Morphotectonic Analysis in the Sombe Lewara Sub-Watershed in the Region of Palu City and Sigi Regency, Central Sulawesi Province

: A study on the tectonic activity index (IAT) was conducted in the Sombe Lewara Sub-Watershed, encompassing the area of Palu City and Sigi Regency, Central Sulawesi Province. The objective of this research was to determine the level of tectonic activity in the Sombe Lewara Sub-Watershed. The analysis employed in this study was morphotectonic analysis, utilizing parameters such as Hypsometric Integral (HI), Basin Shape (BS), Asymmetry Factor (AF), Stream Length-gradient Index (SL), Area Factor (AF), and Valley floor width-to-height ratio (Vf ratio). The tectonic activity index (IAT) value in the Sombe Lewara Sub-Watershed was 1.6, which classifies it within the high tectonic activity category. This high level of tectonic activity in the research area influences the landscape conditions and the types of rocks present. The predominant rock types in the study area are sedimentary rocks such as conglomerate deposits, sandstone, siltstone, and claystone. These rocks form the character of an expansive alluvial fan landscape that nearly covers the entire plain. One of the primary factors contributing to liquefaction in the Balaroa Region is that the area is composed of sand sediment deposits and granite boulders that constitute the alluvial fan.


INTRODUCTION
Sulawesi Island is the product of complex geological processes.Major tectonic plates of the world, such as the northward-moving Indo-Australian plate, the westward-moving Pacific plate, and the southeastward-moving Eurasian plate, along with smaller plates like the Philippine plate (Katili, 1970;Sompotan, 2012; Watkinson dan Hall, 2017) collectively shape the island of Sulawesi, which resembles a 'K' formation (Katili, 1978).In narrower regions, the impact of these tectonic forces is reflected by geological structures that can form unique geomorphological characteristics known as morphotectonics (Doornkamp, 1986).In addition to the formation of these landscapes, the Palu Valley is traversed by the active Palu-Koro fault line (Katili, 1970;Sukamto Et.al., 1973).this region particularly vulnerable to natural disasters, especially those caused by the movements of these plates, which drive the activity of the Palu-Koro fault (Badan Geologi, 2018).
The occurrence of these natural disasters is one of the manifestations of neotectonic events or tectonic processes that are active in the present era (Stewart dan Hancock, 1994;Van Hinsbergen, 2011).In this regard, to minimize the risk effects of such disasters, it is necessary to conduct research with a morphotectonic study approach.Morphotectonic studies investigate everything concerning the relationship between geological structures and landforms, or more specifically, the relationship between neotectonic structures and landforms (Stewart dan Hancock, 1994).

Regional Geology of the Research Area
The research area is situated within the regional geological map zones of the Palu Sheet and the Pasangkayu Sheet (Sukamto Et.al., 1973;Sukido Et.al., 1993), which comprise generally of an alluvial plain extending in a relatively north-south direction and is bordered by slopes and ridges of mountains that belong to the Sarasin Molasse Formation in hilly areas, portions of Intrusive Rocks on the mountain sides, and the Tinombo Formation in mountainous regions.The research area is located in the Palu-Koro fault zone, which is one of the areas with active geological processes.The Palu-Koro fault system is a horizontal fault with movement of the strike-slip or transcurrent fault type (Katili J.A, 1970;Bellier Et.al., 2001).According to Sukamto et al (1973) and Sukido et al (1993) ,the regional geological stratigraphy of the research area is composed of the Tinombo Formation (Tt), Granite and Granodiorite (gr)/Intrusive Rocks (Tpkg), Sarasin Molasse (QTms), and Alluvium and Coastal deposits (Qap)

RESEARCH METHODOLOGY
The methods employed in this study encompass data acquisition and analysis techniques.Data acquisition methods are aligned with their scope, comprising studio and field collection techniques.Studio data procured include regional geological maps (Sukamto Et.al., 1973;Sukido Et.al., 1993)

 Data Processing and Analysis Phase
The data processing and analysis phase involved a variety of processes, including those derived from watershed morphometry and morphotectonic data.Data processing was conducted using ArcGIS 10.

RESULT
The research results in the Sombe Lewara Sub-Watershed include a discussion on morphotectonic analysis.The parameters of this analysis serve as references to explain tectonic activities in the research area.The parameters used include integral hypsometry (HI), stream area index (BS), stream area asymmetry factor (AF), mountain front sinuosity (Smf), stream length gradient index (SL), and the ratio of valley width to valley height (Vf ratio).

 Hipsometri Intergral (𝑯𝑰)
The Hypsometric Integral (HI) principally delineates the elevation distribution within a specified terrestrial area, predominantly within watersheds.The Integral Hypsometry represents the area located along the hypsometric curve portion.The computation of the HI value is derived from the following equation.(Keller dan Pinter, 1987, 1996).The Stream Area Asymmetry Factor (AF) is a quantitative analysis tool for drainage basins used to detect tectonic tilting, applicable across both small and large drainage basin scales (Keller and Pinter, 1996).The asymmetry of a drainage area (AF) is determined by the ratio of the drainage basin or sub-basin area on one side to the total area of the drainage basin or sub-basin.The measurement of the AF in the study area was conducted on two rivers, namely the Daengguni and Lewara rivers.Subsequently, the results were calculated based on the equation (Keller dan Pinter, 1987, 1996).

