An Evaluation of the  Seismic Risk in Istanbul
 

An Evaluation of the Seismic Risk in Istanbul

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    An Evaluation of the  Seismic Risk in Istanbul An Evaluation of the Seismic Risk in Istanbul Document Transcript

    • Istanbul at the Threshold: An Evaluation of the Seismic Risk in Istanbul Jacob H. Pyper Griffiths, Ayhan Irfanoglu M.EERI, and Santiago Pujol Corresponding author: Santiago Pujol Mailing address: 1040 S. River Rd., West Lafayette, IN., 47907 Phone: 765-4968368 Fax: 765- 496-1105 E-mail address: spujol@purdue.edu Submission date for review copies: Submission date for camera-ready copy:
    • 2 Istanbul at the Threshold: An Evaluation of the Seismic Risk in Istanbul Jacob H. Pyper Griffiths a) , Ayhan Irfanoglu a) M.EERI, and Santiago Pujol a) There is no convincing evidence indicating that future ground motion in at least two-thirds of Istanbul, Turkey, shall be less demanding than the ground motions that devastated the city of Düzce, Turkey, in 1999. Comparison of vulnerability indices calibrated for Turkish construction indicates that the structures of the buildings in Istanbul are no better than the structures of buildings in Düzce. On the basis of these arguments, we project that a future earthquake near Istanbul may cause severe damage or collapse of approximately one quarter of a million buildings. Leaving the vulnerable buildings as they are and organizing for emergency response is not an option for Istanbul. INTRODUCTION Istanbul, Turkey, has experienced several earthquake disasters1 . The 1509 earthquake caused the worst recorded disaster. Parsons et al. (2000a) estimate the magnitude and distance between the epicenter and Istanbul were approximately 7.6 and 60 km, respectively. Ambraseys and Finkel (1991) estimate the earthquake caused 5,000 fatalities and collapsed 1,000 houses. At the time, the population of Istanbul was approximately 160,000 (Ambraseys and Finkel, 1991)2 . It is estimated that, today, Istanbul houses approximately 12 million people (DIE, 2005) in about one million buildings (DIE, 2000). Figure 1 shows Istanbul with its European and Asian districts divided by the Bosporus Strait. The number of recorded devastating earthquakes for Istanbul is sufficiently large that we accept that there will be another one soon. Therefore, in this note, we examine the possible consequences of a devastating earthquake striking Istanbul within the next ten years. a) Purdue University, West Lafayette, IN. 1 The historic structures still found in a state of “tolerable preservation” in the Marmara Sea region should not be interpreted as an indication of lack of seismic activity. Many of these structures have in fact sustained damage during past earthquakes (Ambraseys and Finkel, 1991). 2 The 160,000 estimate in 35,000 households was given for year 1480.
