[Type text] Identification of Cimandiri Fault Activity at Sukabumi Area, West Java, Indonesia (Based on Morphometry Analysis) Abs. No. 29 Supartoyo, Imam A. Sadisun, Emmy Suparka, Chalid Idham Abdullah Faculty of Earth Science and Technology, ITB Sri Hidayati Center for Volcanology and Geological Hazard Mitigation, Geological Agency ABSTRACT：Sukabumi is one of the areas in Indonesia which is prone to the earthquake. The earthquake sources are concentrated along the subduction zone which correspond to convergent plate boundaries and the active fault zones. Sukabumi had experienced several destructive earthquakes of shallow depths. In this area a famous fault zone lies along Cimandiri valley called Cimandiri fault. It lengthens from Palabuhanratu to the south of central city Sukabumi and continues to the Cianjur area. Combination of Landsat and Shuttle Radar Topography Mission (SRTM) shows 2 lineaments at the Cimandiri valley which are W-E in the western and NE-SW in the eastern. Further analysis based on landsat, SRTM and field observation, the Cimandiri Fault can be divided into 4 segments, namely segment 1, 2, 3 and 4. Morphometry analysis uses to identify activity of Cimandiri fault compose of six parameters namely drainage basin asymmetry (AF), hypsometric, stream length gradient index (SL), mountain front sinuosity (Smf), valley floor width - valley height ratio(Vf) and drainage basin shape indices (Bs). The result found that AF is far from value of 50 indicates the presence of tectonic tilting, the hypsometric curve shows that most of the Cimandiri Fault belongs to young topography. Meanwhile the SL is more than 300 indicating of active tectonic, Smf value is less than 1,5 indicating straight of mountain front sinuosity as a Cimandiri Fault Zone, Vf less than 2 shows most of V shape valley and Bs mean is more than 2 shows elongated drainage basin along Cimandiri Fault Zone. All the morphometry para- meters analysis indicates that the Cimandiri Fault is an active fault and in the segment 3 and 4 that located in eastern parts of Cimandiri fault are more active than segment 1 and 2. The destructive earthquake in 1982 with magnitude 5.5 RS was probably related to the segment 3 and 4 of Cimandiri Fault. 1. Introduction active fault based on epicenter distribution along Sukabumi area is located at the western part of Cimandiri Fault Zone (Soehaimi et al., 2004 and West Java Province, boundary by Cianjur regency in 2007; Kertapati 2006) and its movement by GPS the east, Bogor regency in the north, Banten prov- (Abidin 2008). In this study the activities of CF ince in the west and Indian Oceanic in the south. The would be analyzed using geomorphic and morpho- area is one of the areas in Indonesia which vulnera- metric analyses. ble to the earthquake. The earthquake sources are concentrated along the subduction zone, which cor- 2. Tectonic and Geological Setting respond to convergent plate boundaries and the ac- The Indonesia archipelago is resulted from inte- tive fault zones. This area had experienced several raction amongst four active plates: Hindia-Australia, destructive earthquakes of shallow depths. These Eurasia, Pasific and Philippine. The Hindia- earthquakes had caused victim and damaged build- Australia plate moves to the northeast with velocity ing in this area. The destructive earthquake events 7 cm/ year, Eurasia plate to the south (0.4 cm/ year), generally when the earthquake has an epicenter lo- Pacific plate to the west (11 cm/ year) and Philippine cated in land and shallow depth, such as occurred in sea plate to the northwest with velocity 8 cm/ year 1962, 1973, 1975, 1977, 1982 and 2000 (Supartoyo (Minster and Jordan, 1978 in Yeats et. al., 1997). and Surono, 2008). The western part of Indonesia is mostly affected by In Sukabumi area, there is a fault zone lies along interaction between Hindia-Australia and Eurasia Cimandiri valley called Cimandiri Fault (CF). Its plate. It causes the formation of Java-Sumatera length stretches from Palabuhanratu bay to the south trench, Java–Sumatera fore arc basin, the Java– of central city Sukabumi and continues to the Cian- Sumatera outer arc partly submerged south of Java, jur area. Previous studies suggested that CF was an Java and Sumatera magmatic arc, foreland basin of Proceeding of 1st International Seminar of Environmental Geoscience in Asia (ISEGA I), Oktober 2013, page 76 – 83. the Sunda continental shelf, pull apart basin in Su- CF as a reverse fault and inactive fault. The field ob- matera and Java–Sumatera back arc basin. Apart servation along Cimandiri valley showed manifesta- from that, it also produced of earthquake source in tion of this fault such as fault scarp, slicken side, the sea along subduction zone and inland from ac- fault breccias, drag fault, broken zones, lineament of tive fault. This condition makes Indonesia prone to spring water, valley and triangular facets. geological hazard especially earthquake and tsuna- mi. 3. Methods Based on the physiographic map