 Mountain Front Sinuosity (Smf)
The Mountain Front Sinuosity (Smf) is an index that reflects the balance between the erosive forces, which tend to cut along the curves of the mountain front, and the tectonic forces that directly create the mountain front, coinciding with zones of active faults that reflect active tectonics (Keller and Pinter, 1996).From the measurements of the length of the mountain front surface (Lmf) and the straight length of the mountain front (Ls) in the Sombe Lewara Sub-Watershed, based on the boundary between the mountain front and alluvial fans (Bellier, 2001) and supported by topographic data (DEMNAS), the Smf value is obtained using the equation (Bull andMcFadden, 1977, in Doornkamp, 1986).The high tectonic activity in the area influences the landscape conditions and the sedimentation types of rocks.The sedimentary rock types in the research area are generally sedimentary rocks consisting of conglomerate, shale, phyllite, and sandstone deposits.
The hypsometric curve indicates that this area is in an old stage, suggesting that it has reached a geomorphic equilibrium between uplift and erosion.The landscape formed in this area consists of extensive alluvial fans that nearly cover all plains and hills.The

ISSN
, DEMNAS 8.5 m (Badan Informasi Geospasial, 2008), including river shapefiles at a 50_K scale, the Palu watershed boundary shapefiles, and the Sub-Watershed of Palu boundary shapefiles (BPDAS Palu-Poso, 2011), along with field data comprising visual descriptions of the landscape and the rock formations in the research area.Research LocationThe research location is situated in the upper watershed area of the Palu River.Specifically, the research area is within the Sombe Lewara Sub-Watershed (BPDAS Palu-Poso, 2011).Geographically, it is located at coordinates 119°50'22.73"E-0°56'23.20"S.Administratively, the research area is located in the Palu City region, encompassing the districts of West Palu, Tatanga, Ulujadi, and the Sigi Regency area, including parts of the Marawola, Kinovaro, and West Marawola districts, in the Central Sulawesi Province.The total area of the research site is approximately 130.04 km 2 Equipment and Materials Equipment used during the research includes: 1. Geological compass 2. GPS (Global Positioning System) 3. Digital camera Materials used during the research includes: 1. DEMNAS (National Digital Elevation Model) (Badan Informasi Geospasial, 2008) 2. Topographic map of Palu sheet 1 : 50.000 (BAKOSURTANAL, 1991) 3. Regional geological map of Palu sheet scale1 : 250.000 and regional geological map of Pasangkayu sheet scale 1 : 250.000 (Sukamto et al., 1973; Sukido et al., 1993).4. River Shapefile 50_K, Palu Watershed Boundary Shapefile, and Palu Sub-Watershed Boundary Shapefile (BPDAS Palu-Poso, 2011). Data Collection Phase The data collection phase encompasses both studio and field data gathering.Studio data collection obtained includes; regional geological data, topographic data (DEMNAS) (Geospatial Information Agency, 2008), Palu watershed river shapefiles 50_K, Palu watershed boundary shapefiles, and Palu Sub-Watershed boundary shapefiles (BPDAS Palu-Poso, 2011).Meanwhile, field data collection consists of rock layer data obtained from surveys or direct reviews of the research area.

3 and
Globalmapper 10 software. Data Interpretation Phase The data interpretation phase employs morphotectonic analysis to ascertain the zonation of tectonic activity levels based on the classification by El Hamdouni et al. (2007) and Dehbozorgi et al. (2010).Field data consisting of rock layers are analyzed to understand the rock formations in sedimentary areas or alluvial fans.

Figure 3 .
Figure 3. Map of Tectonic Activity Levels Based on the Basin Shape Index (BS) : AF = Asymmetry Factor Ar = Area of the right side of the drainage basin (direction of water flow downstream) (km²).At = Total area of the drainage basin (km²) The division of tectonic activity levels based on the Asymmetry Factor (AF) values is categorized into three classes, according to El Hamdouni et al. (2007) as cited by Dehbozorgi et al. (2010).These classes are defined as class 1 (AF values ≥ 65 or AF < 35), class 2 (35 ≤ AF < 43 or 57 ≤ AF < 65), and class 3 (43 ≤ AF < 57).Based on the measurements conducted, the Asymmetry Factor (AF) value in the study area is 65.26.According to this value, the tectonic activity level in the research area is classified within class 1.

Figure 4 .
Figure 4. Map of Tectonic Levels Based on the Asymmetry Factor of Drainage Basins (AF).
Smf = Lmf / Ls Where: Smf = Mountain Front Sinuosity Lmf = Length of the mountain front surface (km) Ls = Straight length of the mountain front (km) According to El Hamdouni et al. (2007) and Dehbozorgi et al. (2010), the classification of tectonic classes based on the Mountain Front Sinuosity (Smf) value is divided into class 1 (Smf value < 1.1), class 2 (Smf value between 1.1 and 1.5), and class 3 (Smf 5).Based on the measurements, the level of tectonic activity in the research area has a value of 1.20, which falls into class 2.

Figure 5 .
Figure 5. Map of Tectonic Levels Based on Mountain Front Sinuosity (Smf)

Figure 6 .
Figure 6.Map of Tectonic Levels Based on the Stream Length Gradient Index (SL) in the Study Area.

Figure 9 .
Figure 9. Sombe river area (a), outcrop of sedimentary rocks showing cross-bedding of sand and conglomerate deposits composed of shale and phyllite fragments (b), landscape appearance of the Lewara mountainous area (c), outcrop of shale deposits in the Duyu hilly area (d), Balaroa liquefaction disaster area (e), and the arrangement of sedimentary deposits in the crown area of the Balaroa liquefaction landslide composed of sand and granite (f).