    • 3 In 1999, two devastating earthquakes shook Düzce, Turkey—a provincial capital city located about 200 km east of Istanbul. In Düzce, the 1999 earthquakes caused almost 600 fatalities and significantly damaged or collapsed approximately 3,500 buildings out of approximately 12,000 buildings (DPT, 2001) 3 . The population of Düzce at the time was approximately 80,000 (estimate based on 1997 census data, DIE, 1997). To examine the possible consequences of another devastating earthquake in Istanbul, we contrast two sets of data—the history of earthquakes and the vulnerability of buildings—for Istanbul and Düzce. We suggest that the ground motion that devastated Düzce in 1999 would be comparable to that expected for Istanbul. We compare data showing that the vulnerability of the buildings in Düzce in 1999 was not higher than the vulnerability of the current buildings in Istanbul. Given these similarities, we conclude that what took place in Düzce probably represents, at a smaller scale, what will happen in Istanbul. THE 1999 EARTHQUAKES IN TURKEY GROUND MOTION In 1999, there were two earthquakes in the Marmara sea region exceeding magnitude 7. Both earthquakes shook Düzce and caused the ground to rupture along parts of the surface trace of the North Anatolian and associated faults. The Marmara earthquake occurred on 17 August. Its epicenter was located near Izmit, Turkey, approximately 100 km from Istanbul. It had an Mw magnitude of 7.4 (USGS) and caused damage from Istanbul to Düzce. The Düzce earthquake occurred on 12 November (Figure 2). Its epicenter was located near Düzce. It had an Mw magnitude of 7.2 (USGS) and caused damage from Bolu (Figure 3) to Düzce (Figure 4). Figure 4 shows the distribution of damage within Düzce and the relative location of the ground motion recording station in the city. The recorded peak ground acceleration (PGA) for the Düzce earthquake was 0.5 g at about 8 km from the surface trace of the fault, and 0.8 g at about 15 to 20 km from the surface trace of the fault (Table 1). The peak ground velocity (PGV) was 90 cm/sec at about 8 km from the fault trace and 60 cm/sec at about 15 to 20 km from the fault trace (Table 1). Differences in soil properties do not explain the differences in 3 The Turkish State Planning Organization (DPT) reported in 2001 that in the provincial capital of Düzce the damage distribution among all buildings, i.e. regardless of the construction material and style, cumulative of the two 1999 earthquakes was: collapsed/heavy damage (3491 buildings), medium damage (3,170 buildings), and light damage (5,366 buildings).
    • 4 the ground motion (Dönmez, 2005). There are probably too few recorded ground motion data to distinguish a relation between distance and PGA or PGV for the Düzce earthquake. However, documented information on the distribution of damage shows no clear trend between distance and damage within 20 km from the fault (Figures 2 to 4). STRUCTURAL RESPONSE Data reported by Dönmez (2005) indicate that approximately 40% of the buildings in Düzce sustained severe damage or collapsed during the 1999 earthquakes. Among buildings with three or more stories, this ratio is 50%. Approximately 90% of the buildings studied were reinforced concrete structures. Dönmez (2005) refers to local structural failures as severe damage (in general, shear and anchorage failures, and buckling or fracture of reinforcement). Dönmez (2005) showed that the distribution of structural damage in Düzce is consistent with a measure of vulnerability proposed by Hassan and Sozen (1997). This measure is named priority index and is computed as the ratio of “effective” column and wall cross- sectional areas to the total floor area of a given building: Priority Index = Column Index + Wall Index (1) ∑ = floor columns A A 2 1 IndexColumn (2) ∑ + = floor walls10 1 wallsRC A A IndexWall masonryA (3) where Acolumns is the cross-sectional area of all columns at base level, ARC walls is the cross- sectional area of reinforced concrete walls at base level, Amasonrywalls is the cross-sectional area of masonry walls filling entire frame bays, and ΣAfloor is the summation of floor areas above the base of the structure. Figure 5 shows that, for buildings in Düzce, priority index decreased with increasing number of stories, indicating that column and wall areas do not increase proportionately with increases in number of stories. Figure 5 also shows that damage concentrated in buildings with priority indices lower than 0.4%.
