The Dynamics Of Eastern Tibet From Focal Mechanism And Seismic Anisotropy

The mechanism to explain the uplift and enlargement of the Tibetan Plateau has been one of the hot topics in the earth science community for a long time.Several deformation mechanisms have been presented,and in recent years,the lateral extrusion of rigid crustal blocks along large strike-slip faults and channel flow of viscous materials in the mid-to lower crust have been in the center of the controversy.Rigid crust model suggests that the crust-scale faulting may play a dominant role,while the lower crustal flow model implies that the lower crust may be decoupled from the upper crust and upper mantle.In this study,we collected the seismic waveforms of earthquakes occurred between 2008 and 2014 in the Longmenshan fault with magnitude larger than 3 from Sichuan Seismic Network.We have determined 1491 focal mechanism solutions by the moment tensor inversion.In addition,we also collected focal mechanism solutions of 695 earthquakes in the study region but outside the Longmenshan fault.According to the fault plane parameters of the focal mechanism solutions,we obtained the high resolution tectonic stress field with the damped linear inversion technique.Meanwhile,we also collected the dense seismic waveforms from portable broadband stations in the study region to study the crustal and upper mantle anisotropy in the eastern margin of Tibetan plateau baed on group receiver function analysis and SKS/SKKS splitting measurement.At the same time,we employed the H-k stacking technique to determine the Moho depth and crustal average Vp/Vs ratio.(1)Tectonic stress field of the upper crust in the study region reflect strong regional variation.In Longmenshan fault zone,the orientation of stress not only vary with different segments,but also show different patterns along back,central and front range faults.In addition,the discontinuous variation of stress orientation may reflect an incoherent movement in Longmenshan fault zone.In other areas,the orientation of stress appear to be in simple pattern.The maximum compressional stresses inside the Sichuan-Yunnan rhombic block appear to be different from that along the Xianshuihe-Anninghe fault.To the east of Anninghe fault and south of Sichuan basin,the maximum compressional stresses vary slightly.Although minimum compressional stresses appear to be continuous in eastern Tibet,it shows complex distribution pattern in Longmenshan fault zone.The Wenchuan-Lushan seismic gap is found to be decoupled around the depth about 18 km from the study of maximum compressional stresses in different depth ranges.(2)Based on the study of crustal anisotropy using receiver function datasets analysis,the fast polarization directions show very complex variation pattern without clear regional characteristics.The fast polarization directions are dominated by the SE directions in Sonpan-Ganzi block,that are obviously different from the directions of maximum compressional stresses in this area.In the Sichuan-Yunnan rhombic block,the fast polarization directions turn to the SEE and SWW directions.The fast directions are basically parallel to the strike of Xianshuihe fault.In Sichuan basin and the areas to the south of Sichuan basin,the fast polarizations are in general in NE and NEE directions.The splitting times range from 0 s to 1.1 s,with an average splitting time of 0.41 s.In the junction area of the Longmenshan fault,Xianshuihe fault and Anninghe fault zone,the splitting times are significantly larger.(3)Based on the study of upper mantle anisotropy by SKS/SKKS splitting measurements,the average splitting time is about 1.2 s,significantly larger than that of 0.41 s in the crust.In Songpan-Ganzi block,the polarization directions are mainly in SSE direction,but are dominated in SE direction in Sichuan basin.The polarization directions show two patterns in southern Longmenshan fault,with one in the NNE direction parallel to the strike of Longmenshan fault,and the other parallel to the fast polarization direction in the Sichuan basin.To the south of Sichuan basin,the fast polarization directions are similar to that in Sichuan basin.The fast polarization directions in Xianshuihe-Anningge fault zone are basically parallel to the strike of the fault.In the Sichuan-Yunnan rhombic block,the fast polarization directions appear to be very complex,which may reflect the complex deformation mechanism,and discontinuous deformation in upper mantle beneath the block.(4)The crustal thickness in the study region show strong regional patterns,but it is not completely bounded by large faults in the study region.The crustal thickness is about 45 km along the Longmenshan fault,and it thickens to about 50 km southward from the Wenchuan earthquake.In northeastern Songpan-Ganzi block,the crustal thickness is basically stable in the range of 48-54 km.While in southern Songpan-Ganzi block,the crustal thickness is between 54 and 64 km,which is close to thickness beneath Sichuan-Yunnan rhombic block.Along Anninghe fault and the areas to the south of Sichuan basin,the crustal thickness is basically less than 54 km,significantly different from that in Sichuan-Yunnan rhombic block.Thus although the Xianshuihe fault is the boundary of Sonpan-Ganzi block and Sichuan-Yunnan rhombic block in upper crust,it may not be the boundary near Moho between the two blocks.The Anninghe fault appears not only to be the geological boundary of Sichuan-Yunnan rhombic block and the Yangtze block,but also the deep boundary of the two blocks.(5)The Vp/Vs ratios show high value in the Xianshuihe-Anninghe fault and the surrounding areas.This may reflect the weak mid-to-lower crust.The Vp/Vs ratios may reach 1.8 in some areas of the Sichuan basin,especially along the Longquanshan faults,which may be a result from the sedimentary layer with the thickness of 6-10 km in these areas.The Vp/Vs ratio is about 1.75 in most areas of Songpan-Ganzi block and Sichuan-Yunnan rhombic block,indicating medium deformation strength.(6)Based on the significant differences between GPS velocity,maximum compressional stresses,crustal and mantle anisotropy in the most areas,we suggest that decoupled movements may appear not only between crust and upper mantle,but also between the upper and lower crust in the eastern Tibet.Thus this fact can exclude the possibility of vertical coherent deformation model and also provide the evidence of multiple layered deformation.The low velocity zones in the mid-to-lower crust which are widely present in the eastern Tibet work as a slipping layer.Because of this layer,in a response to the push force from the interior of the Tibetan plateau,the upper crust may move independently a long distance.This may serve as the major deformation mechanism of the eastern Tibetan plateau.