Preliminary Earthquake Risk Management Strategy Plan of Eskisehir, Turkey by using GIS

Transcription

1 Preliminary Earthquake Risk Management Strategy Plan of Eskisehir, Turkey by using GIS Metin Altan, Ferah Özturk and Can Ayday Space and Satellite Sciences Research Institute Anadolu University, TURKEY Eskisehir, Turkey SUMMARY Earthquake risk is defined as the product of hazard and vulnerability studies. The main aims of earthquake risk management are to make plans and apply those for reducing human losses and protect properties from earthquake hazards. Eskisehir, which is one the biggest urban area, is located in the northwest part of middle Anatolia. The population of the urban area is above half million and it is under the affect of North Anatolian Active Fault. In 1956, an earthquake of magnitude 6.4 took place close to urban area (approximately 15 kms) and caused deaths, injuries and produced serious damage to several structures. Natural risk managers are studying to identify and manage the risk from an earthquake for these highly populated urban areas. They want to put some strategic plans for this purpose. Risk managers need some input about these kinds of studies. Some input for preparation of earthquake risk management strategy plans were tried to find in this study. Tectonic settings, seismicity of the area were studied and magnitudes of historical earthquakes were used to predict the probability of occurrence of major earthquakes. After obtaining the critical earthquake magnitude, the study concentrated on the field data for providing soil dynamic properties. Number of measures are employed to characterise the earthquake s affects in the soil of studied area. Maximum horizontal peak ground acceleration was provided for the area. It is seen that soil property change from place to place. The soil classification map, which indicates soil properties, was prepared. In addition to these outputs, which can be used as an input for strategy plans, some critical properties and lifelines like natural gas lines, interchanges and schools have been indicated on the map. Geographic Information System (GIS) was used for the realisation of damage and loss estimation analyse in this part of the study. The earthquake risk manager can use these outcomes and plans as input for their own studies. KEYWORDS: Earthquake hazard, risk management, vulnerability, GIS INTRODUCTION It is known that, approximately 20 % of the world s population are living in seismically active zones. In 50 years time, half of the urban people in the world s 50 largest cities will live within 200 km of faults that are known to produce earthquakes of Richter magnitude 7 or greater (Tucker et al., 1994). Furthermore, 90 % of that population will be in risk the developing countries. It is also obvious that, socio-economic development of the countries has led to increase of losses due to natural disasters like earthquake. The aims of risk management are making a project and put some strategies to reduce not only human losses but also protect properties, lifelines, etc. due to earthquakes. It is seen that, when the number of unknowns and uncertainties increases in any study, the degree of risk increases dependently. It is known that, it is not easy to prepare an earthquake risk management plans of any urban area due to high number of input parameter needed. The goal of risk management is to identify the risk of the study and develop strategies to reduce them. So, the goal of risk management should be to move uncertainty away from risk and move towards opportunity (Wideman, 1992). The risk manager or the person who works 7 th AGILE Conference on Geographic Information Science 29 April-1May 2004, Heraklion, Greece Parallel Session 1.2- Decision Support Systems/Risk Management I 83

2 for this study must use all necessary information for making the right decision. The result will have perfect outcomes which we can make predictions with a high degree of confidence and low uncertainty. Earthquake risk management methodology is composed of by the integration of two different studies. Earthquake hazard analysis is one of them, which provides information on engineering geology, geomorphology, tectonics, seismicity and soil conditions. These data are used to compute the potential for ground motion and ground failure. Historical earthquake records were used to determine the seismotectonic property of the region. Regional tectonics of the region was studied to predict the type and source location of the earthquake. Dynamic soil property of the studied area, like peak ground acceleration was calculated by using attenuation functions. Using conic penetration test (CPT) results does classification of the soil according to soil type. Vulnerability investigations were done in the second stage. Only limited number of infrastructures and lifelines were included in this study. These critical facilities which are included in this study are major roads, interchanges, railroads, main streets, main natural gas lines, schools, hospitals, governmental buildings. Influences of these infrastructures and lifelines due to the earthquake-induced soil with the proposed earthquake magnitude were analysed in the form of GIS database and maps. The length and area attributes of each property were calculated. The earthquake risk manager can use these outcomes and maps as input for their more detailed studies. The use of GIS allows fast and convenient handling of large data files. The incorporation of GIS techniques especially for risk management study is strongly recommended for these kinds of studies. STUDIED AREA The city of Eskisehir is situated in the northwest of Middle Anatolian. With more than half million population, it is one of the biggest cities of Turkey located between Ankara and Istanbul railroad (as illustrated by figure 1). If you look at it from the seismic view, it is less than 100 kms. to the Figure 1: The location maps of the studied area. North Anatolian Fault (NAF), this is known as one of the most famous active faults like San Andreas. It is defined as sister of San Andreas in some books and articles. In 1956, an earthquake of magnitude 6.4 according to Richter scale took place very close to Eskisehir (approximately 15 km. west of it) and caused deaths, injuries and gave serious damages to several structures. Eskisehir faced with deaths, injuries and 84

