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The Morphotectonics,Seismicity and Origin of Shanxi Graben System,East China

LI Shude (Department of Urban and Environmental Science, Peking University,Beijing,100871)  
The Shanxi graben system is one of the important specific morphotectonic types in the eastern part of China.It is located in the crustal attenuation zone and the region ofintense crustal movement with high intensity,high frequency,shallow focus (10~30km) and Quaternary volcanic activity which is similar to that of Rhine graben and East African rift.The earthquake distribution in the graben system is regular.Large earthquakes (M≥6) occurred inside the graben.Twelve of the seventeen earthquakes (M≥5) in Linfen region in history (about 70%) occurred inside the graben.Seventeen earthquakes (M≥8) occurred in recorded history in China,six of them in North China block and three of the six in fen wei graben.The earthquakes are closely related to active faults:(1)All of the earthquakes occurred along active faults;(2)The earthquakes occurred along strike slip fault having the characteristics of low frequency and high intensity;(3)All causative faults are steep (dip angle≥65°);(4)Strong earthquakesoccurred at the cross section of shear segment and tensile segment or compressional segment. The origin and development of the graben system may be roughly divided into three stages:(1)The large scale uplifts,thrust faults and fractures were formed by left lateral shearing caused by NW SE trend compressional pressure in the Yanshan movement stage;(2)NNE and NE trend tensile fractures were formed and half grabens were produced by right lateral shearing under the action of NE SW trend compressional pressure which were caused by the sudden change of compressional pressure trend;(3)Half grabens keep developing under the action of mantle uplifting and crustal surface extension caused by the subduction of Pacific plate.
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