An Assessment of Cikangkareng Rockfall Due to the Tasikmalaya Earthquake on September 2, 2009

Abstract

The earthquake that shook Tasikmlaya - West Java on Wednesday, September 2, 2009, amounting to 7.3 on the Ritcher scale at 02.55 pm was one of the geological natural disasters that affected Cikangkareng village - Cibinong District, Cianjur, West Java - Indonesia. According to the Bureau of Meteorology and Geophysics (BMG), Among 30 districts in Cianjur, 15 districts have great potential for land movement. The topographic condition is a fairly steep slope of hills with an almost upright angle ± 80 - 90o. In addition, a residential area was located at the foot of the hill. Thus, the geological conditions are fragile. Based on the map of land movement vulnerability (DVMBG, 2004), the Cianjur area is categorized as a zone of high movement potential of soil which means that movement of soil is easily triggered by rainfall and earthquakes. The Tasikmalaya earthquake event triggered a landslide on a large scale and caused rockfall. The material of the site collapse consists mainly of sedimentary rocks, sandstone rocks, sandstone, breccias, sandy tuffs, and yellow spots. The mineral testing results revealed that the existing minerals are Feldspar and Cristobalite. From the result of the slake-durability, the material has medium durability. The main cause is not only the acceleration in the horizontal or vertical direction of the rocks but also the seismic force can cause an increase in the water pressure in the pores and the rock fracture can give a change of pressure in the contact field of rock joint. As a result of this, the earthquake event triggered an avalanche on a large scale and caused rockfall. Due to the contact stress decreasing drastically, practically, the friction resistance of the joint plane could be significantly reduced.