Constrains of upper mantle Vp/Vs ratio variations beneath North China from a new receiver function mapping method

P wave receiver function is a time series consisted of P-to-S converted phases, which are produced when teleseismic P waves transport upward cross velocity discontinuities. The traveltimes of different converted phases on a P-wave receiver function are the traveltime differences between converted S waves and direct P wave. The traveltime of converted phases on the 410- and 660-km discontinuities, which bound the mantle transition zone, intensively rely on the Vp/Vs ratios in the upper mantle. We developed a new mapping method to image the Vp/Vs ration in the upper mantle. Based on an updated receiver function method, the depths of 410- and 660-km discontinuities can be determined by a grid mapping of different receiver functions. Further, according to H-k scanning, we also can obtain the crust thickness and velocity ratio in the crust beneath each station. After the topography of 410-km discontinuity is clearly mapped and the effects of the crust are isolated, we can constrain the Vp/Vs ratios between the Moho and 410 km discontinuities. Taking advantage of raypath crossings like tomography, we map receiver functions along the raypaths to different model grids to invert a high-resolution Vp/Vs ratios image in the upper mantle. The change of mineral compositions in the upper mantle affects the velocity ratio more than the temperature variation. So we can constrain the lateral variation of mineral compositions in the upper mantle using their velocity ratio differences. Using the receiver functions of 120 broadband stations in North China region, we obtained a high-resolution velocity ratio image of the upper mantle. The result suggests there are higher velocity ratios in the upper mantle beneath the Taihang Mountain in the middle of North China, and the highest anomaly reaches 10%. The abnormal velocity ratio indicates the strong lateral variation of the mineral composition in the upper mantle beneath North China. There are two possibilities for the higher velocity ratio. One is the partial melting in the upper mantle beneath the Taihang Mountain, and the other is lower mantle materials upwell and result in the change of the mineral compositions.