GIS based Seismic Stability Zonation of Slope around Lanzhou City The contour theme with altitude attribution was converted to TIN theme in ArcView with 3D analysis extension. Then the TIN theme was converted to grid theme which could generate slope angle theme using slope method in Avenue. For to calculate the safety factor using Koppula method, we need to know the angle of shear strength f, seismic coefficient A and slope angle ß. The other parameters a, d and n could be assigned values by scanning method. Using spatial analysis extension of ArcView, the grid theme of shear strength angle, seismic coefficient and slope angle were generated with 200m×200m cell size. Then formula 1 was used to calculate the safety factor of each cell. This process was carried on by writing program with Avenue in ArcView. The calculated factors of safety were considered in three groups with respect to risk levels as: high for Fs<1, moderate for 1=Fs=1.5 and low for Fs>1.5. 4 Stability zonation results Under 2% probability of exceedance in 50 years condition (figure 3), the slopes at south of Lanzhou and the slopes at north of Chengguan district have high risk level of instability. The slopes at north of Anning district have moderate risk level of instability. There are few slopes with low risk level for instability due to their small slope angle around Lanzhou city. Under 10% probability of exceedance in 50 years condition (figure 4), the slopes with high risk level for instability mainly are located at south of Xigu district and Qilihe district. The slopes around Chengguan district have moderate risk level for instability. The slopes at north of Anning district have low risk level for instability. Under 63.5% probability of exceedance in 50 years (figure 5), no slopes have high risk level for instability. The slopes with moderate risk level for instability are located scatteredly at south of Xigu district and Qilihe district with small area. Most slopes have low risk level for instability.  Figure 3. Seismic stability zonation map of slopes around Lanzhou city (2% probability of exceedance in 50 years)  Figure 4. Seismic stability zonation map of slopes around Lanzhou city (10% probability of exceedance in 50 years) |