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ANAGEMENTVolume 10, Number 3 (April 2023):4445-4456, doi:10.15243/jdmlm.2023.103.4445 ISSN: 2339-076X (p); 2502-2458 (e), www.jdmlm.ub.ac.id
Open Access 4445 Research Article
The impact of digital elevation models resolution on tectonic activity assessment based on morphotectonic indices: a case study of Seulawah Agam Volcano, Indonesia
Muhammad Ronggour Pardamean Siahaan1*, Emi Sukiyah1, Nana Sulaksana2, Agus Didit Haryanto1
1 Department of Geoscience, Faculty of Geological Engineering, Padjadjaran University, Bandung, Indonesia
2 Department of Applied Geology, Faculty of Geological Engineering, Padjadjaran University, Bandung, Indonesia
*corresponding author: [email protected]
Abstract Article history:
Received 30 October 2022 Accepted 6 January 2023 Published 1 April 2023
The Digital Elevation Number (DEM) is the main tool for quantitative geomorphological tests. Furthermore, Shuttle Radar Topography Mission (SRTM) images with a resolution of 30 m have been widely used as a source of DEM data in geomorphological studies, while DEMNAS (National DEM) images with 8 m are rarely used. Both images can identify typical volcanic morphology based on a visual comparison of hillshade with certain variations in slope. The objective of this study was to determine the feasibility of DEM for tectonic activity assessment based on morphotectonic indices. In this study, geomorphological comparisons were carried out on twelve watersheds in the Seulawah Agam Volcano (SAV). The data extraction of DEM resulted in the total area and perimeter, namely 486.8 km2 and 455 km (SRTM), as well as 482.8 km2 and 460.3 km (DEMNAS). The total segments up to the 4th order and the resulting lengths were 290 and 512.8 km for SRTM, while DEMNAS were 527 and 711.7 km. The morphotectonic variables used included drainage density, bifurcation, circularity, and valley floor ratio, as well as basin shape index. The results of these parameter calculations using mean values of SRTM imagery showed very coarse textures, deformed, more elongated, moderate tectonic, and low uplift. On the other hand, those of DEMNAS imagery showed coarse textures, not-deformed, more elongated, low tectonic, and low uplift of the basin’s characteristics.
Keywords:
DEMNAS GIS
morphotectonic Seulawah Agam SRTM
To cite this article: Siahaan, M.R.P., Sukiyah, E., Sulaksana, N. and Haryanto, A.D. 2023. The impact of digital elevation models resolution on tectonic activity assessment based on morphotectonic indices. A study case of Seulawah Agam Volcano, Indonesia. Journal of Degraded and Mining Lands Management 10(3):4445-4456, doi:10.15243/jdmlm.2023.103.4445.
Introduction
The Seulawah Agam Volcano (SAV) is a stratovolcano-type volcano located on the north side of the island of Sumatra, precisely to the northeast of the Sumatra fault. Stratovolcano type is a very dominant type of volcano in Indonesia, which is more than a hundred; this type is a product of the subduction of continental plates and oceanic plates with very high tectonic intensity and volcanic activity. Therefore, potential hazards due to high tectonic activity are very
likely to occur in the area around SAV. The common volcanic hazards are lava flow, ash fall, glowing clouds, direct blasts, lahar (volcanic debris and mudflows), volcanic gases, volcanic earthquakes, ash clouds, landslides and subsidence (Ghosh, 2016).
Particularly in SAV, where there has been no tectonic or volcanic activity for two decades, there is a need for research to increase attention and awareness of potential hazards from tectonic and volcanic activity.
This study aimed to identify tectonic activity based on geomorphological and GIS analysis based on two
Open Access 4446 satellite imagery with different resolution, the
comparison will lead to the ability of the image to identify geomorphological characteristics to explain geological conditions and tectonic affect around the research area (Hancock et al., 2006; Guth, 2010). Even though much DEM data has become available in recent years, differences in resolution affect the interpretability and dependability of the data.
Inaccurate data due to resolution can result in erroneous conclusions; hence, the morphological conditions to be concluded are not in accordance with the actual. Therefore, this research compared the geomorphic indices’ results of two different types of image resolution.
Geomorphological studies such as morphometric, morphotectonic, and morphstructure analyses can identify zones of tectonic activity and the presence of structures as well as surface morphological conditions with certain classifications related to volcanic systems (Sukiyah et al., 2018; Riswandi et al., 2020; Siahaan et al., 2022). Morphometric analysis has been widely used as a technique for identifying young mountains, which has been proven to develop geomorphic features (Sukiyah et al., 2018; Riswandi et al., 2020; Siahaan et al., 2022). On the other hand, morphometric and morhotectonic indications are influenced by the condition of the Earth’s surface, the shape, stream dimensions, elevation, and surface relief. Based on the classification of Dd, Rb, Rc, Bs, and Vf, the existence of an active tectonic zone can be confirmed (Sharma et al., 2017; Sukiyah et al., 2018).
