http://journal.uir.ac.id/index.php/JGEET E-ISSN : 2541-5794 P-ISSN : 2503-216X Journal of Geoscience, Engineering, Environment, and Technology Vol 7 No 3 2022 124 Khayati,N.N. et al./ JGEET Vol 7 No 3/2022 RESEARCH ARTICLE The Effect of Weathered Layer Thickness and Slope on Potential Areas of Landslides in Gerbosari Village, Samigaluh District, Kulonprogo Regency, Indonesia Novia Nurul Khayati1, Sudarmaji2*, Eddy Hartantyo2 1Program S2 Fisika, FMIPA, UGM, Sekip Utara Yogyakarta,55281, Indonesia 2 Lab Geofisika, Dep Fisika, FMIPA, UGM, Sekip Utara Yogyakarta, 55281, Indonesia * Corresponding author : ajisaroji@ugm.ac.id Tel.:+62-81392840341; fax: +62-274-545183 Received: Mar 21, 2022; Accepted: Sep 23, 2022. DOI: 10.25299/jgeet.2022.7.3.9161 Abstract Gerbosari Village has a history of landslides with intensity and risk of 56 occurrences over 5 years. Gerbosari Village, Samigaluh District, Kulonprogo Regency is located at geographic coordinates 7β—¦ 38 '45.33 "- 7β—¦ 41' 35.24" LS and 100β—¦ 9 '20.80 "- 110β—¦ 11' 16.52" BT with topographic conditions at an altitude around 500 - 1000 mdpl. This study aims to determine the subsurface structure of landslide- prone areas in the form of weathered layer thickness and the effect of slope in landslide-prone areas so that it can be used in making micro zonation maps of landslide-prone areas. This study uses 43 microtremor data with a distance between points of 650 m. The microtremor signal was analysed using the horizontal to vertical spectrum ratio ( HVSR) method. From the measurement results, it is obtai ned that the value of the dominant frequency ranges from 1 - 22 Hz, the value of the amplification factor is obtained in the range of 1 - 10.5, the value of the peak ground acceleration ranges from 60 - 300 cm/s2, the thickness of the weathered layer is obtained in the range of 12 - 22 meters. Based on the results of the slope analysis, the study area is on a slope classified as a bit steep - very steep. Keywords: landslides, weathered layer thickness, microzonation, HVSR , Gerbosari Village 1. Introduction Landslides or soil movements are one of the type movements of soil or rock masses, down or out of the slopes due to disturbed soil stability or rock making up the slopes (B). The consequences of landslides are not only casualties but also damage road access so that it can paralyze economic accommodation in the area due to closed main road access in the area. Landslides that occur are caused by several factors, among others, high rainfall, slope, and thickness of the weathered layer. One area that has a history of high landslide incidents is in Gerbosari Village, Samigaluh District, Kulonprogo Regency where the area has a history of large landslide events of 56 incidents during the last 5 years. So it is necessary to analyse the characteristics of the soil in terms of the thickness of the weathered layer and analysis of the slope that can lead to landslides using the microtremor measurement approach. Microtremor is a weak vibration on the surface of the earth which occurs continuously due to vibration sources such as earthquakes, human activities, and traffic (Nakamura, 2000). This phenomenon creates a seismic wavefield, where the field is known as the microtremor (Dal Moro, 2015). Microtremors array observation has been used for estimating deep structure (Goto et.al, 2007). Microtremors measurement could be used for landslide susceptibility assessment too (Abdelrahman et.al, 2021). Signal data from microtremor measurements consist of 3 signals in the form of a vertical component (up and down), a horizontal component (north-south), and a horizontal component (east-west). Then the signal is processed using the HVSR (Horizontal to Vertical Spectral Ratio) method to produce the peak frequency and HVSR amplitude which represent the amplification and frequency of the specific local (Nakamura, 2000). 