Liquefaction potential in the governor's office of West Sulawesi after the 2021 Mamuju-Majene earthquake

. The Governor's Office of West Sulawesi Province is close to an active fault, which triggers its vulnerability to seismic activity. The Mamuju Majene Earthquake destroyed the Governor's office building with a magnitude of Mw 6.2 in 2021. The geological conditions at the site are alluvial deposit formations. In addition to the geotechnical conditions, it has the characteristics of sandy soil with fine to medium gradations. Liquefaction causes a loss of soil strength and an increased pore water pressure caused by vibrations on the earth's surface in the form of seismic waves. It causes a decrease in effective stress, ground settlement, and lateral spreading. This study aims to determine the potential for liquefaction in The Governor's Office of West Sulawesi using a simplified procedure method with a Standard Penetration Test (SPT). The Peak Ground Acceleration (PGA) value is determined using the RSA Human Settlements. The Liquefaction Potential Index (LPI) produces five zones of liquefaction potential levels. The analysis showed that The Governor's Office of West Sulawesi experienced liquefaction at a depth of 12-18 meters with a moderate to very high level.


Introduction
Sulawesi is one of the islands that experiences the most earthquakes.It is due to a large crustal fault known as the Palu Koro fault.Other faults include the Poso fault, Matano fault, Lawanopo fault, Walanae fault, Gorontalo fault, Batui fault, Tolo fault, and Makassar fault [1].Based on the Indonesia Earthquake Source and Prone Map, the Governor's Office of West Sulawesi Province is near an active fault (Makassar Strait), a sea-level fault in West Sulawesi.
One of the impacts caused by earthquakes is liquefaction.Liquefaction is a condition that results in a significant loss of soil stiffness and strength due to seismic vibrations or rapid loading or shaking, causing large soil deformations due to excessive soil pore water pressure [2].
The Rehabilitation and Reconstruction of the Office of the Governor of West Sulawesi Province is a construction activity after it collapsed due to the Mamuju Majene earthquake with a magnitude of Mw 6.2.The epicenter was located six kilometers northeast of Majene Regency at latitudes 2.98 North Latitude and 118.94East Longitude, with a depth of 10 kilometers.
According to Supartoyo [3], the West Sulawesi Governor's Office collapsed due to an earthquake shock of magnitude Mw 6.2.In addition, the inspection showed the presence of collateral hazards such as cracks, liquefaction, and landslide around the location of the Governor's Office.
On the map of liquefaction vulnerability zones, the Governor's Office of West Sulawesi Province is in the medium liquefaction vulnerability zone.These zones may experience uneven liquefaction potential, and the soil structure is generally damaged.The types of damage to the soil structure that occur are in the form of lateral spreading, landslide, and sand boil.[4].
Based on the soil data, the average soil characteristics are sand with medium to loose density and a moderately high groundwater table.This study The analysis of liquefaction potential using the Simplified Procedure method by Idriss and Boulanger [5].
The research location is on the rehabilitation and reconstruction project of the West Sulawesi Provincial Governor's Office Building.The research location is in the liquefaction vulnerability zone.The soil data used is from the Governor's office of West Sulawesi Province.Three test points used SPT, namely BH-01, BH-02, and BH-03.