    • 5 ISTANBUL’S HISTORY OF EARTHQUAKES AND CURRENT VULNERABILITY OF ITS STRUCTURES Although attempts to quantify ground shaking intensity a priori have been carried out for Istanbul, these estimates are yet to be tested against observation. In this study, we rely on seismic catalogs, ground motion records from previous earthquakes, and the consensus of the profession to make an estimate of ground motion intensity for a plausible reference earthquake scenario for Istanbul. EARTHQUAKES The record of historical earthquakes for the Marmara Sea region in Turkey is one of the most extensive in the world (Ambraseys and Finkel, 1991; Ambraseys, 2002). Approximately 600 earthquakes were recorded between AD 1 and 1899. Figure 6 shows a map of the Marmara Sea region with approximate epicentral locations for earthquakes from the last 2000 years and with Ms magnitudes of 6 or higher. Thirty-eight of these earthquakes had Ms magnitudes of 7 or higher (Ambraseys and Finkel 1991), six of which occurred in the 20th century (Ambraseys 2002). But according to Hubert-Ferrari et al. (2000), the 160 km-long segment of the northern branch of the Marmara Sea fault closer to Istanbul has not ruptured in more than 200 years while adjacent segments have. Given the recorded history of earthquakes near Istanbul over the past 2,000 years (Ambraseys 2002), the last century could be considered a seismic lull for Istanbul. The history of earthquakes in the Marmara Sea region is conclusive: 1) earthquakes have happened near Istanbul repeatedly, and 2) earthquakes have shaken and damaged buildings in Istanbul repeatedly. Given the historical record, it would be unreasonable to assume that another earthquake will not strike Istanbul. Furthermore, on the basis of sequences of earthquakes, some researchers suggest that there is a probability of 20% to 65% that an earthquake may cause shaking with an intensity of VIII or greater in Istanbul in the next 30 years (Parsons et al., 2000a; Parsons et al. 2000b; Parsons, 2004). These estimates depend on whether the researchers consider the amount of time between earthquakes and the dependence of one earthquake on another. Geophysicists have yet to predict an earthquake, so the actual likelihood of a significant earthquake occurring in Istanbul may be different; however, we strongly believe that the published estimates are too high to ignore for a city like Istanbul.
    • 6 STRUCTURES Much of the city was built in the last 40 years in a frantic effort to respond to a large demographic explosion (Figure 7).The rush led to an inventory of buildings with poor structures, the vulnerability of which, as we shall discuss, is comparable to the vulnerability of structures affected by the 1999 Marmara and Düzce earthquakes. It is important to recognize that construction of more than half of the buildings in Istanbul is unmonitored and undocumented (Green 2005). Since the 1999 earthquakes in Izmit and Düzce, several surveys have been carried out to estimate characteristics of the building inventory for Istanbul. A census conducted in 2000 provided information regarding number of stories and type of construction for buildings throughout the city. These data have been used by JICA (2002) to make projections about structural vulnerability. Ozcebe et al. (2006) conducted a study of a specific neighborhood (Zeytinburnu) in which data regarding cross-sectional areas of structural elements and floor areas were collected for approximately 3000 reinforced concrete buildings. The data collected by Ozcebe at al. are used in this study. WHAT IS EXPECTED FOR ISTANBUL? EXPECTED EARTHQUAKE The northern segments of the main Marmara Sea fault zone are approximately 5 to 15 km south of the southern shoreline of Istanbul (Figure 8). Approximately two-thirds of the city is within 20 km of the fault zone (Figure 9, adapted from JICA 2002). Therefore, in terms of distance to the seismic source, one cannot make a clear distinction between Istanbul and Düzce (Table 1, Figure 8). The orientation of the Marmara Sea fault zone near Istanbul is different from the orientation of the North Anatolian fault zone near Düzce, and the consequences of these differences in future earthquakes are difficult to estimate. According to JICA (2002), the faults near Istanbul are capable of generating earthquakes with Mw magnitudes of 6.9 to 7.7. It should be noted that the earthquakes considered in the JICA (2002) study are similar in magnitude and intensity characteristics to those given by Parsons (2004), in which earthquakes with magnitude ≥ 7 are considered, and Parsons et al. (2000), in which earthquakes causing ≥ 8 shaking intensity are considered. Parsons (2004) projected that the probability of an event with M≥7 near Istanbul in the period 2004-2034 is
