Seismotectonic Analysis to Interpretation Fault Zone and Subduction Zone in West Java. How much effective?

Transcription

1 Seismotectonic Analysis to Interpretation Fault Zone and Subduction Zone in West Java. How much effective? Nouval Wibowo, N. 1 Rosyadi, H. 1 1 PetroClan, University of Pembangunan Nasional Veteran, Yogyakarta, Indonesia Abstract Indonesia located between Indo-Australian, Eurasian plates, and Indonesia Pacific that prone to impact from earthquake. Strategies and methods are appropriate that handling of geological disasters in earthquake zones. Methods applied in geological disaster mitigation should be on targeted. Geological disaster mitigation activities is done by the Geological Structure Analysis Radar Imagery Interpretation (SRTM), analysis of earthquake, focal mechanism Analysis, source area analysis of earthquakes and other data that support for mitigation. It determine geological structure phenomena s of a region, historical seismicity where determine areas prone to earthquakes. Main structural trends in West Java shown that namely Sumatra trend or direction, Sunda direction, Java direction and Meratus direction. Distribution of seismicity in West Java is dominated by shallow earthquakes and medium earthquakes, where most dominant in Sukabumi, Cianjur, Tasikmalaya, and Bogor. Seismicity schematic cross section shows shallow earthquake on surface (continental), intermediate earthquake under surface of the continent, and earthquake in Beniof or subduction of ocean crust Seismotectonic analysis can make seismotectonic map and mark potential occurrence of an earthquake, or source of earthquake zone that good information for urban development and mitigation. Key Words: earthquake, West Java, structure, mitigation, urban development, Indonesia. Introduction Seismotectonic is a relationship between earthquakes and tectonic processes (Lay and Wallace, 1995). Earthquake as a symptom or branch of tectonic activity (Scholz, 1995). Seismotectonic studies generally associated with understanding about main control of surface deformation what control causes deformation of earthquakes. This issue is very important for assessing seismic risk. Seismotectonic uses earthquake science, geodesy, geology, rock mechanics and engineering models. Many study case of seismotectonic concentrated about distribution for earthquakes areas, time, size and style. Gravity data, seismic, Landsat / aerial photographs and field observations (outcrop) where West Java divided 3 (three) fault patterns that northwest southeast direction (direction Sumatra), east west (direction Java), and north-south very dominant northern part of Java and the Java Sea region (Sukendar Asikin, 1978).Landsat and epicenter of the earthquake has pattern again trending northeast-southwest that very prominent southwestern of Java (Cimandiri / Sukabumi).Earthquake epicenter distribution from medium and shallow, that pattern University Publication (2009) 1

2 considered still active. Northwest southeast trending pattern recorded on gravity data, which means involves bedrock. Fault patterns interpreted from lineation from old trail tectonics in Sumatra. It located nearly west-east where generally fault (north) and involves sediments of Tertiary, while trending north-south in north of Java where seismic data appear unless tertiary rock that control bedrock. One of north-south fault is a segment that separates Bogor and southern mountains. Position as tectonic elements has considered important because their existence has not separated different structural patterns but precipitation patterns between segments with surrounding (Bogor segments and segment exposure of continent). This note gives an introduction to some of the equipment and analysis that necessary to do this. Methodologies Data collected from of IRIS Earthquake Browser, Data Image Radar (SRTM) and base maps of Indonesia. Process and analyze these data is include Seismotectonic analysis which Geological Structure Analysis with Radar Imagery Interpretation (SRTM), Analysis of Earthquake, Earthquake Focal Mechanism Analysis and seismotectonic map. Interpretation was done by pulled the straightness of Citra Radar (SRTM) to obtain appearance of structure pattern in area. Interpretation was done by pulled of Citra Radar (SRTM) to obtain appearance structure pattern. Pattern of this structure can know for regional geological structures and tectonic fabric pattern to determine fault movement. This is done to seismotectonic analysis. Data collected from IRIS Earthquake Browser that based data (depth) then performed using MapInfo GIS software. Data plotted into base map of Indonesia where especially in West Java. Distribution map shows earthquake point with a variety of colors, which corresponds to depth. Seismotectonic studies relationship between tectonic processes with Earthquake. In this case analysis of the structures contained in area to determine the kinds of deformation (tectonic) that associated with fault movement. Then structure of data connected to historical data of seismicity / distribution epicenter for seismotectonic analysis that data analyzed to determine earthquake potential and pattern of the structure. University Publication (2009) 2

