【摘要】 根据龙门山断裂带周边的固定数字地震台网和流动地震观测获得的宽频带地震记录,用多种地震学方法研究该地区的地壳上地幔结构。深部结构研究表明,龙门山断裂带物性分布具有显著的分段特征。用远震接收函数H-k叠加方法计算了各个台站的地壳厚度和波速比。地壳厚度总体变化是,地壳从东向西增厚,最小厚度为37.8 km,最大厚度是68.1 km。从东南向西北横跨龙门山断裂带的地壳急剧增厚,从41.5 km增厚至52.5 km。但是,龙门山断裂带两侧地壳厚度的差异在断裂带的南段和北段是不同的。在南段,地壳厚度急剧变化的分界线在中央断裂附近;在中段,分界线在后山断裂附近;在北段,则断裂带两侧地壳厚度差异很小。泊松比的空间分布是,松潘—甘孜地体北部和西秦岭造山带具有低泊松比(ν<0.26),扬子地台具有低—中泊松比(ν<0.27),松潘—甘孜地体南部、三江褶皱带和四川盆地具有中—高泊松比(0.26<ν<0.29)。除龙门山断裂带南段及其附近,大部分地区均不具有超高的泊松比(ν>0.30)。龙门山断裂带南段地壳具有高泊松比(ν>0.30),而北段地壳则为中—低泊松比。高泊松比可以看成是铁镁质组分增加和/或部分熔融的证据,表明那里的下地壳部分熔融是可能的。松潘—甘孜地体东南部地区的下地壳处于富含流体或温度较高的部分熔融状态,它有助于青藏高原的下地壳物质向东运动。青藏高原东部中、上地壳向东运动受刚性强度较大的扬子地台的阻挡,沿龙门山断裂带产生应变能积累。当应变达到临界值,发生急剧的摩擦滑动,释放积累的应变能,产生汶川Ms8.0地震。汶川地震在龙门山断裂带不同地段,表现出不同的破裂特征和余震分布,可能与断层带的分段深部构造差异有关。更多还原

【Abstract】 With the broadband seismic records acquired from permanent and mobile digital seismic stations around Longmenshan Fault Zone(LMFZ),the structure of crusts and uppermost mantle was investigated by using integrative seismic and geophysical methods.The study of the deep structure shows remarkable subsection diversity in deep structure and in material properties along the LMFZ.The H-k stack of teleseismic receiver functions was deployed to obtain the crustal thickness and velocity ratio of P and S waves.The result shows that the crustal thickness increases from east to west in general.The minimum thickness is 37.8 km and the maximum is 68.1 km.Across LMFZ from southeast to northwest,the crustal thickness is increasing sharply,from 41.5 km in the southeast to 52.5 km in the northwest.The distribution of Poisson’s ratio can be divided into three areas: the northern part of Songpan-Ganzi terrain and the West Qinling orogen,where are of lower Poisson’s ratio(ν<0.26);the Yangtze platform,where is of low-middle Poisson’s ratio(ν<0.27);and the southern part of Songpan-Ganzi terrain,the Sanjiang fold system and the Sichuan Basin,where are of middle-high Poisson’s ratio(0.26<ν<0.29).The distribution pattern shows that this study can not verify the assumption that there is widespread lower crust flow in the east margin of the Tibetan Plateau because except the southern segment of LMFZ and its vicinity there is no very high Poisson’s ratio(ν>0.30) being found in almost of the whole research region.The Poisson’s ratio is higher(ν>0.30) in the southern segment of LMFZ,while lower in the northern.The high Poisson’s ratio(ν≥3.0) in the local area of south segment of LMFZ can be regarded as the evidence of the existence of mafic component increment and/or partial melting.It provided the possibility of the partial melting in the lower crust.The lower crust in the southeastern part of Songpan-Ganzi terrain may have been of liquid-rich or in the status of high temperature and partial melting,which facilitated the eastward movement of lower crust material in the Tibetan Plateau.When the eastward movement of the upper and middle crust was obstructed by the more rigid Yangtze platform,the strain energy accumulated along the LMFZ.As the strain became higher than the critical value,the accumulated strain energy was released with the sharp slip of LMFZ and the Wenchuan Ms 8.0 earthquake occurred.The difference in the behavior of rupture and the distribution pattern of aftershocks may have resulted from the deep structure variation of the fault zone.更多还原