    • 7 approximately 40%. The Düzce and Marmara earthquakes had Mw magnitudes of 7.2 and 7.4. Again, in terms of magnitude, one cannot identify a clear distinction between a plausible earthquake in Istanbul and the 1999 earthquakes. Although Düzce is located on 250-m deep alluvial deposits (Dönmez, 2005), acceleration records obtained in 1999 do not indicate strong influence of soils. In fact, the spectra for the records from Düzce resemble spectra for records obtained on stiff soils in California. In contrast, the 1999 Marmara earthquake made evident the presence of soft soil deposits in the European side of Istanbul. We note that although the properties of soil deposits could vary significantly over a few hundred meters in Istanbul (JICA, 2002), we suggest that uncertainties and variations in ground motion are too large to warrant incorporation of microzonation statistics into our forecasts. On the basis of the similarities described, it is reasonable to expect that the intensity of future ground motions in the southern two-thirds of Istanbul, i.e. within 20 km of the fault zone (Figure 9), will not be less than the ground motion intensity observed in Düzce during the 1999 earthquakes. STRUCTURES Teams lead by researchers from the Middle East Technical University (Ozcebe et al., 2006) conducted a detailed investigation of the building stock in the Zeytinburnu district of Istanbul. This district is located on the south-eastern region of the European part of Istanbul (see Figure 10) and houses approximately 240,000 people (about 2% of the population of Istanbul) in 16,000 buildings (about 1.6% of the total number of buildings in Istanbul). Approximately 3,000 buildings were surveyed. Each survey included information on total column, wall and floor areas. The average column index computed for the buildings surveyed is 0.17%, the average wall index is 0.03%. Observe that the average relative amount of walls is very small. The mean priority index is 0.19% with a standard deviation of 0.08%, indicating that 99% of the buildings in Zeytinburnu are likely to have a priority index of less than 0.40% (Figure 11). Recall that, in Düzce, severe structural damage and collapses concentrated in buildings with priority indices not exceeding 0.40% (see Figure 5).
    • 8 INFERENCES The discussions above lead to two simple conclusions: 1) there is no substantial evidence indicating that future ground motion in the parts of Istanbul that are within 20 km from the fault zone will be less demanding than the ground motions that devastated Düzce; 2) the structures of buildings in Istanbul are no better than the structures of buildings in Düzce. If our conclusions are correct, one should expect the percentage of buildings that may experience severe damage or collapse in the southern parts of Istanbul to be the same as that of Düzce (40%). Assuming that two-thirds of the buildings in Istanbul are within 20 km from the fault zone, we could expect that at least 250,000 buildings may be severely damaged during a future earthquake in Istanbul. To obtain this estimate, we have ignored the fact that there is a higher percentage of buildings with three or more stories in Istanbul than in Düzce (compare the distribution of buildings in Figures 5 and 11). As Figure 5 indicates, those buildings may be more vulnerable. Even if we are wrong by a factor of two, our projection is alarming. We cannot estimate with certainty the number of buildings that may collapse but almost any sensible guess leads to a grim conclusion. CONCLUSIONS There is no convincing evidence indicating that future ground motion in the parts of Istanbul within 20 km from the fault zone will be less demanding than the ground motions that devastated Düzce in 1999. The structures of the buildings in Istanbul are no better than the structures of buildings in Düzce. On the basis of these arguments, we project that a future earthquake near Istanbul may cause severe damage or collapse of approximately one quarter of a million buildings. Given the current state of building inventory in Istanbul and the high probability of a significant earthquake striking in the near future, we believe that Istanbul is facing a tremendous risk of experiencing a catastrophe of unseen proportions. We do not believe that leaving the vulnerable buildings as they are and organizing for emergency response is an option for Istanbul. Efficient mitigation efforts must be undertaken urgently to improve or replace the buildings in Istanbul.
    • 9 ACKNOWLEDGMENTS This work was completed thanks to support provided by the National Science Foundation’s Structural Systems & Hazard Mitigation of Structures Program (award # CMS 0512964). TABLES Table 1. Strong motion information for Bolu and Düzce recorded during the 1999 earthquakes (from Ozturk, 2003) Station Earthquake Distance to Fault PGA PGV [km] [g] [cm/sec] BOLU 17 Aug. 50 --- --- 12 Nov. 15-20 0.8 60 DÜZCE 17 Aug. 10-15 0.4 60 12 Nov. 8 0.5 90
    • 10 FIGURES IstanbulIstanbul Figure 1. True-color photograph of Istanbul and the Bosporus Strait (adapted from NASA/EOS, 2004).
