Analysis On The Cause Of Formation And Mechanism About Ground Fissures Of Longyao, Hebei Province

Longyao ground fissure principally develops in the intense tectonic zone of the NorthChina plate,and also the junction belt of the NWW Ningjin-Hengshui fault arch and the NNELinqing fault arch,with several active faults gone through it.Longyao ground fissure hascaused a severe losses to the agricultural production,industrial development and people’slivehood in the locality on account of its long history of activity and large scale.Hence,theanalysis of Longyao ground fissures’ formation mechanism makes a significant contributionto the improvement of existence environment,disaster prevention and protection of people’slives and properties.The content of this paper is based on geology investigation,analysis of prospectingtrench section and numerical simulations.Combining field work with numerical simulationresults, following conclusions are derived:1.Longyao ground fissure has a total length of36km and thus turns it into the largestground fissure in scale in North China plate.Longyao ground fissure is divided into threesegments,western segment,middle segment and eastern segment.The western segment,whichis comparatively continously exposed and strikes in NE68°,is approximately6km in lengthand30-40cm in width.The middle segment, which is continuously exposed and strikes inNE75°,is approximately10km in length and40-60cm in width.The eastern segment, which isinterrupted exposed and strikes in NE60°,is greater than15km in length and30-40cm inwidth.2.According to trench section,the displacements of soil caused by ground fissuresincrease gradually from bottom to top.Displacement of soil at the top of trench section reaches20cm,displacement at5m deep from top reaches62cm and displacement at bottom reaches2.92m.These signs indicate that ground fissures develop gradually from depths to superficiallayer and closely relate with deep tectonic activity.ESR spectra indicates that Longyao groundfissure has a long history of activity and large scale.The accumulated dispalcement in verticalhas reached2.92m with a activity velocity of1.933×10-3cm/a since15.1±1.0ma.3.Numerical simulation results indicate that the stretching deformations of basementgenerates a dip-slip movement in normal fault and engenders an additional deformation within a certain depth range nearby hanging wall.Thus conduces to the transformations from ahorizontal compressive stress state to a horizontal tensile stress state and becomes a dominantfactor of ground fissures’ development in surface crust.4.With the annually declining of water table and effects of underground waterseepage,flushing and erosion,fissures occur in ground surface.Consequently,activity of waterfurther accelerate the development of Longyao ground fissure.A conclusion based on geology investigation,trench section and ESR results shows thatLongyao ground fissures are the outcropping of an Holocene active fault that developedupward and through soil stratum in geological history.It is regarded as one type of tectonicfissures and indicates an strengthening of regional stress field.