3 Structural interpretation based on image Radar (SRTM) and Regional Geology Image Radar (SRTM) is used for draw lineament structures as indication of fault. (SRTM) is clearly map that appearance of straightness of the ridge, valley, escarpment, river, and hue. This straightness of SRTM identified as an indication of fault. (SRTM) are obtained much straightness of structure which diverse pattern fault that grouped into 4 pattern / dominant direction as trend. Southwestern Northeast is direction of Sumatra, east-west direction as direction of Java, north south predominant in the north direction of Java and Java Sea region also known as Sunda and last direction toward northwest or southeast direction as Meratus. Regional Structur used map from N. Ratman & S. Gafoer, The appearance of the geological structure is based on regional map that compared with field and map. In geological map is seen clearly structure of West Java pattern, but the appearance of structure from geological map is basically as regional outline. Figure 1. Structure lineaments is indication of fault in SRTM which overlaying map can control data interpretation from SRTM and regional map. Digitized pattern of regional structure west Java is overlay with results from image interpretation Radar (SRTM) that next structural developments analysis. University Publication (2009) 3

4 Figure 2. Structural lineaments is data of dominant direction in West Java. It analysed in rosette diagram that showed 4 main structural trends in West Java. It is Sumatra direction, Sunda direction, Java direction, and Meratus direction that dominant structure is Sumatra direction. Overview intensity of lineament structures in study area. Structure pattern obtained from results of image interpretation Radar (SRTM) that corrected by comparing the pattern of regional structures in West Java to get right direction, pattern, and accordance with field condition. Contours areas and colour showed that area there have many geological structures, where as for shallow contours showed that areas have little geological structure. Contour used to look how structure in an area that possible dangers if active fault or have movement. It is presented by display density of contours and pattern structure of West Java. Figure 3. Intensity structures in west java have many structure.the red colour shows high intensity, blue (medium), and green (low). Analysis of Earthquake in West Java West Java is part of the Eurasian plate which moves relative to southeast interact with the Indian plate - Australia, that located in south of Java Island. Java Island is one of in the active plate areas that frequent tectonic activity characterized by high-frequency earthquake events. Based on distribution of University Publication (2009) 4

5 seismicity in West Java (Earthquake distribution maps sourced from IRIS Earthquake Browser), that began in 1973 until July 2009 how looks distribution of earthquakes that located on land and sea. Shallow earthquakes located in Indian Ocean subduction zone that related activities (subduction zone) between the Eurasian continental plates and oceanic plates Indo - Australia. Meeting plates produces a subduction zone is characterized by the presence of ocean trench that is a source of earthquake in the sea. Earthquake derived from subduction zone has potential to cause a tsunami, because magnitude can reach 8 Richter Scale. Tsunami recorded on southern coast of Java occurred on June 3, 1994 due to events with a magnitude of earthquake 7.2 Richter Scale. Pengandaran Tsunami occurred on July 17, 2006 due to the earthquake events how magnitude of 7.7 Richter scale. Thus the southern coast of Java prone for Earthquake and tsunami disaster. Shallow earthquake in West Java related to activity of active faults. Fault is cracks in the rocks which have undergone moved or displacement with each other that a shift or movement. Active fault never moved during Quaternary (1.8 million years ago) until now. Land earthquakes are derived from movement of active faults that very dangerous because potential disaster. Based on distribution shallow inland earthquake have seen in region of West Java where center of location Earthquakes prefer in land that compare with Central Java and East Java. It relates to active faults in West Java where active faults in Banten, active faults in district of Bogor, active faults Cimandiri in Sukabumi, Lembang faults in district of West Bandung, Baribis faults in district of Subang, and Majalengka faults in district of Garut, faults in district of Tasikmalaya. Distribution earthquake in West Java Distribution point of earthquake in West Java collected from 1964 to 2009 that t shallow earthquake and medium earthquakes most dominant in Sukabumi, Cianjur, Tasikmalaya, and Bogor West Java. It is visible areas that have relatively high potential earthquake and activities in this period that possible make a mitigation activities against earthquake disaster-prone areas. Distribution point earthquake in West Java as follows: University Publication (2009) 5

6 Figure 4 Sukabumi region has a most shallow earthquakes from all regions in West Java that has 122 points, moderate (38).Cianjur has shallow (56) and moderate (24). Tasikmalaya has shallow (53) and moderate (13). Bogor has shallow (36) and moderate (25). waves that processed using a particular method to obtain model of region mechanism focals. It used reference for active faults in region that triggered earthquake. Focal seismic data obtained from Global CMT. Traversing area of active fault has potential earthquake or has cause greater destruction than areas that has not structure or active faults. Data from map and graphic explained Sukabumi has a highest mechanism focal that compared to other regions in West Java where this area 6 mechanism focal illustrated 6 point movement of active faults. Mechanism focal generally recorded faults in the area is a reverse-slip fault and strike slip fault. One focal mechanism in Cianjur is a reverse-slip fault. One focal mechanism in Bandung is reverse fault.2 mechanism focal In Bogor are reverse-slip fault. Mechanism focal in Karawang is slip-reverse fault. Focal Mechanism and Faults Analysis Focal seismicity is a structural data of active faults movement where related to activities of earthquake, that focal point of seismicity based on seismographic data. It taken from propagation of P waves and S University Publication (2009) 6