2. Geology Samigaluh Subdistrict, Kulonprogo Regency, which is one of the regencies in the Special Region of Yogyakarta (DIY) is located on a tectonic and volcanic route. The northern part of the DIY Province is bordered by the active volcanic Merapi, in the southern part of the DIY Province it is bordered by the Java Trench which is a subduction route of the Indo-Australian-Eurasia plate (Marjuki and Yogafanny, 2008). Rahardjo et al. (1995) described the Kulon Progo stratigraphy as composed of Quaternary volcanic rocks, alluvial deposits accompanied by the presence of a breakthrough rock complex. The rock formations from old to young consist of the Nanggulan Formation, Kebo Butak Formation, Jonggrangan Formation, Sentolo Formation (Husein, et al. 2007). While the research location in Gerbosari Village, Samigaluh District, Kulonprogo Regency consists of the Kebo Butak Formation (Tmok) and Jonggrangan Formation (Tmj). Gerbosari Village, Samigaluh Subdistrict, Kulonprogo Regency is in The Kebo Butak Formation (Tmok) and the Jonggrangan Formation (Tmj) as shown in Figure 1. The Kebo Butak Formation (Tmok) is a formation that is not aligned above the Gamping-Wungkal Formation and is sediment. The results of volcanic activity are composed of volcanic rocks, either in the form of pyroclastic, plastic, or coherent lava (Bronto et al.). http://journal.uir.ac.id/index.php/JGEET Khayati,N.N. et al./ JGEET Vol 7 No 3/2022 125 The constituent rocks of the Kebo Butak Formation consist of andesite breccias, tuffs, tuff lapilli, agglomerates, and clays. Several measurement points are located in the Jonggrangan Formation (Tmj), which overlaps or hovers over it. The rock composition of the Jonggrangan Formation is in the form of conglomerates, marl, tuff, limestone sandstone with lignite inserts, inset limestone, and limestone, while the upper layer consists of bioherm grey limestone interspersed with marl and layered limestone. The thickness of this formation is 2,540 meters and is estimated to be Miocene-Pliocene (Trianda et al, 2018). Fig 1. Geological Map of the Research Area (Rahardjo, et al. 1995) 3. Method 3.1 Data Acquisition Microtremor measurements were carried out around Gerbosari Village, Samigaluh District, Kulonprogo Regency using 31 measurement points with a distance between points of 650 meters using a sampling frequency of 100 Hz with a measurement duration of approximately 45 for each measurement point. Microthermal measurements are based on the terms of the Sesame European Research Project using the HVSR (Horizontal to Vertical Spectral Ratio) method analysis. The measurement used hardware instruments in the form of a set of Lennarzt 3D/20s Seismometer, Garmin GPS Hand Help, Sesame Log Sheet, and software in the form of DataQ Instrument, Geopsy, Rockwork, and Arcgis. 3.2 HVSR Method (Horizontal to Vertical Spectral Ratio) The Horizontal to Vertical Spectral Ratio was first introduced by Nogoshi and Iragashi (1970) which states that there is a relationship between the horizontal and vertical component comparisons of microtremor measurements which was later developed by Nakamura (1989). The horizontal-to-vertical (H/V) method has the potential to significantly contribute to site effects evaluation, in particular in urban areas (Bonnefoy-Claudet, 2008). The HVSR equation is stated based on the pattern below: 𝐻𝑉𝑆𝑅 = √(𝑆𝑁𝑆) 2 + (π‘†πΈπ‘Š) 2 𝑆𝑉𝑆 (1) 𝑆𝑁𝑆 is the horizontal spectrum component in the surface layer in the north-south direction, π‘†πΈπ‘Š is the horizontal spectrum component in the west-east surface layer and 𝑆𝑉𝑆 is the vertical spectrum component on the surface. 