Important geomorphic indices that define landforms undergoing active deformation and provide information on active tectonic zones can be derived through coarse analysis of topographic maps, aerial photographs, satellite images, and quantification of morphotectonic features (Sukiyah et al., 2018;
Riswandi et al., 2020; Siahaan et al., 2022). DEM resolution will greatly affect the raw data to be worked on due to the importance of geomorphological index results. Most of these indicators can be calculated using algorithms and topographic models from DEM data. Moreover, DEM image modeling is highly dependent on the resolution of the image being tested (Irvin et al., 1997; Hancock et al., 2006; Guth., 2010).
Materials and Methods Geological Setting
The Seulawah Agam Volcano (SAV) is a single and active volcano with a layered cone type. It is a Quarter in age and a continuation product of the Sumatran fault, which is split into two segments on the north side of Sumatra Island. SAV is a volcanic geological unit (Bennett, 1981; Siahaan et al., 2022), which consists of Alluvium (Qh) formations and Lamteuba Volcano Rocks (QTvt) that comprises andesite to dacite, breccia, agglomerates, ash flows, and lava flows. This also includes the rocks of the Lahar (Qvtl), the
Seulimeum formation (QTps), tuffaceous sandstone, and calcareous, and the last one in the southern part is the Padangtiji formation, which consists of sandstone, limestone, and silt (Bennett, 1981; Marwan et al., 2019).
The classifications of the stratigraphic units in the SAV complex are divided into four groups (Bennett, 1981; Marwan et al., 2019), namely:
1) Level-1 Heading of Pre-SAV Group: The pre- SAV group is composed of volcanic and sedimentary rocks. The volcanic rock consists of basaltic andesitic lava, pyroclastic flows, and pyroclastic falls, while sedimentary comprises claystone, calcareous sandstone, conglomerate, and limestone, which are regionally part of the Seulimeum formation.
2) SAV Group: SAV products consist of several lava flows and pyroclastic flows resulting from three different craters. Generally, the andesite lavas have a gray to light gray color and a porphyritic texture. The pyroclastic flows or hot clouds of sediment are composed of lava blocks embedded in a gray to reddish-colored base of volcanic ash.
3) Side Eruption Group: The product of this unit is in the form of phreatic deposits produced from Heutz Crater.
4) Secondary Sedimentary Group: Units in this group are the result of the re-deposition of pre- existing rocks
Methodology
In this research, twelve watersheds that are directly related to the morphology of SAV were identified using Geographic Information System (GIS) tools and analysis. The digital elevation model used was the Shuttle Radar Topography Mission (SRTM) and the National DEM (DEMNAS) image belonging to the Indonesian Geospatial Agency (BGI).
SRTM is a radar image launched into orbit with an altitude of 233 km, a slope of 60° north and 56°
south, covering 80% of the Earth’s landmass. The C- band Spaceborne Imaging Radar and X-Band Synthetic Aperture Radar (X-SAR) hardware have been used aboard the spacecraft since 1994. This technology uses interferometric radar, which compares two images or radar signals taken at slightly different angles (USGS, 2018). The resolution of the SRTM image used in this study was one arc-second (30 m), as shown in Table 1.
BGI released the National DEM (EGM2008 vertical datum) at the end of July 2018, which is the result of interpolation from several data sources, including IFSAR, TERRASAR-X, and ALOS PALSAR, with values of 5 m, 5 m, and 11.25 m resolution, respectively, by adding the stereo-plotted mass point data into the calculation. The validation results using Ground Control Point (GCP) and Geodetic Control Network (JKG) measurements showed that DEMNAS accuracy is better than the
Open Access 4447 mass point data model (BGI, 2018). The resolution of
the DEMNAS image used in this research was 0.27 arc-second (8 m), as shown in Table 1. After both image's data were downloaded for free, the next step was data processing using GIS tools. The GIS toolset using topographic data from both satellite images will produce morphological reconstructions in the form of basins, area, perimeter, contour, landscapes, textures, structures, and stream orders, which are the main
variables in geomorphological character analysis (Yang et al., 2011). A comparison was performed using a combination of visual parameters hillshade, elevation, and slope in both images. Slope was also analyzed and mapped based on DEM topographic map, combined elevation and relief were arranged in slopes analysis, classified into flat/near flat to gently sloping, moderately steep, steep and very or extremely steep (Cheng et al., 2018; Siahaan et al., 2022).