3 serious damages during the last 1999 Marmara Earthquake, which had a magnitude of 7.4. One of the subsidiary faults of NAF, was named as Eskisehir Active Fault also passes just south part of the urban area (as illustrated by figure 2). All these facts indicate Eskisehir is located in seismically active zone in Turkey. Figure 2: Tectonic situation of Eskisehir region (NAF is located on the north) SEISMIC HAZARD ANALYSIS One integrated part of earthquake risk management study is seismic hazard analysis. As it is mentioned above, the degree of uncertainty increases risk value in the earthquake studies. These uncertainties are mostly found around the size, time and location of future earthquakes. Seismic hazard analyses involve the quantitative estimation of ground-shaking hazards at particular site. Seismic hazards may be analysed deterministically, as when a particular earthquake scenario is assumed, or probabilistically, in which uncertainties in earthquake size, location, and time occurrence are explicitly considered (Kramer, 1996). Deterministic analysis was selected for this study and nearly historical seismic records of the earthquakes were used. The historical records have extended from year 2001 to The map coordinates of these records were located on digital map (as illustrated by figure 3). Figure 3: Historical earthquake epicenters around the studied area. 85

4 All earthquake sources which are capable of producing significant ground motion at the site have selected (as illustrated by figure 4). The distance between the source zones and the studied area were measured and the strongest level of shaking which influence the interested site was selected. Level of shaking was assumed to characterise by the peak horizontal ground acceleration (PGA). Appropriate attenuation relationships were used and effective PGA value was calculated for the interested area. Eskisehir Fault Zone that is defined as a line type source was selected as a major source for the studied area. The magnitude of the major source that would be effect the area in the future was determined. The earthquake magnitude 6.4 was selected according to Richter scale and g as a PGA, ground motion parameter for this analysis. Figure 4: Selected earthquake sources areas for seismic hazard analyse. CLASSIFICATION OF SOILS Metamorphic, ultramafic and Miocene aged sedimentary rocks bound Eskisehir on the north and south. Porsuk River flows between these high-elevated mountains from west to east and on the graben type valley. Most of the Eskisehir urban area is built on Quaternary aged soils around Porsuk River. These are the youngest sedimentary deposits of the region. The thickness of alluvial deposit is around 25 to 30 m. The average depth of water table is 3 m in the middle of the valley and close to the river and goes deeper towards to the mountains. The alluvial deposits and rock boundaries are bounded by normal and lateral faults. The soils mainly consist of sand, silt and clay. Sand layers can be seen at 5 m and 7 m depths with a thickness 1-3 m. Silt and clay layers repeat themselves till that sand predominant location. Average shear wave velocity was recorded around 200 m s -1. The soils of the studied area were classified according to their expected response to shaking from earthquakes. Liquefaction is used as the most important earthquake-induced soil failure, which would be expected for this study. Liquefaction potential map was prepared by using GIS techniques and drilling logs from ordinary drilling and CPT data. Iwasaki et. al. (1982) procedure was used for the preparation of liquefaction potential map. The soils of the interested area are classified into three groups according to the liquefaction potential due to shaking from earthquake. Group I, indicates an area which has very high probability of liquefaction at the proposed earthquake magnitude and PGA value. Group II, indicates high liquefaction potential but not much like Group I. The last group is Group III, and shows very low liquefaction potential. Old alluvium observed as cemented gravel, sand and silt was considered as rock in this study (as illustrated by figure 5). 86

