Velocity Structure Of Uppermost Mantle From Pn Tomography And Q₀Tomography Of S Wave Attenuation In Northeast China

Tomography has become one of the most effective methods to explore the deep structure of the earth. Firstly, we introduced the theory of tomography. Then we present the results of the crust and upper mantle structure study and the West Pacific subduction in the recent years in Northeast China. We compare the methods of velocity structure of uppermost mantle from Pn tomography and Qo tomography of S wave attenuation, and focused on the ones of our work.Northeast China is situated partly on the Sino-Korean craton and partly on a portion of the Inner Mongolia fold belt. Characterized by widespread Cenozoic intraplate volcanism, deformation caused by multiepisode extension, Northeast China is an ideal place to study the underlying physical processes of rifting and the subduction and volcanic activity of the western Pacific plate. In our work we have selected lots of ML, amplitudes and travel times of Pn arrivals as reported in the Annual Bulletin of Chinese Earthquakes(ABCE) and regional seismic network of Heilongjiang province, Jilin province, Liaoning province and Inner Mongolia province. A two-dimensional tomography method is employed to find regional variation of crustal attenuation, Pn velocity and anisotropy in the uppermost mantle in Northeast China and its adjacent regions. In the most study area2degrees are well resolved.The major studies and conclusions are as follow:(1) we obtained30648Pn travel time picks from376stations and5768earthquakes to invert for Pn velocity lateral variation and anisotropy images beneath Northeast China. In the most study area2degrees are well resolved. The main results indicate the average Pn velocity is about7.9km/s in this region less than that in entire China. The Northeast region shows distribution of alternating high and low Pn velocity in the direction of NE-NNE. Pn anisotropy was also obtained simultaneously with Pn velocity. Only in Jiamusi massif, Bohai Sea, Changbai volcano and Datong volcan we observe strong anisotropy.(2) we have selected46652ML amplitude reading from6371events recorded by146stations. A two-dimensional tomography method is adopted to find regional variation of crustal attenuation in Northeast China and its adjacent regions. We obtained an average Q0value of323for the entire studied region with regional fluctuations±200. In the most study area2degrees are well resolved. Regions with the highest attenuation (low Qo values) are beneath Bohai Basin, and Songliao Plain and Hailaer Basin also have low Qo values, as these areas have thick sedimentary and strong tectonic activity. The entire Northeast region shows distribution of alternating high and low attenuation.