Landslides are among the most costly and damaging natural hazards in the mountainous terrains of tropical and subtropical environments, which cause frequently extensive damage to property and occasionally result in loss of life. Over the last three decades, numerous efforts have been devoted by many researchers to develop landslide susceptibility maps. These maps describe areas where landslides are likely to occur in the future and classify those areas into different susceptibility zones from very low to very high susceptible zones according to their susceptibility to landslides. Such as landslide susceptibility maps are useful for planners and developers to choose favorable locations for future developments. Roads are the main type of transportation system in Malaysia. About 30% of these roads traversed through or located in hilly and mountainous areas (Jamaludin et al., 2006). During the last two decades landslides are events have increased in Malaysia, especially on cut slopes and on embankments alongside roads and highways in mountainous areas. From 1993-2004, there were 13 major landslides reported in Malaysia, involving both cut and natural slopes with a total loss of more than 100 lives (Huat and Suhaimi, 2005). These landslides which damage roads and highways are still a special problem in Malaysia, that requires more attention of engineers and geologists in order to prevent them and mitigating their effects.
GIS is an effective tool that is used commonly in landslide susceptibility mapping to identify in advance potential landslide-prone areas, by applying different models and approaches. The two main methods are generally applied to landslide susceptibility assessment are qualitative methods, which are a direct hazard mapping techniques and quantitative methods, which are indirect mapping techniques.
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In the literature, there have been many studies applied using probabilistic models (Mancini et al.,2010; Regmi et al., 2010). One of the common multivariate statistical methods applied to landslide susceptibility mapping is the logistic regression model (Nandi and Shakoor, 2010). Frequency ratio method has also been applied (Poudyal et al., 2010; Pradhan, 2010a). Geotechnical model and the safety factor model are quantitative methods used for hazard mapping (Sulaiman and Rosli, 2010). More recently, fuzzy logic and artificial neural network models have also been applied as new landslide susceptibility assessment approaches (Pradhan et al., 2010; Pradhan 2010b).
In this study the frequency ratio model, which is a simple and understandable probabilistic model was used to produce a landslide susceptibility map for the study area and to evaluate the importance of casual factors controlling the landslides.
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Study area: The study area is located in the central northern part of Peninsular Malaysia along the E-W highway which has a length of 117 km and connects the town of Gerik to Jeli. Geographically, it is situated at the co-ordinates between 5[degrees]:24 "6'N to 5[degrees]:45":56.5'N latitude and 101[degrees]:7":53.6'E-101[degrees]:50":26'E longitude, as illustrated in Fig. 1. It covers about 1205 k[m.sup.2], which are characterized by rugged hills and mountain terrains covered by thick rain forest.
The study area is frequently subjected to landslides following heavy rains, especially alongside the high way since it was constructed (Fig. 2). The common types of landslides identified in the area were rock slumps, rock falls, wedge slides, toppling, soil slides and soil slumps.
From the lithological standpoint, the study area is dominated by the three rock types, namely sedimentary, igneous and metamorphic. Igneous and metamorphic rocks cover the middle and eastern part of the area while the sedimentary rocks are commonly found in the west. The igneous rocks are mainly composed of granite except a small area at the east end part which is composed of granite, granodiorite and syenite.
The argillaceous rocks represented by the upper Ordovician to lower Devonian Kroh Formation consist of black carbonaceous shale and siliceous mudstone with chert. The arenacous rocks in the eastern part of the area are represented by Kroh Formation (Baling Group) which consist of metarenite with Ordovician to Silurian age, whereas in the western part of the area is represented by the Mangga Beds consisting of metasandstone and metagreywacke of the Permian age. The pyroclastic rocks consist of metatuffs of rhyolitic composition.
The metamorphic rocks, which are strongly foliated, are...