Table 1. Digital Database of Elevation Model.
SRTM DEMNAS
Name n05_e095_1arc_v3.tif DEMNAS_0421-33_v1.0.tif
DEMNAS_0421-34_v1.0.tif DEMNAS_0421-61_v1.0.tif DEMNAS_0421-62_v1.0.tif
Resolution 1 arc (30 m) 0.27 arc (8 m)
Author https://earthexplorer.usgs.gov/ https://earthexplorer.usgs.gov/
Datum WGS 1984 UTM Zone 46 Northern Hemisphere
The GIS toolset extraction was used to extract the stream network and drainage system, including filling, flow direction, density and drainage. The drainage system indicates tectonic influence in its development and reflects the geological condition of an area (Sukiyah et al., 2018; Riswandi et al., 2020; Siahaan et al., 2022). The identification of stream basins and segments is a basic parameter in assessing geomorphological characters. The image accuracy in identifying segments is indicated by the results of stream order, the number of segments, and the length of the stream (Gentana et al., 2018; Sukiyah et al., 2018; Ummah et al., 2018). Then these variables will be calculated using quantitative formulas based on the required morphological parameters, especially on several parameters that indicate the presence and influence of tectonic or volcanic activity on a watershed. In this study, the parameters used were drainage density (Dd), bifurcation ratio (Rb), basin circularity (Rc), basin shape (Bs) and valley floor ratio (Vf).
Definition, formula and classification
Drainage density (Dd) is a comparison between the total length of the identified stream and the area of the water system. The value of the flow density indicates the surface texture of the watershed, which influences geological and climatic conditions. Furthermore, the Dd classification is divided into six textures, namely very coarse, coarse, medium, slightly fine, fine, and very fine, with a range of 0-1.37, 1.38-2.75, 2.76- 4.13, 4.14-5.51, 5.52-6.89, and 6.9-8.27, respectively (Sukiyah et al., 2018; Ummah et al., 2018). The main calculation parameters included the total length of the stream segment (L) and the area of the watershed (A) under study, as shown in Table 2. The index value of the bifurcation ratio (Rb) is a comparison between the total number of the first (n) and subsequent segments
(n+1), where the results obtained are the level of a deformed water system area or basin (Sukiyah et al., 2018; Ummah et al., 2018). The order of the stream was determined using the Strahler method. Image resolution will greatly determine the accuracy when identifying stream branches from the main river to the tributary. On the other hand, the sharpness of the resolution will affect the calculation of the Rb value.
The classification of stream branching ratio (Rb) is based on Verstappen’s (Sukiyah et al., 2018; Ummah et al., 2018) reference, where the area is deformed with an index >5 and <3. An index value between 3-5 is classified as a non-deformed zone, as indicated in Table 2.
The basin circularity (Rc) index calculation shows the type of water system area affected by the fluid flow velocity on the surface. Therefore, the comparison between the area and the circumference of the water system shows the intensity of hydrological activity. The watershed is elongated, indicating that fluid flow will quickly enter and leave the area when the index is <0.5. However, an index >0.5 means that the water system area is rounded with the character of the fluid being inundated (Asfaw and Workineh, 2019;
Briceno et al., 2020), as shown in Table 2.
The basin shape (Bs) index is a comparison of the maximum length and width of a water system area.
The results will show the tectonic character of the water system area, which describes the geological processes that occur. The Bs equation is an index of
<3, representing low tectonic characters, between 3-4 for moderate, while >4 indicates active tectonic zones (Gentana et al., 2018), as shown in Table 2.
The data acquisition of the valley floor ratio (Vf) has a significant impact on the precision of topographic data. This is because the main parameter of the Vf index calculation is the width of a valley floor to the highest and lowest elevation on each side and
Open Access 4448 a cross-section in a water system area, respectively.
The index indicates the level of uplift experienced by a water system area. A high level of Vf indicates a tectonic influence that may have occurred.
Calculations are carried out based on the Bull equation (Gentana et al., 2018; Ummah et al., 2018), with the criteria of <0.5 indicating a high uplift level, 0.5 to 1 for moderate, and >1 for low, as shown in Table 2.
Table 2. The formula of morphotectonic variables.
Index Formula Ref.