3.1 The Dominant Frequency (π’‡πŸŽ) and Amplification Factor (π‘¨πŸŽ) The dominant frequency is the frequency value that often appears in an area so that it is recognized as the frequency value of the rock layers in that area. It can indicate the characteristics of the rock types that comprise it. The dominant frequency value is related to the depth of the reflected plane subsurface, where the reflected plane is the boundary between loose sediments and hard rock (bedrock). So that if the value of the dominant frequency is small, it comes from the reflection of the wave which shows the deeper of the reflection plane. The amplification factor is the magnification of the wave acceleration that occurs on the surface due to the type of soil in an area. The magnitude of the amplification value can be estimated from the contrast of the wave propagation parameters, namely the density and velocity of the bedrock and surface sediments (Nakamura, 2000). The greater the difference in these parameters, the greater the amplification value of the wave propagation (Gosar, 2007). If the dominant frequency value (𝑓0) is high, it means that the subsurface is composed of hard rock. And if the dominant frequency value (𝑓0) is low, it means that the subsurface is composed of soft rock (sedimentary rock). Dominant frequency (𝑓0) is influenced by subsurface velocity (𝑉𝑆) and sediment layer thickness (h) (Mucciarelli M, Gallipoli MR. 2004). 3.2 Inversion of Dispersion Curve HVSR curve inversion processing was conducted by using Dinver software that a neighbourhood algorithm (Wathelet, 2005). The assumption is that the used microtremor waves are dominated by Rayleigh waves (Molnar et.al, 2022). Processing begins with the HVSR curve Formation Tmok (Kebo Butak Formation) Tmj (Jonggranan Formation) Koluvium Andesite 126 Khayati,N.N. et al./ JGEET Vol 7 No 3/2022 which is used as input in the ellipticity curve by providing a limitation of the initial model parameter values including P wave velocity (𝑽𝒑), S wave velocity (𝑽𝒔), density (ρ), and Poisson ratio so that the error or mismatch value is obtained (misfit) is lowest (0 ≀ misfits <1). Some examples of the ground profile appearance of the processing results are shown in Figure 2, which shows the value of shear wave velocity (𝑽𝒔) and layer depth for each measurement point. Fig 2. Ground Profile of Inversion Result 𝑽𝒔 Value. 4.1 Result and Discussion 4.1. Microzonation Dominant Frequency Value (π’‡πŸŽ) and Amplification Factor Value (π‘¨πŸŽ) The distribution of the dominant frequency values in the study area has a range of values ranging from 1 – 22 Hz. The measurement results show that almost all research areas have a dominant frequency value that tends to be low which is indicated by the blue colour except at 3 points namely N30, V27, and N8B which have high dominant frequency values which are represented by reddish-orange. The research area that has a low dominant frequency value shows that the area is an area with a very thick surface sediment thickness with a value range of 1 - 8 Hz. On the contrary, an area that has a high-frequency value indicates that the area has a surface sediment layer that tends to be thin and hard rock shallow with a value range of 15 – 22 Hz. The micro zonation of the dominant frequency values is shown in Figure 3. Fig 3. Distribution of the dominant frequency values in the study area. The distribution pattern of the distribution shows that the north and south sides of the study area with a history of landslides that are seen directly based on field observations during the acquisition are in an area that has a low- frequency value which indicates that the area has a thick enough surface sediment layer and it is indicated that the rock the base is at a fairly deep depth. Amplification is influenced by shear wave velocity (𝑽𝒔) which is related to the rock density due to reduced rock density which will cause an increase in the amplification factor value. The level of rock density can reduce the Khayati,N.N. et al./ JGEET Vol 7 No 3/2022 127 amplification of shocks because the amplitude of the waves propagating in solid rock is relatively small, while in rocks with soft characteristics it will slow down the propagating waves and increase the amplitude of the waves which can cause the potential level of landslide due to shocks originating from large earth shakes. The distribution of the amplification factor value can by seen in Figure 4. Fig 4. Distribution of the amplification factor value distribution in the study area. Table 1. Variation of 𝑽𝒔 value and layer depth Name Point Value of Layer 