Using China Earthquake Networks To Study Core-Mantle Boundary Structure By SmKS And Image The Mantle Transition Zone Beneath North China By Receiver Function

The Earth's outer core is well mixed due to vigorous convection,and is therefore more or less compositionally homogeneous,there are,however,increasing evidence that suggests stratification may exist within the outermost outer core due to the presence of light elements.It is critical to understand outer core's composition,geodynamic process and evolution.The morphological variation of the mantle transition zone in the depth between 410 km and 660 km can not only reflect the temperature change of MTZ environment,but also the 660 km discontinuity can illuminate the evolution process of the deep subduction plate.Limited data and low sensitivity are two greatest weaknesses that constrain the previous study of deep earth.In recent years,the development of China's seismic network and regional dense-flow seismic arrays has recorded rich and informative seismic data for the study of the deep earth.This paper employs a large amount of seismic data from the China Seismic Network in the North China and Sichuan-Yunnan regions.The research is focused on the structures of the two important discontinuities.The main results are as follows:The SmKS wave(m=2,3,4...)is a useful tool for the study of the core-mantle boundary structure,as they pass through the core mantle boundary and repeatedly reflect from under-side of core mantle boundry.The southeastern part of China is located on the opposite side of South America.The China Array,which was installed in recent years,provides feasible conditions for recording the SmKS phases excited by the deep earthquakes in South America.In this paper,the SmKS waveform anomaly is observed in real data.By comparing with the theoretical SmKS waveform,it is found that the incident angles of SmKS,in a series of refraction-reflection-refraction processes in which P waves are converted into S waves at the core mantle boundary,P waves are repeatedly reflected into S waves in the outer core and finally S waves are coverted into P waves at the core mantle boundary,are distributed near the critical angle,causing the refracting and reflection coefficients to contain imaginary terms,that is,there is a phase shift,which affects the waveform of the SmKS wave.Therefore,it is suggested that the assumption that "the travel time difference of SmKS is only related to the top velocity of the outer core" in the previous study is not exact.The SmKS waveform anomaly observed in the real data is related to the low velocity zone at the bottom of the mantle beneath the South Pacific.In this paper,a large number of seismic data are applied to generate over 90,000 high signal to noise ratio(SNR)receiver functions that can image the structure of the mantle transition zone in North China.In order to improve the result,the EARA2014 three-dimensional model is introduced to correct the time difference of the upper mantle.After the time difference correction,the amplitude of the seismic wave after common conversion point(CCP)stacking,especially the Ps converted wave amplitude of the 660 km discontinuity,increase significantly,which means a higher resolution of the structure image.Our result shows that the 410 km discontinuity has an average depth of 417 km in the study area and the depth is deeper in the west than east.The deepest region is in the southwest aspect of Datong volcano,about 428 km.We imply that there may have some hot material in this area to cause this abnormal phenomenon,which may be related to the SEE upwelling mantle flow below the northwest of Datong volcano.In the study area,the depth of 660 km discontinuity is slightly deeper than the global model.The average depth is roughly 676 km,which indicates that the edge of the stagnant slab of the Pacific subduction plate may extend to the surrounding gravity gradient zone.The influence of the stagnant slab on the bottom 660 km discontinuity is much greater than that on the top 410 km discontinuity.Another 660 km discontinuity(?700 km)is found beneath the Bohai Bay.This may correspound to parts of Philippine Sea plate that detached in the past and Pacific plate that have intersected at this depth.It could also represent the transformation from ilmenite to perovskite in a cold environment,such as in the Pacific plate,or another complicated behavior of Pacific plate at this depth.Combined with the distribution of the thickness of the mantle transition zone,we unveil that the thickness of the local transition zone is thicker in the northeastern part of the Datong volcano(275 km),which may relate to the demolition of the southernmost lithosphere of the Daxinganling.