THE DISASTER MECHANISM OF THE MS6.9 EARTHQUAKE IN MENYUAN,QINGHAI PROVINCE,2022
Earthquake disasters are one of the most significant natural disasters faced by human society.Understanding and mitigating earthquake disasters have always been a key focus of research for seismologists.Conducting investigations on post-earthquake seismic disasters is of great significance for the recovery and reconstruction of disaster-stricken areas,as well as for earthquake prevention and mitigation.Earthquake disasters can be classified into two types based on their mechanisms:one is the destruction caused directly by the seismic vibrations on buildings,lifelines,and other structures;the other is the damage related to geological hazards triggered by earthquakes.The former is mainly related to the density of regional economic layout;the latter seismic geological disasters typically include collapses,landslides,debris flows,ground fissures,ground subsidence,and soil liquefaction.These geological disasters often exacerbate the impact of seismic disasters,posing a more significant threat to human life and property safety.Therefore,it is of great significance to investigate the mechanisms of significant engineering disasters caused by earthquakes,as it can provide important insights for engineering recovery,reconstruction,and site selection.The Qilian-Haiyuan fault zone is an important boundary fault on the northeastern margin of the Qingzang Plateau.It plays a crucial role in absorbing and accommodating the convergence of the Indian Plate towards the Eurasian Plate in a NNE direction.With a total length of approximately 1000km,it is primarily composed of the Tolaishan fault,the Lenglongling fault,the Jinqianghe fault,the Maomaoshan fault,the Laohushan fault,and the Haiyuan fault,from west to east.On January 8,2022,a magnitude 6.9 earthquake occurred near the stepover of the Longling and Tuolaishan faults of the Qilian-Haiyuan fault zone.Although the earthquake occurred in uninhabited,sparsely wooded alpine grasslands and did not cause any casualties,it completely destroyed the Liuhuanggou bridge and the south-side Daliang tunnel on the Lanzhou-Xinjiang high-speed railway,a major artery of the Silk Road transportation network in China.This marks the first time that a mainline of the high-speed railway network,which is a showcase of China's economic achievements,has been entirely disrupted by earthquake damage.Based on the high-resolution orthophoto images and digital elevation models( DEMs) obtained through post-earthquake emergency scientific investigations using the unmanned aerial vehicles,this article conducted another field investigation on earthquake disasters in vehicles;this article conducted another field investigation on earthquake disasters in the isoseismal area.First,by investigating geological disasters such as collapses,landslides,and soil liquefaction in the meizoseismal area,as well as the damage to buildings and structures.Then,based on field surveys,a detailed mapping of the reverse-type surface ruptures formed by the Mengyuan earthquake was conducted,identifying the distribution patterns and geometric and kinematic characteristics of the surface ruptures and determining the distribution of coseismic vertical displacements.Additionally,the development of geological disasters caused by this earthquake was analyzed,and the disaster-causing mechanism of the Liuhuang Bridge was discussed.The research indicates that the Liuhuanggou River,located in the isoseismal area,does not exhibit large-scale earthquake landslides and collapses.Instead,only smaller-scale rockfalls and accumulations of rolling stones,as well as localized occurrences of sand liquefaction in certain riverbeds,are observed,which is clearly inconsistent with expectations.In addition to the formation of two strike-slip surface rupture zones,the earthquake also generated a reverse-type surface rupture zone approximately 7.9km long within the Liuhuanggou river on the northern side of the western section of the Lenglongling fault.The rupture zone exhibits an unstable southward trend and is primarily composed of discontinuous arc-shaped compressional ruptures,mole tracks,tensile ruptures,and seismic scarps.Along the surface rupture zone,a total of 35 vertical displacement measurements were obtained,with the minimum displacement of (8±1) cm and the maximum displacement of (49±3)cm.The average vertical displacement is approximately 24cm,and the displacement distribution along the strike is uneven.The surface rupture zone,which cuts nearly vertically across the Lanzhou-Xinjiang high-speed railway Liuhuanggou bridge,has caused extensive surface deformation and displacement.This is the direct cause of the destruction of the Liuhuanggou bridge.This finding suggests that when implementing seismic engineering design measures for major linear projects crossing fault zones,it is important to consider the extensive shear effects of reverse-type surface rupture zones.
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