The Connection Between The Geophysical Structure Of The Crust And The Distribution Of Earthquake Focals And Its Tectonic Implication In Southen Of The S-N Belt, China

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.