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The Identification of Seulimeum Fault System in Iejue, Aceh Besar (Indonesia) by Using Magnetic Method Muhammad Taqiuddin Zakaria Postgraduate Student, Geophysics Section, School of Physics, 11800 Universiti Sains Malaysia, Penang, Malaysia; e-mail: [email protected] Dr. M.M. Nordiana Lecturer, Geophysics Section, School of Physics, 11800 Universiti Sains Malaysia, Penang, Malaysia; e-mail: [email protected] Dr. Rosli Saad Associate Professor Senior Lecturer Geophysics Section, School of Physics, 11800 Universiti Sains Malaysia Penang, Malaysia; e-mail: [email protected] Dr. I. N. Azwin Lecturer, Geophysics Section, School of Physics, 11800 Universiti Sains Malaysia, Penang, Malaysia e-mail: [email protected] Nur Khairunnisa Aqilah Yunos Postgraduate Student, Geophysics Section, School of Physics, 11800 Universiti Sains Malaysia, Penang, Malaysia; e-mail: [email protected] ABSTRACT Magnetic is a non-destructive geophysical technique that measure the Earth's magnetic field at specific location. It has numerous application in engineering and environmental field such as locating a fault zone. Fault are described as planar or gently curved fracture in a rock mass where there are significant relative displacement along the fracture as a result from the earth movement. The purpose of this study is to identify the Seulimeum fault system in Iejue, Aceh Besar (Indonesia) by using a magnetic method and correlate the result with geological map and also to understand a trend pattern of fault system. Magnetic survey was conducted with the proton magnetometer with 50 m interval of each magnetic station (rover). This survey consist of three difference survey lines with length of 1 km of each line. The magnetic data was processed by utilizing Microsoft excels and Surfer10 software which was displayed in a form of contouring and revealed fault zones. The local magnetic value covers from -150 to 600 nT. The result showed a low residual magnetic value at a north part compared to south part of study area which indicated a pattern of fault zone. Magnetic residual was successfully correlated with the geological map which show the existence of the Seulimeum fault in this study area. KEYWORDS: Magnetic, Seulimeum fault, Iejue, Aceh Besar - 533 - Vol. 21 [2016], Bund. 02 534 INTRODUCTION The Aceh province in Indonesia, located around the Sumatran subduction zone. The Great Sumatran Fault (GSF) is recorded as dextral strike-slip fault zone which contain a result from the oblique convergence of the subduction between the Indo-Australian and Eurasian plates (Bellier et al., 1997). Strike-slip faulting occurs hundred kilometers to the east of the trench and involves slip directed parallel to the trench which is pass through the Sumatra Island. This fault is classified into three major sections which are southern, central, and northern (Aydan, 2009). The Sumatran Fault with 1900 km long structure of the right lateral strike slip is highly segmented and consists about 20 major segments. Most segment are range in length about 60 to 200 km, (Natawidjaja and Wahyu, 2007). From the history of earthquake along Sumatran Fault that has been documented in 1980 shows the correlation of the segmentation of the fault and the location of the earthquake in the past ruptures (Natawidjaja et al., 2006). The sumatran fault runs length of the Barisan Mountains, a range uplifted basements blocks, granitic intrusions and Tertiary sediments, topped by TertiaryRecent volcanism. The Sumatran Fault System in Aceh can be classified into two major segments which are Seulimeum and Aceh fault. Geomorphic features shows the Seulimeum Fault still active compare to Aceh Fault. This fault cut through Plio-Pleistone sediments and volcanic product of the active Seulawah Agam volcano and displaces the axial traces of east-west trending folds. This process cause the Pliocene deposits moved from the southwest side of the fault to northwestwards (Bennett et al., 1981). GENERAL GEOLOGY Aceh is located at northern part of the Sumatra island. There are four major volcanosedimentary sequences, separated by unconformities are identified which is three are pre-Tertiary in age and others is Tertiary to Recent (Cameron et al., 1980). Sumatran Fault system occurs from the Middle Miocene and the opening of the Andaman Sea. Pre-Tertiary basements rocks outcrop mainly along the central spine of the Barisan Mountains, which extend the