of Moho depth; Aobvious low velocity layer is exist at the depth of8-18km. Beneath part of the stations,we found more visible low-velocity lower crustwith gradually thinning from west toeast. The results provide direct seismic evidence of the lower crust thickening in thestudy area; Meanwhile, the amplitude information of Pms phase indicates that theMoho strength is weak near the Ordos block, which may be caused by the invasion ofupwelling hot material of the upper mantle. |
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