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 (Vs) 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 Vs generally increases with depth but varies from one site to another. Based on the Vs profiles, a thick soil layer with Vs 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 Vs 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.
Research has been carried out using seismic refraction in the LIPI area - Bandung, which aims to determine the land of embankment in the area. Retrieval of field data was carried out using geometric Es-3000 tool along 46 meters with a spacing of 2 meters and a 7 shoot punch consisting of 2 phantom shoots beginning and ending. Data processing is done by the first step, namely by geometric editing so that data can be read by the computer. The inversion process is done by seismimager software which consists of pickwin to extract data and plotera for modeling the subsurface layer. The results of the data interpretation show the P wave velocity from 315 - 435 m / s. layer grouping based on P wave velocity is at the first color layer having a wave velocity of about 315 - 342 m / s, the second color layer has a wave speed of 355-382 m / s, and the third color layer has a speed of 359 - 422 m / s and thick layer more than 435. Based on the lithological classification of subsurface rock layers, this study area tends to have a layer of soil type with a depth of 5 meters, and can be said to be a layer of soil deposits because of the formation of soil structures that tend to be new
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