length of the island parallel to the southwest cost. Figure 1 shows the regional tectonic setting of sumatra island. The area from northeast and southwest is overlain by Tertiary sedimentary and volcanic rocks. Rock unit of all ages are transacted by Sumatran Fault which follow the NW-SE trend (Barber and Crow, 2003). The geology of Banda Aceh Quadrangle has been mapped (figure 2) by Bennett et al., 1981. The lithology of the study area, Ieju is dominated by Lam Tuba volcanic which is composed of andesitic to dacitic volcanic, pumiceous breccia, tuffs, agglomerate and ash flows with composition of tuffaceous and calcareous sandstones, conglomerates and mudstones(Bennett et al., 1981). The geological formation formed a topographic depression, occupied with alluvial flat and low flat-topped hills within Barisan Range (Katili and Hehuwat, 1967; Page et al., 1979). Figure 3 shows the location of Iejue in the google earth. Vol. 21 [2016], Bund. 02 Figure 1: Regional tectonic setting of Sumatra. 535 Vol. 21 [2016], Bund. 02 536 STUDY AREA Scale 0km 5km 10km Figure 2: Redraw and simplify from: Geological Map of Banda Aceh Quadrangle, Sumatra (Bennett et al. 1981). Vol. 21 [2016], Bund. 02 537 Legend Geology fault Figure 3: Location of the study area: Iejue (Google Earth, 2015). METHODOLOGY The ground magnetic survey is carried out with proton precession magnetometer device and Global Positioning System (GPS) navigation for real-time measurements. Three difference survey line which is L1, L2, and L3 with length of 1 km and 50 m interval of each magnetic station (rover) is setup in order to identifying the subsurface characteristics. A base station with magnetic homogeneity is carefully selected to record magnetic reading at time interval 60 seconds for diurnal correction affected by the Earth's field. Figure 4 and 5 shows the magnetic line and magnetic station in a survey area. The magnetic data is measured independent grid was later combined to produced single master grid. This single master grid is processed using Surfer10 software to produced full magnetic map (figure 6). All the magnetic data were corrected for diurnal variation and IGRF correction to obtain a corrected magnetic anomaly data for mapping the residual magnetic contour map. Vol. 21 [2016], Bund. 02 538 Legend ML1 : Magnetic Line 1 ML2 : Magnetic Line 2 ML3 : Magnetic Line 3 Scale: 0m 500m 250m Figure 4: Magnetic survey line of the study area (Ieju). Legend Magnetic station Figure 5: Magnetic station on a survey line area. Vol. 21 [2016], Bund. 02 539 RESULTS AND DISCUSSION Magnetic results show lateral view of the faulting system in the study area (Figure 7). The local magnetic value covers from -150 to 600 nT. The total intensity mangnetic anomaly study area shows low magnetic value from -150 nT to -50 nT at northwest part while highly magnetic anomalies from range 200 nT to 450 nT at southeast of study area. The long dislocation from northwest part to southeast direction of study area is interpreted as geological fault. Magnetic Residual (nT) LEGEND Figure 6: Magnetic anomaly of local residual LEGEND Magnetic Residual (nT) Fault line system Figure 7: Magnetic anomaly of fault line system. Vol. 21 [2016], Bund. 02 540 CONCLUSION The magnetic result from this survey shows that a Seulimeum fault trend is from NW-SE direction and also supported by geological map that suggested there are several small fault exist in this survey area. From this survey it shows that magnetic method is suitable in determination and mapping the fault zone beside identify the subsurface characteristic. ACKNOWLEDGEMENTS The authors wish to thank to lecturers, technical staffs and all member of Geophysics group, Universiti Sains Malaysia (USM), Penang, Malaysia, all students and staffs of Faculty of Science, Universitas Syiah Kuala(UNISYIAH), Banda Aceh (Indonesia) for their assistance during the geophysical field data acquisition. REFERENCES 1. Aydan, Ö. (2009). A Reconnaisance Report on the Pariaman-Padang Earthquake of September 30, 2009. Earthquake, (October), 1–36. 2. Bellier, O., Sébrier, M., Pramumijoyo, S., Beaudouin, T., Harjono, H., Bahar, I., & Forni, O. (1997). Paleoseismicity and seismic hazard along the Great Sumatran Fault (Indonesia). Journal of Geodynamics, 24(1-4), 169–183. 3. Bennett, J. D., McC, D., Bridge, Cameron, N. R., Djunuddin, A., Ghazali, S. A., Jeffrey, D. H., Kartawa, W., Keats, W., Rock, N. M. 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