LANDSLIDE HAZARD MAPPING BY USING GIS IN THE LILLA EDET PROVINCE OF SWEDEN Arzu ERENER 1 , Suzanne LACASSE 2 , Amir M. KAYNIA 3 1 Geodetic and Geographic Information Technologies, Middle East Technical University, 06531, Ankara/TURKEY, Phone: +90 312 210 2488, Fax: +903122101002, erener@metu.edu.tr 2 International Centre for Geohazards, NGI, Oslo, Norway, phone: +47 22 02 31 03, Fax: +47 22 23 04 48, suzanne.lacasse@ngi.no 3 International Centre for Geohazards, NGI, Oslo, Norway, phone: +47 22023013, fax: +47 22 23 04 48, amir.kaynia@ngi.no KEY WORDS: Hazard Mapping, FOSM, Probability Likelyhood ABSTRACT Quick clay slides occur frequently in Sweden and cause serious damages and costs. Reliable hazard and risk assessment approaches would help to mitigate the consequences of such landslides. This paper evaluates the hazard of clay slides in the Lilla Edet munucipality in Sweden using Geographical Information Systems (GIS). This tool provides a powerful tool to model the landslide susceptibility accounting for the spatial analysis and prediction. Landslide hazard areas are analysed and mapped using the landslide-occurrence factors, by the probability likelihood ratio method as well as by the first order second moment (FOSM) method. The governing factors such as slope angle, aspect, curvature, plan, and elevation are recognised for the probability likelihood ratio method. These factors are extracted from the light detection and ranging (LIDAR) data. The historical landslide locations are digitized from the hardcopy map of the region. The LIDAR data and aerial photos are used to confirm the historical landslide locations. In the FOSM the application of method a probabilistic slope stability model is integrated into GIS environment. The performance of the landslide mapping is assessed using the actual landslide locations and the relative operating curve (ROC) method. An ROC curve evaluates how well the methods predict landslides. The analyses show satisfactory agreement between the hazard map and existing data on landslide locations and validate the methodology used in this investigation. 1. INTRODUCTION Landslides in quick clay occur frequently in Scandinavian countries and cause damages and cost. After the Tuve landslide, the Swedish government decided to carry out a survey mapping of landslide hazard in built and planned areas within certain landslide susceptible sites (SGI Raport, 2006). The spatial probability of occurrence of clayslide in the Lilla Edet municipality in Sweden is analysed to help to reduce the consequences of such slides. For assessing landslide susceptibility, different methodologies are available in the literature. In this study two different approaches were used. The first approach was based on a univariate probability analysis. In this analysis, often called the frequency ratio model (FRM), the spatial relationship between the landslide locations and each landslide-related factor is analysed (Lee and Tu Dan, 2005; Lee and Pradhan, 2006). Frequency ratio values for each category of the influencing factors are computed using Geographical Information System (GIS) tools. Also the relationship between landslide related factors and the landslide locations are analysed by GIS tools. Then these relationships are used for each factor's rating in the overlay analysis in the FRM method. The ratings of the factors are summed up to give a landslide susceptibility index and susceptibility map. A second method applied in this study for susceptibility mapping is the First Order Second Moment (FOSM) method. A FOSM method is one of the widely used methods due to its relative simplicity in formulation and straightforward mathematics (Chen et. al., 2007). This method was applied recently by Duzgun & Ozdemir (2006); Duzgun & Karpuz, (2006), and Chen et. al., (2007). In this study the shear strength was combined with the slope