| The Xiadian fault is an important buried active fault in the Beijing plain area.It is the seismogenic fault of the Sanhe-Pinggu M8 earthquake in 1679.It has formed an obvious surface rupture zone that is still visible on the surface.The morphological characteristics of the surface rupture zone and the period of ancient earthquake activity have always been the focus of scholars.In addition,there are also adverse geological phenomena such as surface subsidence,uneven settlement,sand liquefaction and ground fissures along the Xiadian fault.Therefore,it is of great theoretical and practical significance for the planning and design of the capital area and disaster prevention and mitigation to study the characteristics of Quaternary activity,paleoearthquake activity,surface rupture zone and fault influence zone.On the basis of collecting and sorting out the previous research results,this paper discusses the geometric distribution,kinematics and dynamics characteristics of Xiadian fault by means of ground investigation,geophysical exploration,trenching,drilling,in-situ stress measurement and numerical simulation,reveals the characteristics of paleo-earthquake activity since 20 ka,evaluates the stability of Xiadian fault,analyzes the characteristics of surface rupture zone of Sanhe-Pinggu M8 earthquake in 1679,and determines the width of fault influence zone,which provides geological safety guarantee for urban planning and construction.The main understanding is as follows :(1)The Xiadian fault strikes NE,dips SE,and is normal faulted.The apparent dip angle is60～80°.It is a Holocene active fault.The main fault is from the south bank of the Chaobai River to the Sanhe River.The upper and lower plates have a large fault distance.The thickness of the Quaternary system in the lower plate is 500 m ～ 600 m,and the thickness of the Quaternary system in the upper plate is 800 m ～ 1000 m.There are obvious signs of deformation around Xiadian Town,and fault scarps,fault plug ponds and other faulted landforms are developed.There is no sign of surface deformation in the Pinggu section of the fault.The thickness difference between the two sides of the fault is reduced,and the thickness difference between the two sides is 50 m ～ 100 m,which still shows the characteristics of Quaternary activity.(2)The stratigraphic faults and seismic wedges shown in the Dahuzhuang South Trough of the Xiadian fault reveal two paleo-earthquake events of the Xiadian fault,which are about(1.61±0.13)ka old earthquake and 1679 Sanhe-Pinggu M8 historical earthquake.The stratiic dislocations are about 0.3 m and 2.5 m respectively.There are seven periods of paleoearthquake events since 20 ka of the Xiadian fault,and the average recurrence period of large earthquakes is about 2716 a.(3)The overall trend of stratigraphic dislocation in the surface rupture zone of the Xiadian fault earthquake is basically consistent with the results obtained from topographic survey data.However,in the Dakangzhuang-Qixinzhuang section,the distance of stratigraphic dislocation is greater than the height of the fault scarp measured on the ground,indicating that the shape of the surface fault scarp may be affected by human activities.The amount of stratigraphic dislocation revealed by the trench is helpful to further understand the surface rupture zone characteristics of the 1679 Sanhe-Pinggu M8 earthquake on the Xiadian fault.(4)The in-situ stress measurement data reveal that the Xiadian fault is less likely to have fault sliding instability under the current in-situ stress conditions.However,as an active fault that has experienced an M8 earthquake in history and still has sliding signs today,it is still necessary to attach great importance to its activity changes.(5)Through the deterministic analysis,the width of the Xiadian fracture deformation fracture zone is about 50 m.Therefore,when the engineering construction is carried out around the fracture,the vertical distance from the fracture should be at least 50 m.Numerical simulation was used to analyze the characteristics of the influence zone of Xiadian fracture deformation and fracture under different engineering sites.It was found that in the engineering sites with the same fault dip angle and different scarp heights,the larger the scarp height,the larger the width of the influence zone of fracture deformation,and the width of the influence zone of hanging wall fracture deformation was larger than that of footwall.In the engineering site with different dip angles of the same scarp,the larger the dip angle,the smaller the width of the rupture influence zone,but the larger the peak relative acceleration,indicating that the larger the dip angle,the higher the seismic fortification requirements. |
PDF Full Text Request