PERSEBARAN KERENTANAN AMBLESAN TANAH DI KECAMATAN SEMANU, KABUPATEN GUNUNG KIDUL, PROPINSI DAERAH ISTIMEWA YOGYAKARTA

The research is area located in Kecamatan Semanu, Kabupaten Gunung Kidul, Yogyakarta Province. The research area has karst morphologywith many sinkholes on it,bothon fresh and weathered rock. These sinkholes may trigger land subsidence on this area which endangers the surrounding residents. The objectives of thisstudyis to identify geological factors that affect land subsidence and to create land subsidence susceptibility map. In order to achieve the research objectives, field observation is conducted and consists of morphological, lithological, structural observations of the area as well as observations of the sinkholes. The first conclusion of geological factors that affect land subsidence from the most important to the least important are as follows: a) lithological parameter, which is weathered, sediment-covered carbonates, b) structural parameter which has less than 200 m in distance, and c) slope angle parameter, which is to be less than 20o. The result is supported by AHP method evaluation which results in weighing lithological parameter as high as 65%, whereas structural and slope angle parameters are weighed 23% and 12%, respectively. In order to create land subsidence susceptibility map, the values of each factor is used in weighted overlay by using ArcGIS software. The result is then validated in order to know sinkholes distribution in each susceptibility level. The second conclusion of this study is that land subsidence susceptibility can be divided into three categories, as follows: a) highly susceptible (red zone), with land subsidence probability as high as 0,033% per km2, b) moderately susceptible (yellow zone), with land subsidence probability of 0,023% per km2, and c) lowly susceptible (green zone) with land subsidence probability of 0,018% per km2