7 Figure 5. West java has 3692 events of Earthquake in this map.it has classified that based on depth which red (0-50 km), green ( km), blue ( km) and greater than 300 km purple. Mechanism focal in Purwakarta is reverse fault. Mechanism focal in Sumedang is reverse fault, Kuningan mechanism focal is reverse-slip fault. 2 mechanism focals in Garut is reverse fault. 1 mechanism focal in Tasikmalaya is reverse-slip fault, and last in Ciamis district normal- slip fault. that beneath surface based on depth with the x-axis (Latitude) of earthquake distribution areas. West java areas divided into 3 parts of West Java area. The distribution is as follows: Sectional schematic interpretation made for subduction zone and the path of Benioff. Schematic cross section of West Java Schematic cross section made to determine distribution point of Earthquakes University Publication (2009) 7

8 Figure 6. Section A,B, C based on depth with the x-axis (Latitude) of earthquake distribution areas.grafik shows interpretation of subduction zone and benioff zone. Acknowledgments Thanks to Arif Rianto BN, ST. Msi as supervisor of GIS who guide writers until finish, Supartoyo, ST. MT. as the supervisor in geological research center and Dedy Wijayanto as friend of discussion. Conclusion Rosette diagram shows 4 main structural trends in West Java.It is Sumatra direction, Sunda direction, Java direction, and Meratus direction that dominant structure is Sumatra direction. Data distribution of seismicity from 1964 until 2009 is dominated by shallow earthquakes and medium earthquakes, where the most dominant in Sukabumi, Cianjur, Tasikmalaya, and Bogor. Sukabumi region University Publication (2009) 8

9 has a most shallow earthquakes from all regions in West Java that has 122 points. Seismicity schematic cross section shows that a shallow earthquake on surface (continental), intermediate earthquake under surface of the continent, and earthquake in Benioff or track to track subduction of oceanic crust. Geological structure analysis, Earthquake analysis and analysis of focal mechanisms can identify zoning or potential earthquake areas and source of earthquake zone. References Asikin, Sukendar Geologi Struktur Indonesia. ITB, Bandung. Martodjojo, S Evolusi Cekungan Bogor. Desertasi, Program Pasca Sarjana, Institut Teknologi Bandung, Tidak Diterbitkan. N. Ratman, & S. Gafoer, 1998, Peta Geologi Lembar Jawa Bagian Barat, Pusat Penelitian dan Pengembangan Geologi, Bandung. Pannekoek, A.J., 1949, Outline of the Geomorphology of Java, reprint from Tijdschrift van Het Koninklijk Nederlandsch Aardrijkskundig Genootschap, vol LXVI, part 3, E.J. Brill, Leiden. Pulunggono, A., dan Martodjojo, S. (1994) : Perubahan tektonik Paleogen-Neogen merupakan peristiwa tektonik terpenting di Jawa, Proceedings Geologi dan Geotektonik Pulau Jawa sejak akhir Mesozoik hingga Kuarter, Seminar Jurusan T Geologi Fak. Teknik UGM, Rusli Yunus, H.M., dkk. (1995), GempaBumi dan Tsunami, Direktorat ESDM, Badan Geologi, Pusat Vulkanologi dan Mitigasi bencana geologi, Bandung, hal Scholz, C.H., and J. Campos. (1995), On the mechanism of seismic decoupling and backarc spreading at subduction zones, J. Geophys. Sungkowo, A, dkk. (2008), Buku panduan praktikum Geologi Citra Penginderaan Jauh, Laboratorium Geologi Citra Penginderaan Jauh, Jurusan Teknik Geologi, Fakultas Teknologi Mineral, UPN Veteran Yogyakarta, Yogyakarta. T. Lay and T.C. Wallace. (1995), Modern global seismology, Academic Press, London, U.K. Van Bemmelen, R.W. (1949), The Geology of Indonesia, Vol. IA, Martinus Nijkoff, The Haque, Netherland. accessed at 1 Oktober WIB accessed at 17 Oktober WIB accessed at 17 Oktober WIB accessed at 19 Oktober WIB accessed at 19 Oktober WIB accessed at 19 Oktober WIB accessed at 19 Oktober WIB accessed at 27 Oktober WIB accessed at 19 Nov WIB accessed at 19 November WIB accessed at 19 November WIB accessed at 29 November WIB adar.htm, accessed at 7 Desember WIB University Publication (2009) 9