Geological Condition and Landslide Vulnerability Zonation Using Frequency Ratio Method in Salaman District, Magelang Regency, Central Java Province

Abstract

Salaman District is an area with an area of 68.87 km², which is included in the Magelang Regency area. The southwest part of this area features hilly topography, forming the slope of the Menoreh Hills. In contrast, the central and eastern areas have predominantly flat topography, with breccia rocks distributed widely. The district has a population of 74,429 people, and the majority earn their livelihoods as farmers. The primary commodities include cassava, as well as fruit crops like mango and durian. Other commodities originating from this area are teak wood, bamboo, mahogany, sengon and waru. From January 2020 to April 2021, rainfall in Magelang Regency ranged from 20 mm to 531 mm. Based on data obtained from the BPPD Magelang Regency website, Salaman District is one of the sub-districts that had the highest intensity of landslides in that period, with a total of 67 landslides. Given the significant number of landslide disasters, it is deemed crucial to conduct mapping and develop a landslide susceptibility map for the research area. The research utilizes direct data, including lithological information, geological structure, and geomorphology, as well as secondary data comprising landslide occurrence, topography, DEM, and land cover data in the research area. The method used in this research is the frequency ratio (FR) method. It is constructed based on the relationship between the location of the landslide incident and the factors that control the occurrence of landslides. The FR method is included in the bivariate statistical method, which is used to determine the relationship between two variables. Based on the geomorphological analysis, it was found that several landforms developed in the research area, namely River Body (F1), Alluvial Plain (F2), Structural Hills (S1), and Residual Hill (D1). The stratigraphy of the research area consists of six rock units, listed from oldest to youngest as follows: Kaligesing Pyroclastic Breccia unit, Kaligesing Andesite Lava unit, Kaligesing Epiclastic Breccia unit, Old Sumbing Laharic Breccia unit, Young Sumbing Tuf-lapilli unit, and Alluvial Deposits. The geological structures that develop in the research area are right strike-slip faults and left strike-slip faults, which have a west-east line. Based on the results of the analysis using the Frequency Ratio method, which uses parameters in the form of slope, lithology, structure distance, river distance, land use, and landslide occurrence points, the zoning in the research area is divided into 4, namely the Very Low Landslide Susceptibility Zone with a Landslide Susceptibility value. Index (LSI) 22.48 – 40.75; Low Landslide Susceptibility Zone with LSI value of 40.75 – 46.72; Medium Landslide Susceptibility Zone with LSI value 46.72 – 52.87; and High Landslide Susceptibility Zone with an LSI value of 52.87 – 67.27. After validating the landslide susceptibility map using the Area Under Curve (AUC) method, the results obtained were 70.95% and included in the good category, so the map was suitable for use.