Jalil et al. Geoenviron Disasters (2021) 8:21 Geoenvironmental Disasters https://doi.org/10.1186/s40677-021-00194-y RESEARCH Open Access Liquefaction in Palu: the cause of massive mudfows Abdul Jalil1,4*, Teuku Faisal Fathani1,3, Iman Satyarno1,3 and Wahyu Wilopo2,3 Abstract The 7.5 Mw tectonic earthquake that hit Palu City on 28 September 2018 was followed by tsunami and liquefaction, triggered massive mudfows 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 confrmed that Bal- aroa, 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 runof that generated the mudfow 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. Keywords: Liquefaction potential, Microtremor, HVSR, Ground shear strain, Mudfow Introduction to the Palu Koro fault line, and it was interpreted that it On September 28, 2018, an earthquake of Mw 7.5 struck fell along the fault as it was close to the fault line. Te Central Sulawesi Province, Indonesia with the earth- mudfow left severe damages on a large part of the soil on quake hypocenter located at a depth of 11 km and the the gentle slopes of Palu City Socquet et al. (2019) within geographic coordinate of 0.18°S and 119.85°E. the earth- a distance of 1 km approximately. Te damage of the quake had prompted extensive damages (Fig. 1) and Gumbasa irrigation channel triggered fash foods which mudfows in Palu City Valkaniotis et al. (2018), Mason turned into wave-like mudfows (Mason et al. 2019). Te et al. (2019). Te mudfow covered the areas of Balaroa, Gumbasa irrigation channel was an anthropogenic haz- Petobo, and Jono Oge, as shown in Fig. 1 b–d, respec- ard of unexpected landslides in Palu that clearly demon- tively. Te enormous life loss due to the liquefaction in strated the need for more proactive assessment (Bradley Palu could probably be attributed to the three mud- et al. 2019). fows, which moved material within hundreds of meters Te epicentral distance to the liquefaction site in Palu downslope parallelly. Two of these mudfows originating City was plotted in an image developed (Ambraseys as lateral spreads directly beneath the Gumbasa irriga- 1988) based on the database regarding shallow earth- tion channel occurred at Petobo and Jono Oge (Fig. 1c, quakes around the worldwide (Fig. 2). As shown in Fig. 2, d). Meanwhile, the mudfow in Balaroa was located close the distance between Palu City and the epicenter of the 7.5 Mw earthquake was 80 km, which indicated consist- *Correspondence: [email protected] ency with the fndings. 1 Department of Civil and Environmental Engineering, Faculty Te Meteorological, Climatological, and Geophysical of Engineering, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia Full list of author information is available at the end of the article Agency (BMKG) reported that the earthquake that hit © The Author(s) 2021. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Jalil et al. Geoenviron Disasters (2021) 8:21 Page 2 of 14 Fig. 1 Liquefaction resulting mudfows in Palu City: a Map of Palu City, b Balaroa mudfows, c Petobo mudfows, and d Jono Oge mudfows the city and coastal area caused enormous damages to out liquefaction and mudfows research to identify and the settlements and buildings (Sadly, 2018). Tis circum- reduce the disaster risk. stance also developed liquefaction, landslide, and mud- Te Palu City soil investigation report mentioned that fows Mason et al. (2019). Tus, it is required to carry the area was composed of alluvium deposits with a sand Jalil et al. Geoenviron Disasters (2021) 8:21 Page 3 of 14 Fig. 2 The maximum epicenter distance to liquefaction sites, Re and the moment Magnitude, Mw (after Ambraseys 1988) layer at the top, silt in the middle, and clay in the bottom (Widyaningrum 2012). Widyaningrum (2012) also con- ducted the semi-empirical and liquefaction potential index (LPI) methods with the results reported a shallow ground- water level in Palu City. Although this study provides valu- able information regarding the liquefaction susceptibility in the Palu area, the researchers only used semi-empirical and geological procedures. Hence, the efect of liquefac- tion that causes mudfow was not explained in this study. In general, previous studies analyzed the liquefaction potential based on the semi-empirical approach proposed by Idriss and Boulanger (2008). Te liquefaction potential index (LPI) was determined by referring to the (Iwasaki Fig. 3 Geological map of Palu (Sukamto 1973) et al. 1981) methods. Te National Center for Earthquake Study released a collateral hazard map due to the 2018 earthquake in Palu, which included fow liquefaction and mudfow Irsyam et al. (2019). Alluvium and Coastal Deposits, Formation of the Cele- Some researches on liquefaction studies apply micro- bes Molasse, Formation of Granite, Metamorphic Com- tremor wave propagation. Futhermore, the researchers plex, and Formation of Tinombo. Te Geological Survey also made numerical models of geotechnical efects of of Indonesia investigated the formations in the 1970s by diferences in seismic parameters (Araujo and Ledezma Sukamto (1973). 2020). Tis study was carried out by correlating the wave Te Palu City area is a quarter sedimentary expanse. characteristics, soil site classifcation, and liquefaction evi- Silty clay, silt alluvial sand and old river channel depos- dence during the 2018 earthquake. Te objective of this its were found along the Palu river at the lower eleva- study is to evaluate the soil susceptibility of Palu City to tions near the valley’s center. Young and old alluvium the liquefaction phenomenon, which might cause fash fan deposits are surfcial sediments in the low relief foods that resulted in the massive mudfows and soil hills, which extend on the west and east sides of the val- dynamic efect. In addition to this, to support the regional ley. Deposits of colluvium debris from gravel sand often infrastructure planning and development. Te soil suscep- occur at higher altitudes Tein et al. (2014). tibility resulting from the dynamic efects was analyzed by Balaroa morphology is approximately the similar to combining the velocity amplifcation values obtained from Petobo, although Balaroa is located near the western hills microtremor measurements with the SPT derived from where the slope is more considerate than Petobo. Tere the site classifcation by using an ArcGIS framework. is no irrigation channel in Balaroa, as opposed to another area afected by soil fow. Te most likely factor that trig- Geological condition of Palu gered signifcant liquefaction in the region is the infu- Te geology of the Palu area is dominated by sedimentary ence of the colluvial fan topographic, which is controlled and volcanic deposits from the Cretaceous and Paleogene by natural shallow groundwater Faris et al. (2019). Petobo (Fig. 3). Palu contains fve formations: Formation of the and Jono Oge are located on a gentle slope in the eastern Jalil et al. Geoenviron Disasters (2021) 8:21 Page 4 of 14 hills. Te morphology of the area is a colluvial deposition factor against liquefaction is determined by comparing area originating from the hill. the cyclic resistance ratio (CRR) and cyclic stress ratio (CSR), which is expressed as: Research method CRRM,σ ′vc Horizontal to vertical spectrum ratio (HVSR) method FSliq = CSRM,σ ′vc (3) Data resulted from the microtremor measurement was analyzed using the Horizontal to Vertical Spectral Ratio where FSliq = the safety factor against liquefaction. Te (HVSR) method. Subsequently, a comparison was carried values were predicted based on the N-SPT penetration out between H (horizontal) and V (vertical) spectrum resistance measured in various depths. data. Te HVSR method was introduced by Nakamura (1989), to estimate the resonance frequency and local Field investigation sediment amplifcation factor of microtremor data. Tis paper uses the results of a microtremor survey in Moreover, it is adopted to estimate the soil susceptibil- 150 measurement stations carried out by Tien (2015). ity index (Nakamura 1997) and the building susceptibility Te measuring instruments include the PIC, Portable index (Sato et al.