【作者】 罗文行；

【导师】 胡祥云； 李德威；

【作者基本信息】 中国地质大学 ， 固体地球物理， 2009， 硕士

【摘要】 南北地震带南段位于青藏高原东缘,是中国大陆东西构造的分界,同时也是各种地球物理特征的急剧变化带。该带地震活动频度高、强度大、周期短,震源浅,是典型的强震多发地带,一直受到地学界的关注。本文通过震源空间分布特征及其与地壳物性结构的研究,对搞清南北地震带地壳深部结构和探讨其形成演化过程具有重要意义。论文主要内容有:第一章,介绍了震源分布的研究意义和国内外研究现状及本文的研究内容和目的。第二章,介绍了研究区区域构造特征,运动场和主要断裂活动特征及地壳结构特征。第三章,通过对收集的地震数据进行统计和分析,得出南北地震带南段震源分布特征。平面上微震的震中呈斑杂状或串珠状分布,与活动断裂的展布相吻合;强震的分布呈弥散状,局部集中,但大多与断裂位置稍有偏离。震源机制在研究区北部以逆冲为主,向南转变为走滑和正断层型为主。断层面倾向以北东和北西向为主,滑动方向以南北向为主。东西向廊带震源深度分布:震源多分布在地壳深度5-33km范围内。微震多集中在10-15km的浅层,强震在10-33km深度均有分布,但20km之下已大为减少。不管是微震还是强震,剖面上均存在“多震层”,不同地段多震层的位置和厚度有变化。横向上疏密变化,强震深度分布图上存在地震“空区”和丛集区,这可能与深部块体边界的构造位置有关。通过三维可视化显示,发现由南到北三个廊带多震层有不断扩展的趋势,越到北部,10km和15km深度的多震层有相互连接成为同一个规模较大的层的趋势。35km是该区强震发生的底界(不同地段底界深度不同,35km为最深底界),该深度以下地震极少。南北向廊带震源深度分布:南北横向上震源点疏密变化,在26°N、27°N、31°N下方一定范围内震源点异常密集,且有强震集中。微震在深度10km、15km和33km左右都有多震层存在,而强震多震层在10km、15km、20km深度存在。不同地段多震层的密度和厚度有变化。第四章,震源分布与地壳物性结构对比研究。地壳速度结构与震源分布:平面上,微震密度分布与15km切片低速异常的分布类似,而强震密度分布与65km深度切片低速异常的分布比较一致,表明微震主要受控于中地壳层次的构造活动,而强震的发生与下地壳构造性质关系密切。本文认为,强震主要受控于下地壳低粘度物质(低速层)的韧性变形,因而“有根的低速异常”上方强震分布密度较高。剖面上,壳内低速层与“多震层”的底界一致,中地壳很薄的低速层与下地壳较厚的低速层都具有这一规律。研究区地壳厚度具有南薄北厚,东薄西厚的特征,地壳较厚的地区中地壳和下地壳广泛存在低速层,这些低速层与多震层的底界相对应。低速层可能是重要的孕震构造,对地震的发生具有控制作用。地壳电性结构与震源分布:研究区电性结构的特征是壳内存在大规模低阻体,与震源密度分布对比可知,低阻体往往被壳内两个不同深度——10-15km和33km左右——的多震层所夹持。震源密度高值点和电性突变带均与地表可见的大规模断裂有良好的对应关系。此外,高热流的地区其震源密度也较高。第五章,分析了低速、低阻层和多震层的构造属性,探讨了研究区大陆动力学过程,并对研究区强震的形成机制做了探索。低速、低阻层、多震层的构造属性:中地壳层次的低速、低阻层很可能是壳内滑脱层,是韧性下地壳与脆性上地壳发生拆离解耦的构造层次;下地壳低速、低阻层是部分熔融、含流体的韧性流变层。壳内多震层是上地壳硬的脆性层,容易发生突然破裂,产生地震。南北地震带南段大陆动力学过程:青藏高原增厚的下地壳向北流动扩展,形成近南北向的下地壳层流构造。下地壳流层向北运动受到塔里木盆地、柴达木盆地强硬块体的阻挡,转向青藏高原东缘川西地区,又被四川盆地阻挡,开始沿接触带——龙门山断裂向北东和南西两个方向扩展,并使得龙门山脉强烈隆升。青藏高原下地壳流层围绕喜马拉雅东构造结顺时针旋转流动,带动上地壳差异运动而形成大规模的走滑断层和正断层。同时,这股旋转流与受北部柴达木盆地阻挡转向四川盆地再次受阻而向云南方向流动的下地壳流层汇集,使得滇西地区上地壳隆升,并形成大量走滑断层和正断层,并拖曳上地壳物质向南方“逃逸”。下地壳流层与上地壳脆性层发生差异运动,在中地壳层次发生剪切拆离,形成滑脱层(对应于中地壳低速、低阻层),并控制了上地壳的构造运动。研究区板内地震形成机制:来自青藏高原的下地壳流层的流动和受阻,是控制南北地震带南段地震活动的重要因素。下地壳流层流动拖曳上部脆性层运动,不仅上下层之间差异运动形成壳内滑脱层,而且不同地段下地壳流速不同,使得上地壳不同块体也做差异运动。而差异运动是形成断层和使先成断层再活动的原因,也即地震活动的原因。对汶川地震形成机制的解释:川西高原下地壳流层向东流动受到四川盆地强硬下地壳的阻挡,下地壳物质汇聚顶托上地壳隆升,在龙门山之下的中地壳层次造成应变和应力的积累。下地壳流层拖曳上地壳脆性层也向东运动,但二者运动速度不同。最初在下地壳流层未与四川地块接触时,下地壳流速应该比上地壳运动速度快,接触而受阻之后,下地壳流速大幅度减慢,物质开始在接触带附近聚集并顶托上地壳隆升。由于四川盆地上地壳主要为沉积物,强度不大,川西高原上地壳的运动受到的阻力没有下地壳流层的阻力大,因而上地壳运动速度减慢的少,继续向四川盆地方向推进,与下地壳汇聚物质的顶托作用一起,使龙门山地区强烈隆升并形成朝向四川盆地的逆冲推覆构造。上、下地壳的差异运动在中地壳层次形成韧脆性剪切带和滑脱层,上地壳和下地壳通过该滑脱层解耦,上地壳脆性层形成断层。龙门山断裂带主要有三条断裂,后山断裂、中央断裂和山前断裂,在川西高原与四川盆地作用过程中,这些逆冲断裂形成闭锁区,应力和能量积累到超过其闭锁极限时,突然释放即形成地震。最后在第六章总结了本文研究得出的结论并进行了简要讨论,指出了本研究的若干不足之处。更多还原

【Abstract】 Southen of the S-N belt located in the eastern edge of Qinghai-Tibet Plateau as the tectonic boundary between the East and West and also the dramatic changes of variety geophysical characteristics of China. It has been of concern to scholars with its frequent seismic activity with high intensity, short-cycle and shallow focal depths. In this paper, the connection between the geophysical structure of the crust and the distribution of earthquake focals are explored, and its tectonic implications are discussed. It has a great significance for clearing the deep crustal structure of the study area and to explore the process of its formation and evolution.Content of the Paper are as follows:ChapterⅠintroduced the meaning of the study on earthquake focal distribution, research