Understanding of flow liquefaction phenomena in Palu City from shear wave velocity profiles

¹ Research Center for Geotechnology, National Research and Innovation Agency (BRIN), Bandung, West Java, 40135, Indonesia
² Faculty of Science and Technology, State Islamic University of Sunan Gunung Djati (IUN), Bandung, West Java 40614, Indonesia
³ Former undergraduate student, Physics Study Program, State Islamic University of Sunan Gunung Djati (UIN), Bandung, West Java 40614, Indonesia

^* Corresponding author: adrin.tohari@gmail.com

Abstract

On 28 September 2018, a strong earthquake (Mw 7.5) struck Donggala Regency triggering flow liquefaction (locally termed Nalodo) phenomena in two residential areas in Palu City. Mitigating such an earthquake collateral hazard requires understanding the geological condition controlling such liquefaction. This paper presents a geophysical study to clarify the geological condition controlling the Nalodo phenomena in Palu City. The objectives of this study were to estimate shear wave velocity (V_s) profiles in the Nalodo and non-Nalodo sites and evaluate liquefaction potential based on the shear wave velocity profiles. The study involved a series of array microtremor surveys in two Nalodo sites and three non-Nalodo sites in Palu City. The results show that V_s generally increases with depth but varies from one site to another. Based on the V_s profiles, a thick soil layer with V_s of less than 175 m/s is present from the ground surface in the Balaroa and Petobo sites. Contrary, this soil layer is detected at a deeper depth in the non-Nalodo sites. Based on the liquefaction potential analysis, the near-surface soil layers in the Balaroa and Petobo sites are clearly liquefiable for all the PGA values used in this study. Although deeper soil layers can also be liquefied, no liquefaction occurred in non-Nalodo sites because a thick medium soil layer exists near the surface. Thus, a thick soil layer with V_s of less than 175 m/s near the ground surface controls the Nalodo phenomena in Palu City. Further studies to clarify other factors controlling flow liquefaction are recommended.