Dd Dd = Ʃ Lu / A (1,2,3)
Rb Rb = Nu + Nu + 1 (1,2,3)
Rc Rc = 4π A / P² (1,2,3)
Bs Re = 2 / Lb * √ (A / π) (1,2,3) Vf Vf = 2Vfw / [(Eld – Esc) +
((Erd – Esc)] (1,2,3)
Results and Discussion Visual landscape comparisons
Visual hillshade on DEMNAS imagery showed more details of the morphological conditions of the volcano dome than SRTM. The calculation of the slope from the two images showed that the difference was not too significant. Furthermore, the combination of hillshade and slopes indicates the lithological boundaries and the character of the volcanic flow. SAV status is active but has not erupted for a long time, making it easier to identify geological boundaries and patterns of volcanic and phreatic material flows that occur, as shown in
Figure 1. Based on the calculation presented in Table 3, the slope classified as flat/near flat to gently sloping (0-15%) on the SRTM image is 315 km2 (57%) is greater than DEMNAS, which has a value of 292 km2 (53%).The morphological zone is rather steep in the SRTM image of 166 km2 (30%) and smaller than the DEMNAS result with a value of 177 km2 (32%). In the steep morphological zone, the SRTM image is 49 km2 (9%), while the DEMNAS image is 59 km2 (11%).
Furthermore, both images show close results in the steep to very steep zone, namely 21 km2 (4%) for SRTM and 23 km2 (4%) for DEMNAS. The DEMNAS image can identify areas with a slope >15% wider than the SRTM image. Generally, both show close and significant calculation results, as indicated in Figure 2.
The combination of hillshade visual unification of certain slopes provides a better interpretation (Keller and Pinter, 2002; Nanda et al., 2020). The interpretation of the morphological characteristics explains the surface structure of the volcano. After analyzing the two images, DEMNAS was shown to identify morphology and structure better than SRTM, as indicated in Figure 1. Some of the main parts of the morphological system resulting from volcanic activity can be seen visually, such as the main crater (Ct), minor crater (Ct), caldera, landslide/debris avalanche (Ld), debris flow or pyroclastic fan flow (Df), mountain streams pyroclastic flows (Pd), and lava or volcanic mudflows (Lm) (Bramantyo and Bandono, 2006) (Figure 1). Strong tectonic activity results in geomorphological characteristics consisting of differences in topography and steep slopes (Hancock et al., 2006; Guth, 2010).
Table 3. Slope classification.
Slope (%) Classification SRTM
(km2) DEMNAS
(km2)
0-15% Flat, Gently Sloping, Sloping 315 (57%) 292 (53%)
15-30% Moderately Steep 166 (30%) 177 (32%)
30-45% Steep 49 (9%) 59 (11%)
>45% Steep, Very Streep, Extreme Steep 21 (4%) 23 (4%)
Segment and basin comparison
Based on the extraction of the two images, twelve basins were named according to their position with respect to the SAV (Figure 2), namely North (N-1, N- 2, and N-3), East (E-1), South (S-1, S-2, S-3, S-4, S-5, S-6, and S-7), and West (W-1). Table 4 shows the calculation results of the basin area and perimeter. A significant difference in the area was found in S-1, S- 3, and W-1 with a deviation greater than 6 km2. In perimeter calculations, S-3, S-7, and W-1 yielded significant data differences, with a length deviation greater than 8 km. Based on geological data and maps, the main fault structures (Seulimeum-fault) and manifestations (crater and hot springs) were in the basin zone where the difference was corrected.
Therefore, the area is indicated to experience tectonic
activity due to active faults, thereby producing a complex morphology.
The morphological complexity will affect the image’s readability, such that the difference in readings between the two images in the active tectonic zone is identified. Table 5 shows the calculation results of the total length and segments of the stream. The results showed that the DEMNAS image could identify streams in more detail than SRTM, as indicated in Figure 3. The identification capabilities were compared by comparing the number of segments and the length of the segments. In the total stream length variable, DEMNAS could identify 712 km, while SRTM identified 513 km or 28% less.
Furthermore, DEMNAS could identify 527 segments in the total segment’s variable, while the SRTM
Open Access 4449 identified 298 segments or 43% less. A significant
difference was seen in N-1, S-1, S-7, and W-1, where the deviation of stream length ranged from 25-45 km and 28-47 for the number of segments, as shown in Table 5. In the identification of the segments based on stream orders, the two images showed the same percentage gain, namely 77% (1st), 18% (2nd), 4% (3rd),
and 1% (4th), but different segment orders.
A significant difference SRTM image showed 229 (1st), 53 (2nd), 13 (3rd), and 3 (4th) segments, while the DEMNAS image showed 408 (1st), 95 (2nd), 20 (3rd), and 4 (4th), as indicated in Table 6. This means that the DEMNAS image extraction is 44% better on orders 1 and 2, 35% on the 3rd, and 25% on the 4th.
