A Study On Wenchuan Earthquake Disasters And The Safety Distance From Active Faults

On May12,2008, a great earthquake of Ms8.0occurred in Wenchuan County,Sichuan Province, which produced surface ruptures and buildings damage, and causedmany landslides, bedrock collapses, and other secondary geological disasters, adding tothe extent of earthquake disasters. As shown in the earthquake damage examples,sudden rupture of active faults is the main cause of the earthquake, and areas along theactive faults are the sites with most serious building damage and casualties. The mainreason is that coseismic surface ruptures, dislocation, strong ground motions and theirsecondary geological disasters can spawn building collapse and destruction.It is generally accepted that crustal earthquakes are caused by sudden displacementon active faults, and many quakes of M>6are accompanied by faulting at the earth’ssurface. However, current science and technology do not allow scientists to predictearthquakes accurately through monitoring active faults and/or seismic precursors. So itis necessary to analyze causes of building damage in the Wenchuan earthquake toimprove capacity of seismic protection of buildings. Tectonic earthquakes usuallyproduce strong ground vibration which makes buildings destroyed in the epicenter area,and rock mass movement on either side of the causative fault can make buildings toreand broken Therefore, on the one hand, people should summarize the reasons frombuildings destruction to improve and strengthen anti-seismic measures, and ensureseismic performance of buildings to meet earthquake-proof requirements according toexisting code for seismic design of buildings. On the other hand, people shoulddetermine a reasonable width of avoidance to seismogenic active faults to mitigatepotential earthquake hazards based on the accurate positions of the active fault. Theobjective of this thesis is to study safety distance range of active thrust faults which areexposed to major earthquakes. The work of this thesis includes analysis of buildingdamage in the2008Wenchuan earthquake by using the method of statistical analysis,and the quantitative relation between co-seismic surface rupture width and buildingdamage characteristics, and based on measurement data of surface rupture deformationin the Wenchuan earthquake and the rupture zone width data of historical earthquakes.The results of this work would provide a scientific basis for buildings avoidance fromactive faults.In this theses, through field measurements and investigations of earthquake disasters, statistics of indoor data and calculation and analysis of data，building damage and itsprevention measures are analyzed, the quantitative relationship between the co-seismicsurface rupture zone width and building damage characteristics is discussed，and safetydistance range of thrust faults is primarily determined by using the method of statisticalanalysis. The the main conclusions are as follows:1. Case analysis of earthquake damage shows that sudden rupture of active faults isthe main cause of the earthquake, and areas along the active faults is the place sufferingfrom most serious building damage and casualties. Avoidance of active faults isnecessary and feasible. If buildings can avoid faults for a distance, the extent of damageof buildings will be largely reduced, as to reduce casualties and economic losses.2. Major or great earthquakes can generate a lot of surface ruptures, which havevarious expressions, regardless of theirs structural complexity and fracture length, andbring strong ground motions and surface deformation which will affect the surface andbuildings to different extents, and cause surface buildings damaged and even collapse.At the same time, secondary geological disasters will also cause building devastationand personnel casualties.3. Through investigations to the damage characteristics of main structure types ofbuildings in earthquake-stricken areas of western Sichuan, earthquake damage ofbuildings is discussed, and damage extents of different types of building can bearranged in an ascending order: reinforced concrete frame structure, masonry structure,wooden structure, brick-wood structure. Therefore, it is recommended to adopt framestructure and masonry structure with anti-seismic measures.4. According to the existing seismic protection requirements of building and codes forseismic design of buildings, typical building damage in the Wenchuan earthquakedisaster and its reasons are analyzed and discussed from buildings location, seismicdesign, building construction and other aspects, and corresponding measures aresuggested, which are stated below.①The building site should be away from active faults which are capable ofgenerating major earthquakes. Reasonable width of avoidance of seismogenic activefaults can be determined according to actual geological situation. Meanwhile, the siteswith potential geological hazards should also be avoided.②The anti-seismic structure system should be correctly selected: to make structuresystem several seismic fortification lines; to avoid the structural system ofdiscontinuous stiffness; to ensure connection between the various components of structure.③New technology about seismic isolation and energy dissipation and new buildingmaterials should be vigorously developed and applied to improve comprehensiveaseismic capability of buildings.5. Quantitative relationship between co-seismic surface rupture zone width andbuilding damage characteristics is analyzed， and surface rupture width data ofmeasuring points in the Hanwang-bailu, Beichuan-Yingxiu and Xiaoyudong surfacerupture zones are statistically analyzed, yielding the following conclusions:①Coseismic rupture of seismogenic faults will cause buildings tore and broken,which is overwhelming with any anti-seismic measures. And coseismic rupture ofseismogenic faults can directly destroy the ground and buildings, which are of localizeddeformation characteristics. Its width effect is limited, generally in the range of30m.The closer to faults, more serious surface rupture is, more serious buildings destructionis. Surface rupture and building destruction on the hanging wall of the seismogenicthrust fault is more severe than that on the footwall of the fault.②From the statistical results about surface rupture width at52measuring points in3Wenchuan earthquake rupture zones, some conclusions can be obtained: the width ofthe Wenchuan earthquake surface rupture is mostly between15-35m, deformation bandwidth of individual observation points is greater than40m, even more than100m, whichis basically consistent with existing statistical results. The results provide a quantitativestandard and scientific basis for determining avoidance width to the thrust fault.6. A lot of data of surface rupture zone width of the Wenchuan earthquake and otherwidth data of surface rupture zones of historical earthquakes of thrust faults arecomprehensively analyzed. As effective width of thrust faults,"avoiding belt" iscalculated to be about30m with statistical analysis methods. Avoiding width of thehanging wall of active fault is about20-22.5m, and avoiding width of the foot wall isabout7.5-10m. These results can provide construction’s with avoiding distance fromthrusting faults and others types of faults as references and quantitative standard.7. It is suggested that further attention should be paid to the research on fault’s"avoiding belt" width and the probing of active faults, and “safety distance” from activefaults should be taken as a legal regulation for constructions. So that earthquake hazardscan be positively and effectively reduced.