Liquefaction in Palu: The Cause of Massive Mudflows

Abstract The 7.5 Mw tectonic earthquake that hit Palu City on 28 September 2018 was followed by tsunami and liquefaction that triggered massive mudflows in Balaroa, Petobo, and Jono Oge areas. Extensive damages to infrastructures occurred as the result of these earthquake-triggered disasters. This study explores the causing factors of the massive mudflow in Balaroa, Petobo, and Jono Oge areas as it is a quite rare phenomenon. This study focuses on the causing factors of liquefaction such as the condition of soil lithology, depth of water table, distance to the fo[1]cal mechanism, and the thickness of soft sediment. To carry out the liquefaction analysis, important data, such as microtremor data which included the Horizontal Vertical Spectral Ratio (HVSR), geological condition, and borehole data, were examines. Additional data i.e. ground layers slopes and other factors were also investigated. Normally, these data are not considered when observing common liquefaction. However, for the case of massive mudflows in Balaroa, Petobo, and Jono Oge, they become the key factors. Based on the microtremor data, the analysis results show that the distribution of ground shear strain values in Palu City ranges from 0.75×10-4 to 2.56×10-4. The distribution of the locations of the liquefaction corresponds to the distribution of ground shear strain values. High ground shear strain values were discovered in Palu City valley. Such high value and groundwater level indicate that liquefaction in Palu City will certainly take place. The semi-empirical method confirms that Balaroa, Petobo, and Jono Oge have high potential for large-scale liquefaction to occur at a maximum depth of 16 meters below the ground surface. Having loose soil grain with high water content, the soil will turn into a massive amount of mud during the liquefaction. In addition, ground slopes and ground vibration due to the earthquake will create massive mudflows similar to flash flood. However, the mudflows movement is slow since the slope inclination is slight.