actuality at home and abroad and the content and purpose of this study.ChapterⅡdescribed the characteristics of regional tectonic of the study area, GPS and the characteristics of the active fault and characteristics of the crust structure.ChapterⅢ, statistical analysis of the collected seismic data and distribution characteristics of earthquake focals of the southern north-south seismic belt. The epicenter distribution of slight earthquake was plaque-like or beaded, and fit close with distribution of active faults; strong earthquakes showed diffuse distribution and local focus, but in most cases with slightly deviated from the distribution of the fracture position. Focal mechanism in the northern part of the study area are mainly thrust, but into the south and strike-slip fault is the main type. Fault plane inclined to the north-East and North-West to the main sliding direction of the main north-south. Focal depth distribution in East-west corridor: a majority of focals distributed in crustal depth range of 5-33km. Slight earthquake concentrated in the shallow 10-15km and the strong 10-33km, but it has been greatly reduced under 20km; There are seismogenic layers located on the sections, and the thickness of layers are changed with different location. Horizontal changes in density, there is an earthquake "empty areas" and the cluster area in the depth distribution of earthquake which may be associated’ with the location of the tectonic block borders. The seismogenic layer seems to be the trend of ever-expanding from south to north through the three-dimensional visual display. 35km is the bottom depth of the strong earthquakes occurred in the area (it is different from of the depth of different sections, 35km maybe the deepest). The focal depth distribution in North-south corridor: there is strong focus under the point of 26°N, 27°N, 31°N. In the depth of 10km, 15km, 20km and 33km have about the existence of seismogenic layer.ChapterⅣ, the comparative study of the earthquake focal distribution and the Geophysical Structure of the Crust. Crustal velocity structure and earthquake focal distribution: density distribution of the slight earthquakes was similar to the low-speed anomalies in 15km depth section and the density distribution of strong earthquakes similar to the low-speed anomalies in depth of 65km section of earth’s crust, mainly controlled by tectonic activity levels , and the occurrence of strong and close under the tectonic nature. This article holds that the main earthquake and controlled by the ductile deformation of low-viscosity lower crust (low-velocity layer) , where "the low-speed anomalies with a root" has higher density of earthquakes. Profile, the crustal low-velocity layer is identical with the bottom of the seismogenic layer. There is a broad low-velocity layer in the areas of thick crust, these low-velocity layer corresponds to the bottom of the seismogenic layer. Low-velocity layer may be important seismogenic structures, and it controled the occurrence of earthquakes. Resistivity structure of the crust and the earthquake focal distribution: the existence of large-scale low-resistivity body is the main characteristic of resistivity structure of the study area. Regularly, the