Study On The Shunyi Buried Active Fault And Its Induced Ground Fissure Disaster In Beijing

The buried active faults in plain areas are the potential geological factors causing disasters in many developed cities in China.When an earthquake occurs,the damage along the active faults is often the most serious,and the active fault will also cause geological disasters such as ground fissures,collapses and ground subsidence,which seriously threaten the urban geological safety.Therefore,it is of great theoretical significance and practical application value to study the geometric location,movement mode,activity nature and disaster control effect of buried active faults.Shunyi fault is a buried Holocene active fault in Beijing plain area.To find out its activity and reveal the mechanism of geological disasters such as ground fissures and land subsidence can provide geological basis and scientific guidance for land space planning and urban disaster prevention and mitigation in Beijing.The main achievements and understandings are as follows1.Shunyi fault is a Holocene Active fault dipping to the southeast with a strike of 45 degrees north by East and a dip angle of 75 degrees.Since Quaternary,the activity has obvious space-time difference: in terms of time,the activity has been the strongest since Holocene;Spatially,the activity of the southern segment of Shunyi buried active fault is stronger than that of the northern segment since Holocene.The vertical activity rates of Sunhe area in the southern segment and Beixiaoying area in the northern segment since Holocene are 1.90mm/a and 0.51mm/a respectively.According to the sliding instability friction criterion of byerlee,the ratio of shear stress to normal stress on the fault plane of Shunyi buried active fault is more than 0.5,which shows an increasing trend,reflecting the increase of fault activity intensity.2.The development characteristics of ground fissures in Shunyi(Airport)are that the distribution direction is mainly along the strike of Shunyi fault,and the influence width varies from 30 to 100 m.It is in the form of tension and clockwise torsion,which is consistent with the activity mode of Shunyi fault.Since2011,the spatial-temporal evolution trend of deformation and failure of airport ground fissures basically corresponds to the activity risk(? value)of Shunyi fault.The monitoring of ground fissures in an airport in the capital from 2017 to 2018 shows that the horizontal tensile displacement is generally in an increasing state,the shear deformation has a slow increasing trend,the ground fissures show clockwise torsion deformation and failure,and the hanging wall(South East wall)of the ground fissures is in a downward trend.The monthly average horizontal tensile displacement change rate of the ground fissures is about 1.7mm,and the deformation in August 2018 is the largest,reaching 15.52 mm..3.Shunyi fault has a controlling effect on Shunyi ground fissure.The shallow part of the fault has obvious vertical dislocation,which is consistent with the development position of ground fissures on the surface,while the deep faults gradually converge to the main faults.Since the Holocene,the activity of the southern segment of Shunyi buried active fault is stronger than that of the northern segment,which may be the reason for the expansion of the ground fissure from north to south in Shunyi area.When the subduction of the Pacific plate is strong or abrupt,the near surface E-W longitudinal extension in North China is strong,and the near surface E-W tensile effect,the activity intensity of Shunyi buried active fault increases,and vice versa.At the same time,groundwater extraction,aircraft dynamic load and Quaternary sediment compaction may also induce the aggravation of ground fissure activity.4.The three-dimensional numerical simulation and physical model test of the ground fissure show that the width of the influence zone of the ground fissure is 40 ? 70 m,the pit and tension fissure are formed in the hanging wall of the fault,and the uplift is squeezed in the footwall of the fault.The vertical settlement displacement of airport runway increases gradually from footwall to hanging wall.When Shunyi buried active fault moves 1cm(prototype 50cm)in the bedrock normal fault,it can induce 0.6cm(prototype 30cm)normal fault effect ground fissures on the surface.The longitudinal tensile strain of the footwall at the top of the middle runway underpass increases with the increase of the vertical displacement,and reaches the maximum when the vertical displacement is 4cm.It is suggested that the minimum safety avoidance distance of the structures along the ground fissure of an airport in Beijing should be 50 m from the hanging wall and20 m from the footwall of the fault.Meanwhile,high strength flexible materials and reducing the size of the cover plate should be used to prevent the ground fissure disaster.In order to provide support services for the safe operation and disaster prevention and mitigation of the airport,the ground fissure deformation monitoring of an airport in Beijing should be carried out,and the dynamic monitoring and early warning system of optical fiber and ground stress was constructed.It is expected to monitor the ground fissure deformation and build a dynamic monitoring and early warning system of optical fiber ground stress,so as to provide guarantee services for the safe operation of the airport and disaster prevention and mitigation.