of West Java In this study we analyze the activities of CF using (Bemmelen,1949) and geological setting (Martodjo- morphometry parameters. Morphometry is defined jo, 2003), West Java can be divided into three trend- as quantitative measurement of landscape and land- ing belts from north to the south which are the low form shape (Keller and Pinter, 1996). Quantitative plain of Jakarta reflecting the shelf basinal area; the measurement allows geomorphologist to objectively Bogor zone a shelf edge with deep basinalturbidite compare different landform and to calculate less sediments were accumulated, folded and thrusted; straightforward parameters (geomorphic indicates). the southern mountains of West Java. The Cimandiri This information is very useful for identifying a par- Fault Zone (CFZ) is located south of Bogor zone and ticular characteristic an area to identify of tectonic mostly covered by young volcanic deposits. Based activity level. This method can be used for rapid of on morphogenetic unit the Sukabumi area is sepa- tectonic evaluation for large areas. Also this method rated by volcanic folded hills and mountain and vol- is very easy to do and the data can be obtained from canic arc of Sunda/ Banda system (Verstappen, topography maps and aerial photographs (Keller and 2000). This separated lies on CFZ. Pinter, 1996). Field observation divided morphology of Suka- Morphometry analyses have been applied to ana- bumi area into three zones, which are flat, moderate lyze the activities of several faults, such as to ana- and steeply hills. The flat occupies the coastal area, lyze fault activities in Vulcanic Trans-Mexican along Cimandiri valley and northeast of Sukabumi. based on Digital Elevation Model (DEM) resolu- Elevation of the coastal area is less than 10 m com- tion 50 m and topography map scale 1 : 50.000 prising Palabuhanratu, Simpenan, Ciemas, Cisolok, (Szynkaruk et al., 2004), to analyze activities of Ciracap, Surade, Cibitung and Tegalbuleud. Several Kompotadesdan NeaAchialos fault in Yunani based coastal areas are hilly with elevation about 50 m and on topography map scale 1 : 50.000 (Zovoili et al., steeply with exposed lava and sandstone in Simpe- 2004), to analyze active deformation on Pannonian- nan, Ciemas and Cisolok. Hungaria basin based on topography map scale Steeply to moderate hills are dominant around 1 : 200.000 and 1 : 50.000 (Pinter, 2005), to analyze Sukabumi area where Cimandiri River runs perpen- activities of North Anatolian Fault topography map dicular through the area to Palabuhanratu Bay. The scale 1 : 25.000 (Gurbuz and Gurer, 2008). Cimandiri valley lies in west-east direction, starting In this analysis topographic map with scale from Palabuhanratu Bay to Warungkiara and bend to 1:25.000 is used to calculate the morphometry pa- southwest- northeast from Cikembar to the southern rameters. This map was published by Badan Koor- of Sukabumi. Mostly of Sukabumi area is covered dinasi Survei dan Pemetaan Nasional (Bakosurtanal) by quaternary deposit such as young volcanic depo- and composed of 11 sheets, namely Balewer, Cida- sits and alluvial. These quaternary deposits are vul- dap, Cigenca, Jampang Tengah, Nyalindung, Tako- nerable to earthquake, due to loose and unconsoli- kak, Palabuhanratu, Cigombong, Cibadak, Sukabu- dated. mi and Gegerbitung. The main manifestation along Cimandiri valley is CFZ. This zone stretches from Palabuhanratu Bay to 4. Geomorphic and Morphometric Analyses Bandung area and occupies between Bandung zone The topography of the CF in the study area is a and the southern mountains. Using landsat image, valley lengthening from Palabuhanratu bay to the Suwijanto (1978) identified CF from the lineament southern of Sukabumi city. In the western part the and several earthquake events along the Cimandiri trend of valley is west – east and in the middle turn valley. Dardji et al. (1991) measured fault slip in to northeast – southwest. Cimandiri valley occu- CFZ and resumed that CF is a sinistral strike slip pied by fluviatil deposit composed of boulder, sand, fault and northeast–southwest directions. Martodjojo silt, mud, and clay. On the south of western part of (2003) resumed that CF is a normal fault and part of Cimandiri valley is bordered by Jampang Formation Meratus trend with northeast–southwest directions. which consist of volcanic breccias and lava as a fault Kertapati (2006) said that CF was a normal fault scarp. On the north of eastern part it is covered by type and trending in southwest–northeast direction Quaternary deposits consist of lahar, lava, and ande- sit basalt originated from Pangrango volcano. with component of strike slip which generated de- Landsat and Shuttle Radar Topography Mission structive earthquake along Cimandiri valley. Hall et (SRTM) of Sukabumi area show a lineament along al., (2007) and Clements et al., (2009) suggest that the Cimandiri valley as a CFZ and as seen in yellow [Type text] dash line in Figure 1. This zone aligned in west-east Pinter 1996).