The Investigation Of Seismic Damage Anomaly In Yutian Based On Spectral Element Method

Yutian low intensity anomaly region is located in the central and northern part ofYutian County, Hebei province. The same anomalous phenomenon was shownevidently both in1679Sanhe-Pinggu earthquake and1976Tangshan earthquake in thisarea, which was a typical low intensity anomaly region. After Tangshan earthquake,many domestic and foreign scholars made a thorough research on the causes ofanomaly, and achieved abundant research results. Based on existed research results,this paper uses the spectral element method (SEM), as an exploration, to make a morein-depth research on the anomalous reasons. The SEM belongs to the generalized finiteelement method, which is an improved seismic numerical simulation method and hasnumerous advantages such as high computation precision, fast convergence and easycalculations on parallel computers. It may be extremely reduce the memoryrequirements and computation time for simulating by this method. In recent years,SEM has made rapid development, and becomes a significant tool of simulatingseismic ground motion for complex geological structure model with a large scale. Fordestructive earthquakes, the investigation on mechanism of seismic damage anomaly isa key issue to study the effect of site conditions on ground motion. This paper providesan improved method for investigating the mechanism of seismic damage anomaly,which has a great theoretical and practical significance in promoting the study onmechanism of seismic damage anomaly and the effect of site condition on groundmotion.This paper first makes a detailed summary on the generate reasons and researchprogress of seismic damage anomaly. Then through the analysis of existed researchresults and collected data about Yutian low intensity anomaly region, mainly from thebedrock terrain and soil characteristics, the author utilizes SEM to simulate the groundmotion of3calculation sections in Yutian, investigates the mechanism of anomaly, andobtains numerous meaningful conclusions. The main research achievements are asfollow.(1) Establishing the soil velocity structure model of105meters depth in Yutianregion. Based upon the collected68borehole data, this paper makes a detailed analysisabout the soil characteristic from the drilling depth, cover thickness, soil shear wavevelocity and site category, and fits the empirical formula showing the change of shearwave for cohesive soil, sand and gravel soil with the variation of soil depth. Combining the velocity features of different soils, and making a hierarchical statistical,this paper uses the average shear wave velocity as classification index, and establishesthe soil velocity structure model of105meters depth in Yutian region.(2) Investigating the effect of bedrock terrain characteristics on the surfaceintensity of ground motion. Based on a theoretical analysis about the simple bedrockconvex terrain model, and through the numerical simulation of the simple bedrockterrain model, this paper discusses the effect of bedrock terrain on the surface intensityof ground motion from multiple aspects. The results indicate that the presence ofbedrock with concave or convex terrain changes the incident angle of seismic wave,which makes the reflection and refraction appear more than once, and the seismicwaves converge or diverge at the surface of bedrock terrain, thus leading the surfaceintensity of ground motion strengthened or weakened, and forming the ground motionintensity relative enhancement or weakening zone. The range of the relativeenhancement zone is smaller than the bedrock concave area, and the ratio of tworegions decreases gradually with the increase of bedrock topographic dip and coverlayer thickness. While the range of the relative weakening zone is larger than thebedrock convex area, the ratio of two regions increases gradually with the increase ofbedrock topographic dip and cover layer thickness.(3) Investigating the effect of Yutian bedrock terrain on seismic damage anomaly.Through the numerical simulation of6bedrock terrain models (3sections) in Yutian,this paper discusses the effect of each section’s bedrock terrain on the intensity ofground motion. The results show that the surface intensity of ground motion varieswith the change of the Yutian bedrock terrain. It may be more obvious in the directionfrom south to north. The bedrock convex terrain under the Yutian City reduces theintensity of ground motion in most of the City and forms the ground motion intensityrelative weakening zone, while the bedrock concave terrain in the northern Yutian Cityincreases the ground motion intensity and forms the ground motion intensity relativeenhancement zone in this area. Compared with the relative weakening zone, the grondmotion intensity in the relative enhancement zone is about2times. The relativeweakending zone is corresponded to the seismic damage anomaly region, which rangeis larger than the bedrock convex area.(4) Investigating the effect of Yutian cover layer on seismic damage anomaly.Through the numerical simulation of the2soil models inside and outside the Yutiananomaly region, this paper discusses the similarities and differences of ground motionresponse from the two models under the effect of the same earthquake, using theintensity of ground motion and frequency spectrum characteristics as indicators. The results show that, in the depth range of0to105m, the two models have roughly thesame intensity of ground motion and frequency spectrum characteristics. The soilinside and outside the Yutian anomaly region have obvious amplification effect onseismic wave on the low frequency (Inside:0.67to3.17Hz; Outside:0.67to2.94Hz),but have certain absorption effect on seismic wave on the high frequency (about6Hz).The phenomenon of amplification and absorption tends to be obvious in the horizontaldirection.(5) Investigating the combined effect of Yutian bedrock terrain and cover layer onseismic damage anomaly. Through the numerical simulation of3actual sections inYutian, this paper discusses the comprehensive effect of each section’s bedrock terrainand cover layer on the intensity of ground motion using PGA values as indicators. Theresults show that, the interaction of bedrock terrain and soil not only increase thesurface ground motion intensity, but also extremely strengthen the convergence ordivergence effect of seismic wave on bedrock concave or convex terrain, turning out tobe a distinct low intensity anomalous phenomenon. Though the range of low intensityanomaly region is larger than the bedrock convex region, compared with the0.2envelop of equal seismic damage index, the range is smaller in the direction from southto north, and approximately equal in the direction from east to west.In summary, based on the premise of rational consideration of anisotropy,geological structure background, site conditions and other factors, to investigate themechanism of seismic damage anomaly by establishing a correct calculation modeland utilizing SEM is feasible and effective. Yutian low intensity anomaly region is theresult of joint action of bedrock terrain and soil. The change of the bedrock terrain notonly increases the intensity of ground motion in the northern Yutian City, but alsoreduces the intensity of ground motion in most of the Yutian City, thus forming theground motion intensity relative weakening zone. Due to the existence of deep coverlayer, the predominant frequency of soil in Yutian are mainly located in the lowfrequency range, which are contrary to the building’s self vibration frequency in theperiod of Tangshan earthquake. This not only avoids the resonance phenomenon, butalso reduces the degree of seismic damage on buildings. The joint action of bedrockterrain and soil decreases the degree of seismic damage in most of the Yutian City, andthen forms the typical low intensity anomaly region.