Some Lessons from Yogyakarta Earthquake of May 27, 2006

The earthquake of moment magnitude, Mw 6.3 in Yogyakarta and Central Java on May 27, 2006 caused widespread death and destruction to the heavily populated and relatively prosperous region. More 5,800 people were killed and robbed hundreds of thousands of residential buildings, and hundreds other structures were also destroyed. This paper presents some observations of the earthquake effects in geological and geotechnical aspects. Yogyakarta region has located on a soft sediment deposit. Therefore, low frequency contents of the seismic wave may have been amplified. The vertical and horizontal PGA at a seismograph station YOGI was 0.183 to 0.303 g and 0.197 to 0.336 g respectively. Based on the high ground peak acceleration on soft soil, several severity levels of observed infrastructure damages were geotechnical related. From field observations, it was found that the geotechnical effects included major landslides, liquefactions and fluctuations in the water levels and quality of wells. Local liquefactions were found in several sites in which the water table was relatively shallow. Ground settlements and horizontal displacement were also observed in several locations where several signs of liquefaction were found nearby. The region affected lies on debris of a subduction zone, hence amplification of horizontal shaking, as observed by the high amplification ratios have played a significant role in the widespread destruction observed