We employed a double-difference algorithm （hypoDD） to relocate earthquakes within the region bounded by 66°E-78°E and 32°N-42°N in the period of 1964-2003 reported by the International Seismological Center （ISC）. The improved hypocentral locations delineate a double-layered Wadati-Benioff zone in the eastern Hindu Kush intermediate seismic belt. Based on this feature and other evidences, we propose that the intermediate-depth earthquakes beneath the Pamir-Hindu Kush region may occur in two collided subduction zones with opposite dip directions.
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.
The great Tancheng earthquake of M81/2 occurred in 1668 was the largest seismic event ever recorded in history in eastern China. This study determines the fault geometry of this earthquake by inverting seismological data of present-day moderate-small earthquakes in the focal area. We relocated those earthquakes with the double-difference method and found focal mechanism solutions using gird test method. The inversion results are as follows： the strike is 21.6°, the dip angle is 89.5°, the slip angle is 170°, the fault length is about 160 km, the lower-boundary depth is about 32 km and the buried depth of upper boundary is about 4 km. This shows that the seismic fault is a NNE-trending upright right-lateral strike-slip fault and has cut through the crust. Moreover, the surface seismic fault, intensity distribution of the earthquake, earthquake-depth distribution and seismic-wave velocity profile in the focal area all verified our study result.
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 subduetion 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.
P-wave arrival times of both regional and teleseismic earthquakes were inverted to obtain mantle structures of East Asia. No fast （slab） velocity anomalies was not find beneath the 660-kin discontinuity through tomography besides a stagnant slab within the transition zone. Slow P-wave velocity anomalies are present at depths of 100-250 km below the active volcanic arc and East Asia. The western end of the flat stagnant slab is about 1 500 km west to active trench and may also be correlated with prominent surface topographic break in eastern China. We suggested that active mantle convection might be operating within this horizontally expanded ＂mantle wedge＂ above both the active subducting slabs and the stag- nant flat slabs beneath much of the North China plain. Both the widespread Cenozoic volcanism and associated extensional basins in East Asia could be the manifestation of this vigorous upper mantle convection. Cold or thermal alaomalies associated with the stagnant slabs above the 660-km discontinuity have not only caused a broad depression of the boundary due to its negative Clapeyron slope but also effectively shielded the asthenosphere and continental lithosphere above from any possible influence of mantle plumes in the lower mantle.
