We investigated the upper mantle anisotropy beneath China by applying teleseismic shear wave splitting measurements at 119 seismic stations from CDSN and GSN/IRIS networks in China.The splitting observations are characterized by apparent diversity of anisotropy pattern in adjacent tectonic domains,including the Tianshan orogenic belt,Tibetan plateau,the Yangtze craton,the North China craton and northeastern region.In western China(Tianshan orogenic belt and Tibetan plateau),fast polarization directions of split SKS waves coincide strikingly well with the dominating trend of deformational crustal features and delay times range from 0.5 s to 1.6 s.While in eastern China,seismic anisotropy deduced from shear wave splitting reveals a homogeneous NW-SE trending structure,almost perpendicular to the strike of large-scale surface structures.The observed delay times of 1.5 s to more than 2 s favor consistent mantle flow over large mantle thicknesses.Based upon the straightforward relationships between seismic anisotropy and the development of lattice preferred orientation of mineral in upper mantle rocks,we interpret the splitting results in terms of tectonic fabric within the upper mantle.Since the lithosphere is less than 100 km thick beneath eastern China and the observed fast directions are subparallel to the trend of the absolute plate motion(APM) of Eurasian plate,we propose that the asthenosphere may mainly contribute to the anisotropic effects beneath eastern China.However,the upper mantle anisotropy beneath western China may have developed more recently in the subcrustal lithosphere because of rather small delay times and thick lithosphere.We also use the opportunity of the dense geodetic measurements available in China to discuss the coupling between the crust and mantle.In the Eurasia-fixed reference frame,GPS and shear wave splitting both depict a similar trend beneath eastern China,suggesting a lithospheric block "escaping" toward the east that could orient olivine [001] axis in the upper mantle.There is a s
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.
During a 4-month period starting from 21 January, 1997, an earthquake swarm of seven major events (Ms≥6.0) struck the Jiashi region at the northwestern corner of the Tarim Basin in Xinjiang,, China. Previous relocation studies suggested that these strong earthquakes had occurred along at least two parallel rupture zones. According to the relocated hypocenters and focal mechanisms of the events, we have constructed fault models for these seven earthquakes to calculate the Coulomb stress changes produced by each of these events. Furthermore, we extended our model calculations to include an ad- jacent 1996 Ms=6.9 Artushi earthquake, which occurred one year before the Jiashi earthquake swarm. Our calculations show that the Coulomb stress change caused by the preceding events was around 0.05 MPa at the hypocenter of the 4th event, and higher than 0.08 MPa at the hypocenters of the 2nd, 3rd, 5th and 6th events. Our results reveal a Coulomb stress interactive cycle of earthquake triggering between two adjacent normal and strike-slip faults.
