# A Combined Method for Preparation of Landslide Susceptibility Map in Izmir (Turkije)
## 3 Materials and Methods
### Geo-Environmental Factors: Definition and Statistical Analysis
that resist shear and, as a result, mass movement occurs along the surface in the direction of the slope [89]. **Slope aspect:** The importance of slope direction on landslide formation is still under debate. While some researchers suggest that the slope aspect is an important factor for landslide incidence [90, 91, 92], some other researchers think that the slope direction is not that important [52]. Many other parameters such as the groundwater level, the water content of the rock mass and the vegetation growing on the land are causative factors affecting stability. Average precipitation also has a significant effect on these parameters. As it is known that the direction and amount of precipitation in each region may differ according to meteorological conditions, slope aspect in this study is considered an important factor that can affect susceptibility to landslides. **Lithology:** This is considered one of the most important factors in LSMs due to its impact on the geo-mechanical properties of a land [93]. Rock type and structural differences usually lead to differences in the strength and permeability of formations [94]. As a result of these differences, the physio-mechanical properties of the rock change. This change will directly affect the shear strength of the rock. On the other hand, the chemical properties and mineralogy of rock affect the rock's resistance to weathering. Rock that loses its physical properties by decomposition will create a higher potential for landslide formation. For this reason, lithology was chosen as the most important geo-environmental factor in preparing the LSMs. The geological map prepared from GDMRE [82] was used for the study area and this map was digitized with ArcMap [95] and converted from \"vector\" format to raster format to be used in the LSM. **Slope curvature:** This is usually obtained by taking the second derivative of a line that occurs at the intersection of the land surface and a plane. A negative value (A) demonstrates that the surface is upwardly convex. A positive profile (B) shows that the surface is upwardly concave. A value of zero means that the surface is linear (C). According to [PERSON] [96], curvature strongly affects the shear and resistance stresses of landslides, in addition to the water convergence or divergence (drainage) of material in the direction of the landslide's movement. However, researchers do not fully agree on the effect of the curvature parameter on the landslide in the same slope aspect map. Curvature maps are divided into three types: plan curvature maps, profile curvature maps and general curvature maps, a combination of the previous two [89]. **Elevation:** This is expressed as the height of the slope relative to the sea level; it is one of the parameters commonly used to determine sensitivity to landslides. Altitude may affect vegetation type and precipitation levels [97], therefore elevation directly affects landslide susceptibility. The general trend in landslide susceptibility mapping studies is that areas at higher levels are more sensitive to landslides and as the elevation decreases, the landslide sensitivity decreases. **Density of discontinuity:** A discontinuity can be defined as a plane of weakness in rock masses due to bedding with low shear stress, faults, schistosity, etc. When the past landslides are examined, it can be seen that the movement generally follows these planes of weakness, whether at the landslide boundaries or in the slip plane. For this reason, regions where discontinuity planes are concentrated have high potential to create landslides. **Stream power index (SPI):** Erosion force is calculated in stream flows using SPI. A stream with a high flow power will erode the surrounding slopes much more along the direction of flow. As a result of the abrasion of the toe, the stability of the slope will deteriorate and the slopes will become more sensitive to landslides. Therefore, landslide susceptibility is much higher in slopes in regions with high stream power index compared to other areas [98].