Regularity And Risk Analysis Landslides Induced By Wenchuan Earthquake

5.12Wenchuan earthquake induced tens of thousands of landslides. Comparedwith the landslides induced by normal gravity, seismic landslides have significantdifferences of morphological characteristics, dynamic characteristics, formationmechanism and so on all. At the same time, under the influence of5.12Wenchuanearthquake, rock and soil mass is prone to geological disasters. According to theactual survey, seismic landslide, collapse and unstable mountains can occur furtherdeformation, which most probably formed a complete chain of disasters in stormconditions, and cause significant economic losses in severely damaged regions. So thepaper studied on seismic landslides of distribution law, forming mechanism,controlling factors, disastrous activity trend of geological hazard and its duration,zoned for seismic landslide in severely damaged regions, which can be as a directeddisaster relief for government reducing property damage as well as providing bettertown plan, this paper is of great theoretical and practical significance.This paper mainly took Pingtong-Shuiguan section as the study area (area of1700km~2), took a number of36seismic landslides of area greater than50000m~2asthe study objects, while drawing on related study outside the study area of this paper,paying close attention to the transformation and excitation between the kinds ofdisaster and improving related laws of seismic landslides in the study field. In thecourse of the study, the paper took a variety of means, including practicalinvestigations in the field, numerical simulation by UEDC, remote sensinginterpretation, engineering geological analogy, AHP method, ARC-GIS platform. Thepaper obtained the following main study results. (1)The paper took "Earthquake Fracture (Yingxiu-Beichuan Fracture)" as thebasic reference point, put forward the "General Seismic Landslide" concept,formulated seismic large landslide distribution law in the study area. Seismic largelandslides exist the following effects: Distance Effect; Effect of Clavicular Section;Hanging Wall Effect; Directional Effects; Effect of Terrain Zoom etal.Using UDEC software simulate back-slope effect, simulation results displays:acceleration peak values (vertical and horizontal direction) of slope body induced byearthquake wave, and values of back slope is greater than welcome slope surface;Increasing distance from seismic source, acceleration peak values of welcome slopetakes on a declining trend, and back slope also meets the law; Slope body exists effectof terrain zoom in earthquake wave role.(2) The paper supplied5types of typical genetic models of seismic landslidesinduced by strong earthquakes, which predecessors put forward, and put forwardLadder-Shaped Landslide of Deep Residual Layer. Others include Crack-Disperse,Crack-Horizontal Sliding, Crack–Slipping Along Bedding, Crack-Shear. Above6types of typical seismic landslides, the paper makes each type with a brief description.(3)The paper analyzed movement characteristics of100seismic landslides withthe parameter “μ” in the study area,“μ” values of8seismic landslides only less than0.3,“μ” values overall are on the high side. Compared with the continent high-speed,long-distance landslide (μ∈0.1~0.6) in the world, movement characteristics ofseismic landslides are obviously weaker in the study area.(4)The paper took Pingxi village landslide as a typical case, introduced itsdetailed basic features, used UDEC software simulate and analyze its formationmechanism and whole motion process. At the same time, the paper used Xie DegeFormula estimate the movement speed of debris flow. The average speed of debrisflow reached25m/s, and the whole process from rock sliding to stopping stacked onlysustained16s. The numerical simulation results have a very good match with XieDege Formula estimating results.(5)The paper from6aspects presented in kinds of factors influencing of slopeseismic geological disaster, took (Seismic Landslide, Seismic Collapse, Elevation,Slope Gradient, Slope Aspect, River, Rock Groups, Fracture, Human EngineeringActivities, Rainfall)10types effect factors as hazard assessment indicators of seismiclandslides.(6)The paper defined combination weights of10factors with AHP method, andbased on the ARC-GIS platform to divide study area into several areas to the risk of seismic landslides. The paper used seismic landslides hazard risk index to quantitativerisk evaluation. According to the factors superposition results of the study, this paperestablished a geological hazard zoning criterion: low-risk zones is Wj∈(1.000-1.602); danger zones is Wj∈(1.602-2.205); high-risk zones is Wj∈(2.205-2.807); high risk zones is Wj∈(2.807-3.409).(7) According to zonation division standards of seismic landslide hazard andcombining the character of post-earthquake seismic landslide, the paper has got a finalrisk zoning map with seismic landslide. Accounting for7.29%of the total area,highest risk area is119.88km~2; Accounting for14.89%of the total area, higher-riskarea for244.85km~2; Accounting for the total area of21.62%, middle danger area is355.47km~2; Accounting for56.19%of the total area, low-risk area for923.76km~2.According to partition characteristics, it put up some advises on prevention.(8)Combining with previous studies, the paper puts forward that the trend ofdisasters activities takes on4or5years a peak in the study area. According to thetotal amount of the slump geological hazard, moving loose materials source,"hazardprovenance amount", debris flow out, the paper estimated the active period ofgeological disasters is about20.22a in the study area after the earthquake.(9)The paper analyzed the disaster evolutionary trend after the earthquake.There formed a more complete secondary disasters chain: from landslides, avalanches,shattered the mountain�'the rainstorm�'mudslides�'housing, roads, river. Thischain shows the characters of the geological disasters have a fundamental change afterearthquake. Before the earthquake, there is mainly slope disaster, but there changedinto mudslide after the earthquake. And mudslide characteristics are large-scale,centrality and successiveness. For the study area, there formed46debris flowsinduced by heavy rain after the earthquake. There is3zones in research area withcentrality and successiveness. The3zones is in Extremely high risk areas of riskdivision. And it also verifies the result exactness basing on the evaluation of theARC-GIS platform.