Indonesia Conference Directory

Wonogiri Dam is one of the largest dams in Indonesia, located ± 2 km south of the Wonogiri City, in the province of Central Java. Wonogiri Dam has a volume ± 400 million m3 with a surface area of 73,6 km2 which has many benefits such as for agriculture, tourism, and power plant. Wonogiri is a part of the East Java Southern Mountain Zone. There are many rocks types found at Wonogiri, such as volcanic breccias, andesite lavas, tuffs, and many igneous intrusive rocks of the Miocene Mandalika and Semilir Formations, unconformably underlie Quaternary volcanic rocks of Lawu and Merapi Volcanoes (Sutarto, 2013). This research aimed to determine Ground Shear Strain (GSS) value along main dam and the surrounding area and to predict phenomena based on GSS values. From dominant period (TG) value, we can predict site class based on the classification of Zhao et al (2004). The microtremor data was taken on 31 observation points which is divided from 14 observation points with 100 m spacing in main dam and 17 observation points with 1.000 m spacing in surrounding area from Wonogiri Dam. Microtremor data was analysed using Horizontal to Vertical Spectral Ratio (HVSR) method to get amplification factor and dominant frequency. Those data were used to determine the seismic vulnerability index (Kg). Kanai empirical equations was used to calculate Peak Ground Acceleration (PGA) value based on earthquake event on 27th of Mei, 2006 with a moment magnitude of 6,3 and and 11,8 km depth. Seismic vulnerability index and peak ground acceleration value are used to determine the GSS value. The results of this research showed that GSS value in research area was about 3,4 x 10-4 to 7,2 x 10-3. The phenomena that can happen were ground movement as the form of surface cracking and settlement. Seismic vulneribility index was below 20 (0,35 to 16,97) which means surrounding area from Wonogiri Dam and main dam have low-risk, but potential for damage to building is still possible. Along main dam area, we need to notice the point at 400m and 600m. The value of the dominant period in research area ranged from 0.07 to 1.11 second. Based on the classification of Zhao et al (2004), there are 3 classes of site class in the research area.