1 𝑽𝒔 Value of Layer 2 𝑽𝒔 Value of Layer 2 𝑽𝒔 Layer Depth 1 Layer Depth 2 Layer Depth 3 N1 357.05 897.11 1446.24 8.4097 51.002 104.29 N2 353.52 862.10 1431.61 11.222 60.405 104.12 N3 357.08 812.65 1208.58 7.536 58.04 103.647 N4 353.54 1185.01 1322.95 12.52 77.43 104.28 N5 357.05 1094.90 1418.02 7.613 53.07 102.83 N6 357.03 820.26 1432.11 8.928 80.6 102.26 N7 353.37 788.25 1000.48 5.994 56.901 103.29 N8 353.42 952.30 1197.30 4.6287 93.587 104.17 N12 352.55 888.26 1258.75 8.26 76.696 104.31 N13 353.52 812.08 989.63 7.1 55.783 104.14 N14 357.02 888.35 1221.81 10.7847 54.684 104.31 N15 356.96 905.83 1073.09 6.89 91.74 104.06 N16 357.00 796.13 1460.71 7.463 72.975 103.57 N19 357.10 844.98 1445.46 6.949 63.484 102.35 N20 353.52 764.91 1233.88 1.8716 97.381 104.304 N21 353.47 915.12 1446.56 9.666 58.048 106.668 N22 353.5 780.41 1446.35 6.4281 52.546 72.216 N23 353.5 820.3 1489.85 8.493 64.761 104.25 N26 353.38 871.36 1475.52 22.745 72.24 104.29 N27 353.51 888.04 1322.59 9.29 62.234 104.31 N28 353.53 780.58 1490.10 5.113 62.252 104.25 N29 353.49 820.28 845.16 4.769 77.462 108.14 N30 357.03 1127.84 1431.89 7.383 55.383 103.28 N31 353.504 828.59 1460.86 12.645 55.226 102.2 N24 360.68 888.39 1514.66 8.164 55.231 123.21 N25 360.62 780.48 1514.92 5.994 91.744 113.76 N18 353.55 828.55 1545.69 6.756 56.34 137.49 N17 353.52 836.92 1560.77 7.166 57.44 128.71 N11 360.66 780.08 1607.92 7.7176 52.04 137.49 N10 357.01 897.11 1560.64 9.7627 54.14 117.25 N9 353.52 812.76 1530.1 17.558 67.35 110.45 The value of the amplification factor in Gerbosari Village has a range of values from 1 – 10.5 and is classified into 3 classes, namely low with a value range of 1.19 – 3.12 indicated by blue. The areas with moderate amplification values have an amplification factor value range of 3.12 – 6.24 indicated by green and areas with high amplification factor values have a range of values ranging from 6.24 - 10.55 which is indicated by dark purple. When we see the distribution of the amplification factor value distribution in the study area, it shows a pattern in the form, on the north side and west side the amplification factor value tends to be medium-high which indicates that 128 Khayati,N.N. et al./ JGEET Vol 7 No 3/2022 the area has a large contrast of bedrock and surface sediments. Meanwhile, the western and southern sides of the research area show a low amplification factor value. 4.2 Ground Profile 𝑽𝒔 Ground Profile 𝑽𝒔 value is a method that can be used to determine the characteristics of the constituent material so that it can be used to determine subsurface lithology. To obtain parameter values is adjusted to the geological conditions in the study area, namely, the value of 𝑽𝒔has a range between 350 – 1500 m/s, the value of 𝑽𝒑 has a value range of 250 – 2500 m/s, and the value of Οƒ has a range of values between 0.2 – 0.5 and a density of 1200 – 2800 kg/m3. It can be assumed that there are 3 layers in the area. And the ground profile identifies 3 layers with varying depths as being shown in Table 1. The first layer has a thickness variation of 1 – 22 meters, the second layer varies in thickness from 51 to 97 meters and the third layer varies in thickness from 72 to 123 meters. 4.3 Microzonation of Weathered Layer Thickness Value Based on the inversion results, the coating value for each point as shown in Table 1 can be used to microzone the weathered layer thickness value in the area by using the thickness value in the first layer as being shown in Figure 5. Based on the results of microzonation in Figure 3, it can be seen that most of the research areas have a high enough thickness value of the sediment layer on the Northeast side, the middle side of the research area, and the southern part of the study area with a value range between 12-22 meters which is shown by green and red colour. Thick weathered layers are around points N31, N26, N23, N16, N12, N11, N10B, and N9 with thicknesses of 12.6, 22.7, 8.4, 7.4, 8.2, 7.7, 9.7, and 17.5 meters. Meanwhile, the areas with thin weathered layer thickness characteristics have values ranging from 4.6 - 7.3 meters which are indicated by the blue colour. 