Figure 1. Visual comparison of SRTM and DEMNAS using DEM data to volcano morphology.
Open Access 4450 Figure 2. Slope and elevation classification.
Figure 3. Segment and basin comparison.
Open Access 4451 Table 4. Comparisons of watershed area and perimeter.
BASIN BASIN
(km2) PERIMETER
(km)
SRTM DEMNAS SRTM DEMNAS
N-1 105.3 105.6 65.5 67.7
N-2 31.3 29.6 32.9 33.6
N-3 46.5 43.3 38.5 39.8
E-1 46.5 43.3 38.5 39.8
S-1 74.4 82.7 42.4 47.1
S-2 11.1 7.6 26.4 22.7
S-3 8.9 2.9 25.9 11.6
S-4 9.2 11.7 21.6 27.0
S-5 7.5 6.9 18.5 18.9
S-6 18.1 18.3 34.9 38.2
S-7 58.4 54.8 48.9 40.5
W-1 69.4 76.3 60.9 73.4
TOTAL 486.8 482.8 455.0 460.3
Table 5. Comparisons of stream length and segments.
BASIN LENGTH
(km) SEGMENT
(pcs)
SRTM DEMNAS SRTM DEMNAS
N-1 114.5 155.7 64 107
N-2 32.2 37.8 22 27
N-3 38.6 53.5 29 46
E-1 38.6 53.5 29 46
S-1 87.0 130.4 40 87
S-2 15.4 10.9 4 8
S-3 10.5 5.0 4 5
S-4 7.0 20.1 5 15
S-5 6.3 10.8 3 10
S-6 23.1 34.2 12 23
S-7 67.9 93.6 37 65
W-1 71.6 106.0 49 88
TOTAL 512.8 711.7 298.0 527.0
Based on the results of slope analysis and the number of segments in order of the stream, the area with a flat/near flat to gently sloping produces a significant level of difference between the two satellite images, which is 4% or about 23 km2. This is evidenced by the significant difference in the number of segments in the first order as well (Table 6). The ability of radar imagery with a more detailed resolution is able to classify areas with flat/near flat to gently sloping morphology better; therefore, the acquisition of more valid data will result in more precise conclusions. On the contrary, low-resolution imagery will produce conclusions with a low level of confidence.
Morphotectonic identification
The influence on geomorphology was the formation of a basin in the SAV. The drainage pattern was very sensitive to processes such as uplift, folding, faulting,
and slope. These characteristics resulted from identifying stream incisions, basin asymmetry, drainage geometry, and stream deflection (Sharma et al., 2017; Sukiyah et al., 2018; Riswandi et al., 2020).
The identification of geomorphic characteristics indicates past and future occurrences of geological phenomena. Dd shows that the research area is a basin type with very coarse (0-1.37) and coarse (1.38-2.75) textures. The SRTM image identified eleven “very coarse” textured basins and one “coarse” (S-2). On the other hand, the DEMNAS image yielded three “very coarse” textured basins, namely N-1, N-2, and E-1, while others were coarse textured. The SRTM image produces a data range of 0.76 – 1.39, with mean values of 1.1 (coarse), while DEMNAS is in the range of 1.24- 1.87, with mean values of 1.5 (very coarse), as indicated in Table 7. The results of the Dd calculation showed that the “coarse” to “very coarse” texture in
Open Access 4452 the SAV is affected by typical volcanic structures,
indicating that this area is dominated by hard resistance rocks (Siahaan et al., 2022). These include the main crater (Ct), minor crater (Ct), caldera, landslide/debris avalanche (Ld), debris or pyroclastic fan flow (Df), mountains pyroclastic flows (Pd), and lava (Lm). High Dd values occur in regions with high relief, and weak of resistant and impermeable subsoils,
while low Dd is found in regions with low relief and highly resistant and permeable subsoils (Briceno et al., 2020). On the calculation of the Dd parameter of both images, showing that the better resolution of the image results in a coarser watershed texture, these landform conditions explain that the flow of sedimentary material transported by streams will be smaller (Siahaan et al., 2020).
Table 6. Comparisons of segments stream order.
BASIN SRTM DEMNAS
1rd 2nd 3th 4th 1rd 2nd 3th 4th
N-1 48 11 4 1 82 19 5 1
N-2 18 3 1 - 21 5 1 -
N-3 23 5 1 - 39 6 1 -
E-1 23 5 1 - 39 6 1 -
S-1 29 8 2 1 63 18 5 1
S-2 3 1 - - 7 1 - -
S-3 3 1 - - 4 1 - -
S-4 4 1 - - 11 3 1 -
S-5 2 1 - - 9 1 - -
S-6 9 2 1 - 18 4 1 -
S-7 29 5 2 1 49 13 2 1
W-1 38 10 1 - 66 18 3 1
TOTAL 229 53.0 13.0 3.0 408 95.0 20.0 4.0
Table 7. Comparisons of morphometric parameters.