5 Figure 5: Earthquake liquefaction potential map of the studied area VULNERABILITY INVETIGATIONS Vulnerability is defined (Blaikie et al., 1994) as the characteristics of a person or group that affect their capacity to anticipate, cope with, resist and recover from the impact of a natural hazard. The degree of vulnerability is defined by factors such as socio-economic status, wealth, ethnicity, gender, disability and age. Vulnerability is one of the major parameters used for loss estimation studies. Loss estimation is defined a very useful tool for developing preparedness plan and for promoting seismic risk mitigation. This study must be done before the earthquake happens. It is not easy to estimate the earthquake damage and loss. Data collection is the biggest difficulty that researchers could face with. Nowadays, with the advance of information technology, it is easy to overcome the difficulties in data collection. In addition to that, development in computer technology and software about GIS can help to identify and manage risk for earthquakes. The old models have been based on conventional type maps and making visual analyses on them. It was impossible to overlay more than five thematic maps and make a decision about the problem by the conventional methodology. Today, unlimited number of even different scaled digital maps can be used at the same time by using GIS. In the application of vulnerability investigation, a significant effort goes into building inventory data. Generally, inventory data are structured into two classes. Occupancy class data provides information on the use and function of the building environment. The second class provides engineering construction types of similar damage potential for each construction (e.g. concrete, masonry, wood, etc.). In this study, occupancy type classification is used for vulnerability investigation. But only hospitals, schools, factories, gas stations and governmental buildings were selected as critical facilities. Railroads, roads, interchanges, main streets were defined as transportation facilities and natural gas line as a lifelines. All these elements were digitised for a GIS environment as lines and polygons. Some of them were attributed, if any information about them could be obtained. Overlay analysis was applied by using each element layer with liquefaction potential map of the interested area. An example of this overlay analyse is seen in the figure 6. The areal extend of very high probability of liquefied areas in the liquefaction potential map, which defined as Group I were calculated 857 hectares ( m 2 ). The areas and lengths of critical facilities for earthquake risk management studies were measured (as illustrated by table 1). It was concluded that, 12 hospital buildings, 10 schools, 10 factories and 90 governmental buildings were under the danger of earthquake hazard. The total area of governmental buildings, which would be effected by the earthquake, is around m 2. More than m length main natural gas line exists into this risky area. 87

6 Figure 6: Overlapped result of very high probability of liquefied areas and some critical facilities for earthquake risk management. Table 1: Inventory data of some critical facilities for earthquake risk management Facilities Number of facility Area (m 2 ) Length (m) Hospital School Factory 10 - Gas station 6 - Governmental build Railroads Highways Natural gas line CONCLUSIONS AND RECOMMENDATIONS It is concluded that, many critical utilities, lifelines are under the influence of earthquake hazard in the liquefiable zone of the studied area. Approximately hospitals have m 2, schools have m 2 and governmental buildings have m 2 areal coverage in the very high liquefiable soil. These facilities must be taken into consideration as soon as possible due to earthquake hazards. Railroads, highways and natural gas lines are also passed into this soil zone. More detailed earthquake risk management studies must be put into action in this region. These kind of studies can provide city planners and emergency risk managers key information on potential damages and losses to buildings, critical utilities and transportation systems. City planners, risk managers can use these outcomes and make their strategic plans according to these results. Development in softwares like GIS will increase the developments in loss estimation tools. This developments will influence the race of earthquake risk mitigation in the highly populated urban areas. 88

7 BIBLIOGRAPHY Blaikie, P., Cannon, T., Davis, I. and Wisner, B., 1994 At Risk: Natural Hazards, People s Vulnerability and Disasters, Rootledge, London and Newyork. Iwasaki, T. and others, 1982, Microzonation for Soil Liquefaction Potential Using Simplified Methods, Third International Earthquake Microzonation Conference Proceedings, USA, vol.3, pp Kramer, S.L., 1996 Geotechnical Earthquake Engineering, Prentice Hall, Upper Saddle River, New Jersey, pp 653. Tucker, B.E., Erdik, M. and Hwang, C.N., 1994 Issues in Urban Earthquake Risk, NATO ASI Series, Series E: Applied Sciences, Vol. 271, Kluwer, Dordrecht, Boston and London. Wideman, R.M., 1992 Project and Program Risk Management, A Guide to Managing Project Risk and Opportunities, A Publication of the Project Management Institute, Four Campus Boulevard Newtown Square, Pennsylvania USA, ISBN