GIS based Seismic Stability Zonation of Slope around Lanzhou City Wang Ya-qiang, Zhang Xiao-ye State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, CAS, Xian 710075, China; Wang Lan-min Lanzhou Institute of Seismology, China Seismological Bureau, Lanzhou 730000, China) 1. Introduction Landslides are one of the most damaging collateral hazards associated with earthquakes. In fact, damage from triggered landslides has sometimes exceeded damage directly related to strong shaking and fault rupture [Jibson et al., 2000]. Therefore, many previous studies were related to the evaluation methods of landslide hazard. In general, there are mainly three types of evaluation model: deterministic model, statistical model and heuristic model. Deterministic model was often used in a large scale area and the hazard in absolute values in the form of safety factors, or the probability of failure could be provided [Jibson et al., 2000; Luzi and Pergalani, 1996; Miles and Ho, 1999; Refice and Capolongo, 2002]. In statistical model, the combination of factors that have led to landslides in the past are determined using bivariate or multivariate statistical analysis, and then the result are used to predict future landslides [Clerici et al., 2002; Dai and Lee, 2002; Donati and Turrini, 2002; Gupta and Joshi, 1990]. This model is suitable for medium scales. In heuristic model, the contribution of the factors to landslide is evaluated by the experts and then the landslide hazard is forecasted [Barredo et al., 2000; Esmali and Ahmadi, 2003]. In this study, we used deterministic model to predict landslides of the slopes around Lanzhou city. A pseudo-static analysis method proposed by Koppula was chosen to analysis the stability of the slopes [Koppula, 1984]. Geophysical information system (GIS) database and spatial analysis techniques were used during the data integration and analysis process. 2. Method for calculating safety factor of slopes Siyahi and Ansal developed a zoning method for slope instability by modifying a method proposed by Koppula [Technical Committee for Earthquake Geotechnical Engineering, 1999]. The mothod originally proposed was a pseudo-static evaluation of slope stability utilizing a seismic coefficient A to account for the earthquake induced horizontal forces [Koppula, 1984]. The variation in shear strength with depth is assumed linear and potential failure surface is taken as a circular arc. A linear variation with depth is assumed regarding the shear strength of normally consolidated soils. The safety factor, Fs, can be defined as: 
where, a is the center angle, d is the obliquity of the secant of circle arc, n is distance from the bottom of slope to the bottom of circle arc, ß is the angle of slope and f is the angle of shear strength. Thus the safety factor depends on the angle of shear strength and stability number, N1 representing the configuration of the slope and failure surface. The minimum values of the stability number are determined by carrying out a parametric study in terms of a?d and n to find out the most critical failure surface. 3. Building GIS database Lanzhou city is located in the area covered by big thickness loess. The three main type of the seismic hazard of loess are subsidence, liquefaction and landslide. Geographical map, micro zonation map of seismic acceleration, loess thickness map, geomorphologic map, ground water depth map, slope type map and geotechnical data were collected and digitalized to build GIS database. Subsidence and liquefaction prediction zonation has been described by Wang et al [2001]. The data related with landslide were mainly seismic acceleration, contour, slope distribution and geotechnical data. Microzonation result of seismic acceleration [Sun and Chen, 1991] was shown in figure 1. The corresponding seismic acceleration of each zone under three probability of exceedance (2%, 10% and 63.5% probability of exceedance in 50 years) was summarized in table 1.  Figure 1. Microzonation map of seismic acceleration at Lanzhou city [Sun and Chen, 1991]. The main slope types around Lanzhou city are loess slope, loess and gravel slope, clay rock slope, grit rock slope, rock bits slope and un-integrity slope. The distribution of the slope types were plotted in figure 2. The corresponding shear strength angle vales were assigned to each slope type based on the geotechnical data.  Figure 2. The distribution map of slope types around Lanzhou city.
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