BASIN SRTM DEMNAS
Dd Rb R1-2 R2-3 Dd Rb R1-2 R2-3
N-1 1.1 4.4 4.4 2.8 1.5 4.4 4.3 3.8
N-2 1.0 6.0 6.0 3.0 1.3 4.6 4.2 5.0
N-3 0.8 4.6 4.6 5.0 1.2 6.6 6.1 7.0
E-1 1.1 5.1 5.1 3.3 1.3 3.8 5.2 3.6
S-1 1.2 3.6 3.6 4.0 1.6 4.0 3.5 3.6
S-2 1.4 3.0 3.0 0.0 1.4 7.0 7.0 0.0
S-3 1.2 3.0 3.0 0.0 1.7 4.0 4.0 0.0
S-4 0.8 4.0 4.0 0.0 1.7 3.3 3.7 3.0
S-5 0.8 2.0 2.0 0.0 1.6 9.0 9.0 0.0
S-6 1.3 4.5 4.5 2.0 1.9 4.3 4.5 4.0
S-7 1.2 5.8 5.8 2.5 1.7 4.1 3.8 6.5
W-1 1.0 3.8 3.8 10.0 1.4 4.2 3.7 6.0
MEAN 1.1 3.8 4.1 2.7 1.5 4.9 4.9 3.5
Based on the results of the calculation of the Bifurcation Ratio (Rb), the total Rb, Rb1-2 and Rb2-3
(Table 7) show significant data between the two images. The SRTM image produces data ranges from 2-6 (Rb1-2), 2-10 (Rb2-3) and 2-6.9 (Rba) in a total of seven basin’s (N-1, N-2, E-3, S-5, S-6, S-7 and W-1) were identified as deformed area. The DEMNAS image produces a data range of 3.5-9.0 (Rb1-2), 3.0-7.0 (Rb2-3) and 3.3-9.0 (Rba), a total of six basin’s (N-3, E-
3, S-2, S- 5, S-7, and W-1) were identified as having deformation.
The ability of DEMNAS imagery to identify stream segments better than SRTM does not significantly affect the calculation of Rb. In general, the calculation results of the two images showed that eight basins had been confirmed to be deformed.
Higher values of Rb by DEMNAS indicate greater soil erosion, while low values of Rb by SRTM imply that
Open Access 4453 it developed on an almost homogenous topographic
structure (Briceno et al., 2020; Siahaan et al., 2020).
These two contradictory results occur due to the ability to identify stream order, where image resolution is important in determining the level of geological confidence. Basin Circularity (Rc) calculations for both images yielded two data ranges, namely strongly elongated (< 0.24) and more elongated (0.24 – 0.51).
Meanwhile, the SRTM image yielded four basins in the strongly elongated range, namely S-2, S-3, S-6, and W1, while DEMNAS produced five basins, such as S- 2, S-4, S-5, S-6, and W1 (Table 8). The other basins of both imageries are classified in the “elongated to
“more elongated” classes, accounting for < 0.53. The character of the basin in the strongly elongated range
in the form of bird feathers in the southern part is influenced by topographic conditions, rock units resistant to weathering, and morphological characteristics of a typical volcano (Asfaw and Workineh, 2019; Briceno et al., 2020).
The morphotectonic analysis related to the shape of the basin is the Bs index. The calculation results on the SRTM image showed a data range of 1.4-6.9 and mean values of 3.0. Furthermore, S-2 and S-6 have active tectonic geology criteria, while S-3, S-4, and S- 5 have moderate tectonics. The DEMNAS image produces a data range of Bs 1.0-5.9 with mean values of 2.8. Two basins, namely S-2 and S-6, have active tectonic activity, while S-5 and W-1 have moderate tectonic indications.
Table 8. Comparisons of morphotectonic parameters.
