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A landslide susceptibility assessment for _ Izmir city (Western Turkey), which is the third biggest city of Turkey, was performed by a logistic regression method. A database of landslide characteristics was prepared using detailed field surveys. The major landslides in the study area are generally observed in the field, dominated by weathered volcanics, and 39.63% of the total landslide area is in this unit. The parameters of lithology, slope gradient, slope aspect, distance to drainage, distance to roads and distance to fault lines were used as variables in the logistic regression analysis. The effect of each parameter on landslide occurrence was assessed from the corresponding coefficients that appear in the logistic regression function. On the basis of the obtained coefficients, lithology plays the most important role in determining landslide occurrence and distribution. Slope gradient has a more significant effect than the other geomorphological parameters, such as slope aspect and distance to drainage. Using a predicted map of probability, the study area was classified into five categories of landslide susceptibility: very low, low, moderate, high and very high. Whereas 49.65% of the total study area has very low susceptibility, very high susceptibility zones make up 11.69% of the area.

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Waviness or undulations of a discontinuity plane due to folding result in changes in the orientation of the discontinuities. This creates difficulties in determining the general trend of the discontinuity planes on both sides of wedges and the plunge angle of wedge-type slides. However, it is possible to draw great circles representing the discontinuity planes on both sides of the wedge with strike and dip angle changes on a stereographic net. Such a wedge may have failed in the past but may be stable presently. Frequently, back analyses of the specific failure surfaces are carried out to establish the shear strength parameters that were effective at the time of the slope failure. In this way, a better appreciation of the engineering behavior and the strength parameters of the rock mass may be obtained for the design of future slopes in similar conditions. Based on these fundamentals, this study is intended to solve this problem, which is often encountered during the back analyses. Furthermore, this new approach can also be used to determine the orientations of the wedgeforming discontinuity planes and their intersection lines using the projection techniques with high accuracy. The new technique is shown to provide better results in the back analyses conducted on wedge-type slides of sedimentary rocks such as shales.