Tectonic, geological, and geoformological conditions
The West Sulawesi Governor's office is close to the active fault is called the Makassar Strait, which is a type of fold-thrust belt consisting of four segments: the northern segment, the middle segment, and the Mamuju segment, and the Somba segment, as presented in Fig. 2  [1].
Based on the Earthquake Intensity Map of the Ministry of Energy and Mineral Resources of West Sulawesi Province, the Governor's office is in a high earthquake-prone area with a scale of VIII MMI (Modified Mercally Intensity).The earthquake's epicenter with a magnitude of Mw 6.2 was in Majene, as shown in Fig. 3 [4].Referring to Indrastomo [6], the lithology of the Mamuju sheet is generally composed of volcanic rocks, volcanoclastic sedimentary rocks, and limestone above volcanic stones.Mamuju Regency comprises quaternary deposits in coastal alluvial deposits, rivers, and alluvial fans.Quaternary deposits and older rocks (pre-tertiary and tertiary) that have undergone substantial weathering are generally loose, decomposed, soft, not yet compact, and strengthen the effects of shocks.The geomorphological condition of the Governor's Office of West Sulawesi Province is on an alluvial plain with soil density conditions and weak or fracture rock hardness and a groundwater table with a depth of more than four meters.These facts lead to the features of a potential liquefaction area [7].Fig. 3. Earthquake intensity map of the Mamuju-Majene earthquake.

Gradations of liquifiable soils
According to Ishihara [8], liquefaction occurs in fine to loose sandy soils and sands containing small amounts of fine-plastic grains.
Tsuchida [9] deduced the grain sizes of liquefiable and non-liquefiable soils based on a grain distribution curve, as shown in Fig. 4. The area within the curve describes sand and silty sand grains in the zone of the liquefiable soil boundary and the potentially liquefiable soil boundary.

Peak ground acceleration (PGA)
The earthquake data used in the study refers to SNI 8460-2017 [10] on Geotechnical Design Requirements.The Peak Ground Acceleration value uses RSA human settlement for the seismicity of buildings and non-buildings.The researcher used a 50-year service period, a probability of exceeding 2%, and a recurrence period of 2500 years [11] for earthquake design criteria.
In the liquefaction analysis, the peak ground acceleration (PGAM) for liquefaction analysis used the peak acceleration of the MCEG by adjusting the site class.This theory is included in SNI 1726-2019 [1], which describes how to plan earthquake resistance for building and non-building Equation 1 is the formula for obtaining the PGAM value.

Liquefaction
Liquefaction is a phenomenon that causes a loss of soil strength and an increase in pore water pressure due to vibrations that occur at the earth's surface.The vibrations are in the form of seismic waves due to the sudden release of energy from within.It can lead to liquefaction of the soil mass, which decreases effective stress, ground settlement, and lateral spreading.
The purpose of analyzing liquefaction potential is to evaluate the liquefaction potential's safety factor (SF) in each soil layer.The liquefaction potential analysis using the Simplified Procedure method by Idriss and Boulanger [5] with the Standard Penetration Test (SPT) based on research project data [12].
The steps in calculating the liquefaction potential analysis are as follows: 1) Calculation of Cyclic Stress Ratio (CSR) The value of the Cyclic Stress Ratio (CSR) is the ratio of the average cyclic shear stress (cyc) with the effective overburden pressure (v'), as shown in Equations 2-5.
2) Calculation of Cyclic Resistance Ratio (CRR) The Cyclic Resistance Ratio (CRR) is the soil resistance during liquefaction with influencing factors such as the relative density of the soil, the duration of the earthquake shaking, and the effective overburden pressure.Idriss and Boulanger [5] suggested a correlation between CRR for an earthquake magnitude value of Mw 7.5 and an effective overburden pressure of 1 atm by adjusting the N-SPT for equivalent clean sand with Equation 6-10.
CRRM, v' = CRRM=7.5,v'=1atm .MSF .K MSF = 6.9 exp 3) The factor of Safety for Liquefaction The safety factor (SF) against liquefaction is the ratio between the cyclic shear stress required for liquefaction and the equivalent cyclic shear stress generated by the earthquake.The SF value is obtained by the following Equation 15 [5].

Liquefaction potential index (LPI)
In conducting a liquefaction analysis, evaluating the potential for liquefaction and the degree to which it occurs is necessary.This step uses the Liquefaction Potential Index (LPI) method to express the level of liquefaction potential.According to Mase [13], sites with relatively high soil resistance have a low susceptibility to liquefication, while those with low soil resilience have an increased vulnerability.The Equation for determining the LPI value is proposed by Sonmez [14].