BASIN SRTM DEMNAS
Rc Bs Vf Rc Bs Vf
N-1 0.3 1.8 2.0 0.3 2.2 0.6
N-2 0.4 3.0 1.9 0.3 3.1 0.5
N-3 0.4 2.5 0.6 0.4 2.7 0.4
E-1 0.4 2.0 2.5 0.4 2.6 0.2
S-1 0.5 1.3 1.0 0.5 1.0 4.5
S-2 0.2 5.3 1.0 0.2 4.3 0.8
S-3 0.2 5.2 2.8 0.3 2.2 8.5
S-4 0.2 4.6 9.8 0.2 3.2 5.7
S-5 0.3 4.2 10.1 0.2 3.6 30.3
S-6 0.2 6.0 4.1 0.2 5.9 5.2
S-7 0.3 1.9 3.5 0.4 1.4 3.5
W-1 0.2 3.1 0.7 0.2 3.0 2.9
Mean 0.3 3.0 6.0 0.3 2.8 6.9
The relationship between the parameters in the resulting data is that the smaller the Rc value, the greater the Bs or the flatter the basin’s shape, and the greater the indication of tectonic activity. Therefore, the images in both parameters identify an active tectonic zone in the southern part of the SAV, where there is indeed an active fault (Seulimeum fault), craters, and solfatara.
The last morphotectonic calculation in the basin area in the SAV is the Vf ratio. Table 8 shows that the Vf value of the two images has very significant differences. This is due to differences in the interpretation of elevation in the image. Furthermore, the SRTM image identified only one basin experiencing moderate uplift conditions (N-3) and eight classified as low uplift, namely N-2, N-3, E-3, S- 1, S-2, S-5, S-7, and W-1. The DEMNAS identified three basins in the high uplift level category, namely N-1, N-2, and W-1, S-1 and S-7 in the moderate category, and five basins were classified as low, namely N-3, E-3, S-2, S-4, and S-6.
Vf conceived to discriminate between V-shaped and U-shaped, old and young drainage systems.
V-shaped representing the active down-cutting characteristic of areas subjected to high uplift and observed, indicating the role of tectonics (Sharma et al., 2017; Gentana et al., 2018). The difference in data classification results from the two satellite images shows a different understanding of the condition of the study area. SRTM explains that the area has experienced a high level of erosion with a dominant U- shaped and low uplift, while DEMNAS illustrates that the area has experienced low erosion, high uplift and high tectonic intensity.
Conclusion
A comparison of SRTM and DEMNAS images was carried out to determine the difference, both visually and quantitatively. The quantitative geomorphological studies provided a complete picture of the accuracy of the image to be tested. Visually, there is no significant difference between the SRTM and DEMNAS. It is proven that both images can provide an overview of the typical morphology of volcanic activities that
Open Access 4454 occur, such as the main crater, minor crater, caldera,
landslide/debris avalanche, debris flow or pyroclastic fan flow, mountain pyroclastic flows, and lava flows.
The DEMNAS image has good quality during comparison; hence, it provides better dimensional characteristics than the SRTM image in visual interpretation. Therefore, both images are visually feasible to use in interpreting landforms, morphological characteristics and studying volcanic eruption patterns.
Based on quantitative testing, the morphometric categories of Dd and Rb showed the same level of similarity to the data. Dd provided an overview of the study area morphology with a rough texture, which means both images indicate the youth of the geomorphic stage with a moderate level of tectonic control. On the other hand, Rb showed deformation in which the zone is strongly influenced by tectonic activity and is proven by the presence of active fault systems (Seulimeum-fault), as shown in Table 9.
If the Dd and Rb result from both images is used as criteria for determining potential areas of tectonic and volcanic activity hazards, with a minimum number of 4 criteria. Then the results are three basins, namely S-2, S-7 and W-1 with 4 criteria and only one basin namely S-5 with 5 criteria (Table 9). This result is proven by the presence of geological evidence such as minor faults and hot springs in basin S-2, major faults and hot springs in basin W-1 and major faults, hot springs and solfatara in basin S-5, as shown in
Table 9. Rc and Bs indicated that the southern zone has a strongly elongated character with moderate to active tectonic in both images, while the difference was significant in the interpretation of Vf. Furthermore, the SRTM image did not show any tectonic activity with uplift, while the DEMNAS image showed high uplift characters in the west, north, and east, as shown in Table 10.
The same calculation for determining potential areas of tectonic and volcanic activity hazards using Rc, Bs and Vf with a minimum number of 4 criteria, the results from both images are used as criteria for only three basins, namely S-2, S-6 and W-1 (Table 10).
The three basin areas are proven by the presence of geological evidence, such as minor faults and hot springs in basin S-2, major faults in basin S-6 and major faults and hot springs in basin W-1, as shown in Table 10. SRTM and DEMNAS images are equally good in horizontal accuracy, for example, when used in the initial interpretation of visuals, slope, basin area, and perimeters. The use of vertical accuracies, such as geomorphic indices of the two images, will produce different and very significant conclusions. Therefore, in interpreting characteristics based on tectonic activity in an area, it is recommended to use image resolution above 8 m. This is due to the significant possibility of erroneous interpretation when the tectonic activity that should constitute a potential hazard does not appear in the results of the geomorphological index.
