The Research Of Seismic Damage Anomalies In Hanyuan During Wenchuan Great Earthquake

Wenchuan Ms8.0great earthquake, which occurred on May12,2008wasthe most destructive, widespread one with the biggest difficulty to providedisaster relief since the founding of People’s Republic of China. Rarely seen inhistory, the destruction, shocked the whole nation and attracted the attention ofthe whole world. Hanyuan County, about250km away from epicenter, is the onlyabnormal intensity Ⅷ in the zoneof intensityⅥ.It is also typical and rareteleseismic abnomal intensity area.This phenomenon has caused widespreadconcern from domestic and foreign scholars after Wenchuan earthquake. Theexploration of the reason of Hanyuan county’s abnomal intensity, as well as thesummarization of its forming conditions and ground motion characteristics, is ofgreat theoretical significance and engineering application value in promoting thestudy of site conditiong on anti-seismic engeering and impoving anti-seismicengeering level.In this paper,the forefront results and literature of this field have beensummarized and commented.On the basis of scientific investigation in Hanyuanduring Wenchuan earthquake, the preliminary explanation of anbormal seismicdamage has been given by the method of damage investigation,scientificinvestigation after earthquake, survey, test, theoretical analysis and numericalsimulation. Researches have been carried out in the following areas.(1)Damage investigation,scientific investigation and site reconnaissancehave been carried out. Investigation group formed by teachers from Institue ofDisaster Preventionhave carried on comprehensive and deeply investigation.More detailed information about earthquake damage, geological structure,seismic geological disasters and topography data is acquired from pre-fieldwork.Site soil structure and soil dynamics parameters are got by the survey andtest.Combined with earthquake damage of scientific investigation, Hanyuanbuildings natural vibration periods are analyzed by empirical formula method.The natural vibration periods of building structural type that are more seriouslydamaged are about0.1s~0.6s.(2)The method for determining input ground motion is proposed. Twomethods of synthetic ground motion and inversion of strong motion records areused for determining Hanyuan city input ground motion. The method of syntheticground motion,the bedrock attenuation relations of peak accelerration and respnsespectrum are fitted,which is loacated on seismogenic fault footewall duringWenchuan earthquake.Analysis shows that the bedrock attenuation laws of peakacceleration and response spectrum on seismogenic fault footewall are wellreflected by the article’s attenuation relations. Hanyuan site peak acceleration andresponse spectrum value are determined according to the attenuation relations.Combined with empirical parameters,5acceleration time histories are given by synthetic ground motion.The method of inversion of strong motion records isproposed,and Jiuxiang strong motion is taken as inversion station in this paperThe station is22km away from Hanyuan town.As there is detailed site data aswell as precious strong records acquired during Wenchuan earthquake in Jiuxiangstrong motion station, this paper will use these data, and calculate EW and NSinput ground motion by one-dimensional equivalent linear frequency-domainanalysis method.At the same time, spectrum analysis is carried out by forwardand inversion method.The results showed that the inversion of bedrock motion iswith high reliability, which can be taken as Hanyuan input ground motion.(3)The effect of soil layer construction on Hanyuan abnormal damage isstudied.On the basis of site invertigation,site soil and test dara soil dynamicsparameters are acquired. The effect of the thickness of topsoil, the depth of gravellayer, the thickness of the gravel layer,gravel layer shear wave velocity and themost unfavorable combination of conditions on Hanyuan surface ground motionare studied by One-dimensional equivalent linear seismic response analysismethod. The results show that special soil layer construction has a significantamplification effect on Hanyuan county, which plays an important impact on theHanyuan county earthquake abnormal damage.(4)The effect of topography on Hanyuan abnormal damage is studied.According to scientific investigation of the earthquake field investigations andgeological data, Hanyuan topography characteristics were analyzed, and theactual mapping of Hanyuan three-dimensional topographic map is acquired. Theeffect of topography is numerical simulated by the principle of two-dimensionalfinite difference method and artificial transmitting boundary, while the transferfunction amplitude spectrums of each observation were acquired, and amplitudeamplification effect is reflected by transfer function amplitude.The computationresults show that Hanyuan terrain amplification effect is significant, andtopographic amplification is one of the main causes of abnormal seismic damage.(5) The effect of fault structure on Hanyuan abnormal seismic damage isstudied. The effect of fracture crushing belt and Jiuxiang fracture on Hanyuanabnormal seismic damage are analyzed by two-dimensional finite-difference andtransmitting boundary theory. The results show that the fracture which has aneffect on Hanyuan abnormal seismic damage is possibly Jiuxiang fracture. It isbecause if Jiuxiang fracture, as a buried fault, went through Hanyuan town, theground motion could be amplified in the band of4~9Hz significantly.In summary, abnormal seismic damage is caused by amplification effect onground motion of soil structure, terrain effects and fault. The results show that theamplification effect of Wenchuan earthquake ground motion is remarkable in theband of2.0to10Hz. The amplified peak ground acceleration value in HanyuanCounty exceeds rare earthquake peak acceleration specified in the Code forseismic design of buildings (version of1989). This makes buildings in Hanyuancounty suffer from seismic inertia force which is more than design specifications,and results in building destruction. At the same time, the band of significant amplification is consistent with building vibration periods, and the earthquakedamage is increased by resonance effect, which caused abnormal seismic damagein Hanyuan.The extent of site conditions on grond motion is comprehensively reflectedby abnomal seismic damage of destructive earthquake. It is closely related to siteconditions, but also to propagation of seismic waves and source physicalprocesses.Thus it is a highly exploratory research topic.This paper has made apreliminary summary and explanation on the phenomenon of Hanyuan seismicabnormal damage with part of data acquired by scene investigation and scientificinvestigation after Wenchuan earthquake. Because of the complexity of abnomaldamage, the study in this paper is superficial. Please feel free to correct theinadequacies.