Abstract
The 7.5 Mw tectonic earthquake that hit Palu City on 28 September 2018 was followed by tsunami and liquefaction, triggered massive mudflows in Balaroa, Petobo, and Jono Oge areas. This study focuses on the generating factors of liquefaction such as the condition of soil lithology, depth of water table, the distance to the focal mechanism, and the thickness of soft sediment. Microtremor data, including the Horizontal Vertical Spectral Ratio (HVSR), geological condition, and borehole data, were examined to conduct the liquefaction analysis. The analysis results based on the microtremor data showed that the distribution of ground shear strain values in Palu City ranged from 0.75 × 10–4 to 2.56 × 10–4. The distribution of the locations of the liquefaction was correlated to the distribution of ground shear strain values. High ground shear strain values and a shallow groundwater level were discovered in Palu City valley, which indicates that liquefaction in Palu City will undoubtedly occur. The semi-empirical method confirmed that Balaroa, Petobo, and Jono Oge had undergone large-scale liquefaction at a maximum depth of 16 m below the ground level. The average peak of water runoff that generated the mudflow was estimated to be at 11.31 cm3/s. Since the soil has loose soil grain with high water content, the soil will turn into a massive amount of mud during the liquefaction.
Highlights
On September 28, 2018, an earthquake of Moment magnitude (Mw) 7.5 struck Central Sulawesi Province, Indonesia with the earthquake hypocenter located at a depth of 11 km and the geographic coordinate of 0.18°S and 119.85°E. the earthquake had prompted extensive damages (Fig. 1) and mudflows in Palu City Valkaniotis et al (2018), Mason et al (2019)
The phenomenon of massive liquefaction led to mudflows in the Balaroa, Petobo, and Jono Oge areas
Liquefaction is controlled by the geological formation of alluvial deposits that are dominated by gravel sand and muddy sand
Summary
On September 28, 2018, an earthquake of Mw 7.5 struck Central Sulawesi Province, Indonesia with the earthquake hypocenter located at a depth of 11 km and the geographic coordinate of 0.18°S and 119.85°E. the earthquake had prompted extensive damages (Fig. 1) and mudflows in Palu City Valkaniotis et al (2018), Mason et al (2019). The mudflow covered the areas of Balaroa, Petobo, and Jono Oge, as shown in Fig. 1 b–d, respectively. The enormous life loss due to the liquefaction in Palu could probably be attributed to the three mudflows, which moved material within hundreds of meters downslope parallelly. Two of these mudflows originating as lateral spreads directly beneath the Gumbasa irrigation channel occurred at Petobo and Jono Oge (Fig. 1c, d). The epicentral distance to the liquefaction site in Palu City was plotted in an image developed (Ambraseys 1988) based on the database regarding shallow earthquakes around the worldwide (Fig. 2).
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