Soil characteristics and stratigraphy
The soil stratigraphy of the West Sulawesi Governor's Office, referring to the three boreholes, is presented in Fig. 5.The depth of the groundwater table in the research area was at a depth of 6-9.5 meters below the ground surface.
According to SNI 1726-2019 [1], the site classification of BH-01 and BH-03 is obtained with the category of Soft Soil (SE) and BH-02 with the type of medium soil (SD).

Grain size analysis
Referring to Fig. 4, The analysis of grain gradation using the sieve analysis test results in data [9].
Based on the grain size distribution curves in Fig. 6-8, points BH-01, BH-02, and BH-03, the soil conditions with saturated soil types with uniform gradation are generally susceptible to liquefaction.As evidenced in the curve, the soil is in the zone of the liquefiable and potentially liquefiable soil boundaries.

Liquefaction potential analysis
Reference to previous research, the peak ground acceleration (PGAM) uses Equation 1.In this study, the PGAM results ranged from 0.7486 g to 0.7491 g, according to Table 3 [18][19].
This study used some earthquake magnitudes ranging from Mw 6.2 to Mw 7. The most significant earthquake magnitude is a potential fault from the Makassar Strait with an earthquake potential of up to 7 MW.Using the formula suggested by Idris and Boulanger [5], this study found that Factor of Safety (FS) values for earthquakes Mw 6.2 to Mw 7 were as presented in Table 4.
The results of the liquefaction potential analysis calculations were carried out on the three boreholes, with an example calculation on BH-01 with an earthquake magnitude of Mw 7, as presented in Table 5.Based on the calculation results, the potential soil liquefaction layer (with FSliq value < 1.0) was only found at depths below the water table with passive soil type and N-SPT value > 30.In BH-01, with a scenario of three earthquake magnitudes, the liquefiable layer was located at depths of 12-18 m and 24-30 m.
At BH-02, with three earthquake magnitudes, the liquefied layers were at depths of 9.5-10 m and 24 m, while at a depth of 16 m, liquefaction potential occurred with an earthquake strength scenario of Mw 7.    In BH-03, with the scenario of three earthquake magnitudes, the liquefied layers were located at depths of 6-8 m and 24-30 m.
According to Toprak and Holzer [20], liquefaction potential analysis should be conducted at depths up to 20 meters because liquefaction impacts of liquefaction at depths below 20 meters on the soil surface are rare.
The value of the factor of safety (FSliq) affects the liquefaction susceptibility in the research area.The lower the FSliq value, the higher the LPI value.It indicates a higher level of liquefaction susceptibility (Table 6).Points BH-01, BH-02, and BH-03 produced moderate to high classification values using three earthquake scenarios.The very high level of liquefaction susceptibility was partly due to the shallow elevation of the groundwater table around the area with small N-SPT values resulting in low FSliq values.The highest LPI value was at BH-03, with a very high level.

Conclusion
Based on soil investigation data in the area of the Governor's Office of West Sulawesi Province, the analysis of grain gradation, the observation area was dominated by sandy soil with loose to medium gradation and supported by a reasonably shallow groundwater table.So, these characteristics indicated the liquefaction potential.
According to the analysis, The Governor's Office of West Sulawesi Province has liquefaction potential.The layer most prone to liquefaction is at a depth of 12-18 meters, with a level of liquefaction potential from moderate to very high.
This research used a general method in analyzing liquefaction potential.Further analyses need to be carried out with various methods so they can validate and add to the findings obtained in this study.

Fig. 1 .
Fig. 1.Location of three boreholes reconstruction project in the governor's office of West Sulawesi Province.

Fig. 5 .
Fig. 5.The stratigraphy in the research area.

Fig. 9 -
11 are the FSliq values from the liquefaction potential analysis.

Table 4 .
Result of liquefaction potential analysis.

Table 6 .
Result of liquefaction potential index (LPI) analysis.