low-resistivity body was nipped by two seismogenic layers with different depths - 10- 15km and 33km around. High heat flow areas corresponding to focal areas of high density.ChapterⅤ, The tectonic properties of low-velocity& resistivity layer and seismogenic layer, the continental dynamics the study area was discussed and the study area and the formation of strong earthquakes’ mechanisms were explored. The low-velocity& resistivity layer is likely to be slippage layer between ductile lower crust and brittle upper crust and detachment occurred in the level of the structure of decoupling; The low-velocity& resistivity layer maybe produced by partial melting, containing rheological fluid layer of toughness. The seismogenic layer is a hard brittle upper crust layer, prone to sudden rupture, resulting in an earthquake. The continental dynamics of the southern section of north-south seismic belt: the expansion of the thickening lower crust of Qinghai-Tibet Plateau flowed northward. The flowed layer under the crust was stoped by the Tarim Basin, Qaidam Basin block, then turn to the eastern edge of The Plateau in western Sichuan. The Sichuan Basin blocked also stoped the flowed lower crust at Longmenshan fracture, and the lower crust flow into both directions East and West along Longmenshan, and makes a strong uplift of Longmen Mountains. The lower crust of Qinghai-Tibet Plateau clockwise flow around the East Himalayas(Namjagbarwa). The movement of the upper crust driven by lower crust flow makes differences in the formation of large-scale movement of strike-slip faults and normal faults. At the same time, this rotating flow together with the the flow blocked by Sichuan Basin, flow to Yunnan direction. The upper crust of western Yunnan uplift, and a large number of strike-slip faults and normal faults are formed. The lower crust flow draged on crustal material escape to the South. Differential movement between the lower crust and the upper crust brittle layer to form a slip layer in the crustal shear detachment level (corresponding to low-velocity& resistivity layer in middle crust), and controls the tectonic movement of upper crust. Mechanisms the of intraplate earthquakes: an important factor is the lower crust flow and delay from the Qinghai-Tibet Plateau to the study area, it controls the seismic activity in southern section of north-south seismic belt. Flowed lower crust dragging the upper brittle layer movement, differences in movement occur not only between the upper and lower crust to form slip layer, but also between different blocks. The difference in movement is the causation of faults activities and that is the reason for seismic activity. Mechanism of the Wenchuan Ms8.0 earthquake: the eastward lower crust laminar flow under the West Sichuan Plateau blocked by the Sichuan Basin at Longmenshan bring on the accumulation of strain and stress. There are three main fault rupture in Longmenshan, there formed a closed thrust zone in these faults, when the accumulation of stress and energy achieve to more than latch-up limit of its sudden release, the earthquake occured.Finally, ChapterⅥsummarizes the conclusions of this paper and a brief discussion, pointing out a number of shortcomings in this study.更多还原