4.4 Analysis of Slopes The slope of an area that has the potential for landslides is a major factor, especially in areas with a slope of more than 15Β°. The slope in an area is the main controlling factor that causes landslides. A steep slope or cliff will increase the driving force. And when the driving force of a rock on a slope is greater than the bearing force, a landslide will occur. The determination of value of the slope is determined by using height data from DEMNAS, the classification of slopes is divided into 7 classes regarding the slope classification by (Sitanala,1989) as shown is in Figure 6. Based on Figure 6, the slope of Gerbosari Village is dominated by a rather steep and very steep classification on the north and south sides with a history of landslides that are quite frequent. This shows that this side of the area has a high enough potential due to the effect of the slope. Besides the slope, the north side has a weathered layer thickness with a high classification with a value of 16-22 meters. The soil characteristics in the area are at a frequency value of 1 - 5 Hz which indicates that the area has soft soil types. Meanwhile, on the middle side, it shows the slope with a slightly sloping rather steep classification and it is can be known that the area has the potential for landslides which is quite low when compared to the north and south sides. Fig 5. Microzonation of weathered layer thickness values in Gerbosari Village. Khayati,N.N. et al./ JGEET Vol 7 No 3/2022 129 Fig 6. Map of the slope of the village of Gerbosari slopes. Fig 7. The layer of weathered thickness around the N30 and N26 points. 130 Khayati,N.N. et al./ JGEET Vol 7 No 3/2022 4.5 Landslide Potential Analysis Based on the history of landslides that often occur in Gerbosari Village, landslides often occur on the north side of the research area and are around points N30 and N26. The results of the analysis show that the area is on a rather steep slope as shown in Figure 6. Based on the results of the inversion and a cross-section of the incision was performed to determine the thickness of the weathered layer in the area as shown in Figure 7. Figure 7 shows, the area is on a slope of 30 Β° with a weathered layer thickness of 17m as the driving factor. The existence of a driving factor disrupts slope stability, especially on steep slopes, which can cause landslides to occur. Landslides in this area are also caused by the influence of high rainfall intensity. Where the slope instability is due to periodic rain which causes the water in the soil to flow laterally so that the degree of saturation of the soil increases and reduces the pore water pressure resulting in decreased shear strength. This decrease in soil shear strength and increase in shear stress causes landslides to occur. 5. Conclusions The result of microtremor measurement shows that the dominant frequency is in the range of 1 - 22 Hz. In the research area, the dominant frequency value tends to be in a low class which indicates that the area has soft soil characteristics. The amplification value in the study area shows the pattern on the north and west sides of the study area have a high enough value which indicates that the area has a small rock density which causes a high magnification of waves. The value of the amplification factor in Gerbosari Village has a range of values from 1 - 10.5. The study area is in a condition with a slope classification that is rather steep - very steep with a slope of 30-> 65 Β°. The north and south sides, of the study area, have a very steep slope classification where in this area, there is a history of previous landslides. Then, based on the inversion results for each point, the value of each layer is obtained where the layer which is assumed to be weathered is used for the analysis of the thickness of the weathered layer as a driving factor for landslides in steep areas. 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