The northeastern (NE) Tibetan Plateau is an ideal place for investigating the far field effects of collision between the Indian and Eurasian plates. By what ways the Tibetan crust is thickened and extended is a long-term debated issue for absence of convincing evidence for proving the existence of the inner-crustal low velocity zone and its role. Using teleseimic P wave data from the China Seismograph Network in Qinghai and Gansu provinces recorded from 2007 to 2009, the crustal S-wave velocity structure beneath the NE Tibet plateau is resolved. The receiver function waveform inversion technique (PROGRAM330) is used to analyze the crustal S-wave velocity structure below seismic stations. The receiver functions are estimated by a time-domain iterative deconvolution method of Ammon (1991) with four different Gaussian coefficients (alpha=1. 0, 1. 5, 2.0 and 2.5). Firstly, receiver functions, which are much similar in waveforms and within a narrow range of back-azimuth (less than 10 degrees) and ray parameter (less than 0. 002), are stacked to enhance the main signal characteristics. To maintain the intrinsical details of the receiver functions, they are stacked without normal moveout. Results of Tian et al. (2013) and Li et al. (2006) are used as the constraints in the inversion process to reduce the uncertainty. The results show thata relatively low velocity layer (LVL) exists between the upper and lower crust in the region between the East Kunlun fault and the Haiyuan fault. The depth of the LVL shallows northeastward from similar to 35 km to similar to 20 km along the surface movement direction, while the Moho uplifts. The variation in thickness of the upper crust is more obvious than the lower crust. The thickness of the lower crust (15 similar to 20 km) beneath the Kunlun-west Qinling orogenic belt is thinner than that beneath the adjacent Qilian block (25 similar to 30 km). Beneath the NE Tibetan plateau, as well as the Alashan and Ordos blocks, the S-wave velocity in the lower crust increases with depth. The whole crustal S-wave velocity increases with depth beneath the eastern part of the west Qinling orogenic belt, the Ordos and Alashan blocks. It can be concluded that the observed LVL in the NE Tibetan plateau can act as an intra-crustdecollement/detechment to decouple the deformation between the upper and lower crust. The geometry of the LVL and the Moho indicates that the NE Tibetan crust is growing northeastward, and is predominated by upper-crustal thickening at present. The lower crust of the Kunlun-west Qinling orognic belt may be more rigid than the adjacent Qilian block and thus has experienced less deformation and crustal thickening. The lower crust of the NE Tibetan plateau is normal and possess high viscosity, so that is not conducive to the flow of the lower crust.
