We have updated the lateral variations of the quality factor Q0(Q at 1 Hz) beneath the crust of North China using ML amplitude tomography with near three times data.The data were selected from the Annual Bulletin of Chinese Earthquakes(ABCE) in 1985-2009,including 26 283 ML amplitude readings from 4 204 events recorded by 38 stations.The result is similar with previous research but has higher resolution.Estimated Q0 values are consistent with tectonic and topographic structure in North China.Q0 is low in the active tectonic regions having many faults,such as Bohai bay,North China basin,the Shanxi and Yinchuan grabens,while it is high in the stable Ordos craton.Q0 values are low in several topographically low-lying areas,such as the North China,Taikang-Hefei,and Subei-Huanghai Sea basins,whereas it is high in mountainous and uplift regions exhibiting surface expressions of crystalline basement rocks:the Yinshan,Yanshan,Taihang,Qinling and Dabie mountains,Luxi and Jiaoliao uplifts.Quality factor estimates are also consistent with Pn and Sn velocity patterns.High velocity values in general correspond with high Q0 and vice versa.This coincides with a common temperature influence in the crust and uppermost mantle.
This is the first of two papers that describes a regional tomography investigation,which combines P-wave arrival times of both regional and teleseismic earthquakes to obtain 3D mantle structures of East Asia up to 1 000 km depth.The most important findings of this tomography study are reported in this paper as follows.(1) No fast P-wave velocity anomalies can be related to subducted oceanic slabs beneath the 660 km discontinuity;instead the subducted oceanic slabs become flattened and stagnant within the transition zone.(2) The high velocity anomalies in the transition zone extend up to 1 500 km to the westward of the active trenches,which is a unique feature in the worldwide subduction systems.(3) Slow P-wave velocity anomalies are visible up to ～250 km underneath most of the East Asia on the east of 115°E,similar to the area of the stagnant slabs.These observations have important implications for the geodynamic process at depths beneath the East Asia,which might in turn control the widespread Cenozoic volcanism and associated extensional tectonics seen at the Earth's surface.
