Liquefaction due to the 2006 Yogyakarta Earthquake: Field Occurrence and Geotechnical Analysis

The event of earthquake may induce the occurrence of liquefaction hazard. Liquefaction could cause damage to buildings and infrastructure due to the sudden increase of pore pressure at the loose uncompacted layers causing the loss of support or bearing capacity. The Yogyakarta earthquake on May 27, 2006 of magnitude 6.2 had resulted in liquefaction phenomenon to occur in several affected areas. During this earthquake liquefaction occurrences were observed near the Opak Fault area. Mapping of liquefaction along with geotechnical investigation and analysis had been carried out in Bantul region, Yogyakarta. Geotechnic al investigation consisted of a series of cone penetration tests and drillings was carried out in Patalan region, Bantul, Yogyakarta. Analysis of liquefaction potential was conducted by analyzing CPT data by Robertson and Wride (1989) method. Analysis results revealed that the loose layer consists of silty sand and sandy silt at the depth between 0.2- 12.8 m are potentially liquefied during the earthquake. Results of liquefaction analysis indicated that the thickness of potentially liquefied soil layer vari es between 0.2 -5.2 m and total settlement induced by the liquefaction is varied from 1.0 - 10.8 cm concentrated at the middle part of Opak Fault. Geotechnical analysis suggested that the liquefaction had occurred within the upper Quaternary sediment fillin g the Bantulgraben.