MEKANISME GERAKAN MASSA BATUAN AKIBAT GEMPABUMI; TINJAUAN DAN ANALISIS GEOLOGI TEKNIK
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More than 150 events of rock mass movement (rock slides and rock falls) were recorded at the west and north parts of Southern Mountain, as the impact of earthquake last May 27, 2006 in Daerah Istimewa Yogyakarta. The susceptible areas for those rockslides and rockfalls were formed by steep volcanic rocks such as interbeded tuff sandstone – tuff claystone, andesitic breccia and limestone. One major fault with the direction of NE-SW with several corresponding micro faults were apparent to be the significant control on the stability of rock mass, which then lead to the movement through mechanism of rock falls and rock slides. One of the largest event occurred at the slope of tuffaceous sandstone situated in Desa Sengir, Kecamatan Prambanan where 2 hectars of land moved and resulted in 16 houses damaged. Interpretation of detailed aerial photograph ( 1 : 5,000) and geological site investigation by drilling through three bore holes with the total depth of 46 m were carried out to preliminary investigate the mechanism of rock mass movements and the major controlling factors. The movement of rock mass in Sengir occurred as the wedge rock sliding through the intersection of two joint sets (N 100 o E and N 175 o E) and the bedding dip of tufaceous sandstone – mudstone with the direction of N 210oE/ 10o. The major controls of this rock sliding were rock joint existence and orientation, the direction of bedding plane at the slope and the difference of rock weathering grade. Those major controlling factors are important to be considered for establishment of landslide susceptibility map at the surrounding area.