摘要
对布设在南北构造带南段的中国地震科学探测台阵项目一期350个宽频带流动台站和中国地震台网90个宽频带固定台站记录的远震XKS(SKS、SKKS和PKS)波形资料作偏振分析,采用最小切向能量的网格搜索法和"叠加"分析方法求得每一个台站的XKS波的快波偏振方向和快、慢波的时间延迟,获得了南北构造带南段上地幔各向异性图像.结果显示研究区的各向异性具有明显的南北分区特征,北部的快波方向为近N-S方向,而南部主要表现为近E-W方向,且北部的平均时间延迟小于南部.分析表明,具有厚岩石圈的北部的各向异性主要由岩石圈变形引起,是一种垂直连贯变形模式;具有薄岩石圈的南部的各向异性主要由软流圈地幔流引起,缅甸和巽达板片的后撤/回转作用产生了指向西南的软流圈地幔流,在岩石圈底部和软流圈之间产生了一个水平差异运动,产生了一个与简单剪切一致的软流圈变形结构,从而产生了南部观测的各向异性.
The North-South tectonic belt(NSTB)is located in the eastern margin of the Tibetan Plateau.It is a north-south tectonic boundary between the eastern and western Chinese mainland with a very complex structure,showing a significant change in geology,geomorphology,and geophysical field characteristics on both sides.Meanwhile,the NSTB is a strong earthquake belt,where half of strong earthquakes greater than M8.0in Chinese mainland have occurred based on historical records,so also named the North-south seismic belt.Thus,the NSTB provides a unique nature laboratory for understanding continental interiors and lithosphere′s deformation.An effective technique for understanding crust and mantle deformation is the analysis of seismic anisotropy.Here we present new shear wave splitting observations from dense temporary seismic array and permanent seismic stations to map out variations in the dynamics of the southern segment of NSTB.Mantle anisotropy is constrained with 440 XKS(SKS,SKKS,and PKS)shear wave splitting observations,350 from portable deployments in the NSTB(2011—2013,the ChinArray Phase Ⅰ),and 90 permanent stations from the Chinese National Seismic Network(2003—2012).We determine the XKS fast wave polarization directions and delay times between fast and slow shear waves for 440 new seismic stations in the southern segment of NSTB usingboth the grid searching method of minimum transverse energy and stacking analysis methods.To obtain a reliable estimate of splitting parameters,the following were used as diagnostics for successful splitting parameter estimations:(1)clear XKS arrivals and distinct tangential component,(2)the horizontal particle motion is elliptical when anisotropy is present,(3)the two horizontal fast-and slow-component waveforms are coherent,(4)the particle motion becomes linear following correction for anisotropy,and(5)successful removal of tangential energy in the case of core phases.The results at most stations are good,the error of azimuth is less than 10°,and the error of delay time is less than 0.2s.The fast polarization directions and delay times do not depend on back azimuth,thus a single layer of anisotropic fabric is able to sufficiently explain the data without the need for additional layer.Based on polarization analysis from XKS data,we developed an anisotropic image of upper mantle in the southern segment of NSTB.In the study region,the fast polarization directions and delay time show partition characteristics from south to north.The fast polarization directions trend near N-S in the north,while the fast polarization directions rotate to near E-W in the south.The average delay time at stations in the north is 0.8s,whereas it is 1.1sin the south.Thus,the average delay time at the stations in the north is obviously less than that in the south.By analysis the anisotropic characteristics of the study region,this study has shown lithosphere deformation plays a major role in observed anisotropy in the north,and is undergoing vertically coherent deformation.In the south,observed anisotropy is mainly attributed to asthenospheric mantle flow beneath thin lithosphere.The subduction and rollback/retreat of Burma/Sunda slabs generate a southeastward flow which produces a differential flow between the lithosphere and subasthenospere.The differential shear is sufficient to generate the observed anisotropy in the south.