Table 9. Result of morphometric classification (I).
BASIN
Drainage Density (Dd)
Bifurcation Ratio
Rb(1-2) Rb(2-3) Rb(a)
DEMNAS SRTM DEMNAS SRTM DEMNAS SRTM DEMNAS SRTM
N-1 Coarse Very
Coarse
Not- Deformed
Not- Deformed
Not- Deformed
Deformed Not- Deformed
Not- Deformed
N-2 Very
Coarse Very
Coarse Not-
Deformed Deformed Not-
Deformed Deformed Not-
Deformed Not- Deformed
N-3 Very
Coarse Very
Coarse Deformed Not-
Deformed Deformed Not-
Deformed Deformed Not- Deformed
E-1 Very
Coarse
Very Coarse
Deformed Deformed Not- Deformed
Not- Deformed
Not- Deformed
Not- Deformed
S-1 Coarse Very
Coarse
Not- Deformed
Not- Deformed
Not- Deformed
Not- Deformed
Not- Deformed
Not- Deformed S-2 Coarse Coarse Deformed Not-
Deformed
Unclassified Unclassified Deformed Not- Deformed
S-3 Coarse Very
Coarse Not-
Deformed Not-
Deformed Unclassified Unclassified Not-
Deformed Not- Deformed
S-4 Coarse Very
Coarse Not-
Deformed Not-
Deformed Not-
Deformed Unclassified Not-
Deformed Not- Deformed
S-5 Coarse Very
Coarse
Deformed Deformed Unclassified Unclassified Deformed Deformed
S-6 Coarse Very
Coarse
Not- Deformed
Not- Deformed
Not- Deformed
Deformed Not- Deformed
Not- Deformed
S-7 Coarse Very
Coarse
Not- Deformed
Deformed Deformed Deformed Not- Deformed
Not- Deformed
W-1 Coarse Very
Coarse
Not- Deformed
Not- Deformed
Deformed Deformed Not- Deformed
Deformed Note : red text indicates the influence of high tectonic activity on the watershed based on quantitative geomorphologic parameters Dd and Rb from SRTM and DEMNAS images.
Open Access 4455 Table 10. Result of morphometric classification (II).
BASIN Circularity Ratio (Rc) Basin Shape Index (Bs) Valley Floor Ratio (Vf)
DEMNAS SRTM DEMNAS SRTM DEMNAS SRTM
N-1 More Elongated More Elongated Low Tectonic Low Tectonic
High Uplift Uplift Moderate N-2 More Elongated More Elongated Low Tectonic Low
Tectonic
High Uplift Low Uplift N-3 More Elongated More Elongated Low Tectonic Low
Tectonic
Low Uplift Low Uplift E-3 More Elongated More Elongated Low Tectonic Low
Tectonic Low Uplift Low Uplift
S-1 More Elongated Elongated Low Tectonic Low
Tectonic Uplift
Moderate Low Uplift S-2 Strongly Elongated Strongly Elongated High Tectonic High
Tectonic Low Uplift Low Uplift S-3 More Elongated Strongly Elongated Low Tectonic Moderate
Tectonic
Not-Uplift Not-Uplift S-4 Strongly Elongated More Elongated Low Tectonic Moderate
Tectonic
Low Uplift Not-Uplift S-5 More Elongated More Elongated Moderate
Tectonic Moderate
Tectonic Not-Uplift Low Uplift S-6 Strongly Elongated Strongly Elongated High Tectonic High
Tectonic Low Uplift Not-Uplift S-7 More Elongated More Elongated Low Tectonic Low
Tectonic
Uplift Moderate
Low Uplift W-1 Strongly Elongated Strongly Elongated Moderate
Tectonic
Low Tectonic
High Uplift Low Uplift Note: red text indicates the influence of high tectonic activity on the watershed based on quantitative geomorphologic parameters Rc, Bs and Vf from SRTM and DEMNAS images.
There are two main benefits from the results of this study. First, it can be used as a basis for determining satellite imagery that will be used as a tool in a geological study, the level of confidence in the results of image interpretation and the potential data errors that may occur in remote sensing applications. Second, basin areas around SAV with indications of high tectonic and volcanic activity can be a reference for local governments to determine land use plans. With all the risks that may occur, such as volcanic and tectonic earthquakes, volcanic dust, lava flows or other geological disasters, whether the area is suitable for residential use, plantations, other public facilities or just an area isolated from human activities.
Acknowledgements
The authors are grateful to the Faculty of Geological Engineering Lecturers, Padjadjaran University, for the support provided during the Gunung Seulawah volcano study.
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