    • 11 Figure 2. Locations of Düzce and Bolu with respect to fault of 12 November 1999 earthquake (from Dönmez, 2005) Figure 3. Bolu and the location of the ground motion recording station (from Dönmez, 2005).
    • 12 Figure 4. The distribution of damage in Düzce and the relative location of the ground motion recording station (from Dönmez, 2005) PRIORITY INDEX VS. NUMBER OF STORIES 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2% 1.4% 0 1 2 3 4 5 6 7 NUMBER OF STORIES PRIORITYINDEX Severe Damage and Collapse No Damage to Moderate Damage Figure 5. Priority index for the buildings surveyed in Düzce (adapted from Dönmez, 2005).
    • 13 Figure 6. Estimated Spatial Distribution of Earthquakes in the Marmara Sea region during last 2000 years (adapted from Ambraseys and Finkel, 1991). Note: the 1999 events are not included in this figure. 0 2,000,000 4,000,000 6,000,000 8,000,000 10,000,000 12,000,000 1927 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1997 Year Population Figure 7. Growth of population in Istanbul (adapted from IMM, 2005). DIE (2005) projects the population of the city as approximately 12 million for 2006.
    • 14 Figure 8. Bathymetric map of part of the Marmara fault zone (adapted from Le Pichon et al., 2001). Main active faults are shown with thick black lines. Figure 9. Istanbul and the area within 20 km of the Marmara fault (adapted from JICA, 2002).
    • 15 Figure 10. Zeytinburnu, Istanbul (large circle) (adapted from JICA, 2002). ISTANBUL (ZEYTINBURNU) 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2% 1.4% 0 1 2 3 4 5 6 7 NUMBER OF STORIES PRIORITYINDEX 99% Figure 11. Zeytinburnu number of stories versus priority index distribution.
    • 16 REFERENCES Ambraseys, N., and C. Finkel, 1991, Long-term seismicity of Istanbul and of the Marmara Sea region, Terra, 3, 527-539. Ambraseys, N., 2002, The Seismic activity of the Marmara Sea region over the last 2000 years, Bulletin of the Seismological Society of America, 92, 1, 1-18. Devlet Istatistik Enstitusu (DIE), 1997, General Census, Republic of Turkey, Prime Ministry State Institute of Statistics, http://www.die.gov.tr. Devlet Istatistik Enstitusu (DIE), 2005, Mid-year Population Projections by Provinces, 2000-2010, Republic of Turkey, Prime Ministry State Institute of Statistics, http://www.die.gov.tr. Devlet Planlama Teskilati (DPT), 2001. Düzce ili raporu. eds. Özaslan, M., G. Ersahin, D. Akkahve, and A. Sabuncu. The Prime Ministry of Republic of Turkey, State Planning Organization. http://ekutup.dpt.gov.tr/iller/duzce/2001.pdf (in Turkish) Dönmez, 2005, “Spatial Distribution of Damage Caused by the 1999 Earthquakes in Turkey,” Earthquake Spectra, EERI. Green, P., 2005, Disaster by design: corruption, construction and catastrophe. British Journal of Crimonology, vol. 45, 528-546. Hassan, A.F. and M.A. Sozen, 1997, Seismic vulnerability assessment of low-rise buildings in regions with infrequent earthquakes. American Concrete Institute Structural Journal, vol. 94, no. 1, 31-39. Hubert-Ferrari, A., A. Barka, E. Jacques, S.S. Nalbant, B. Meyer, R. Armijo, P. Tapponnier, and G.C.P. King, 2000, Seismic hazard in the Marmara Sea region following the 17 August 1999 Izmit earthquake, Nature, 404, 269-273. Istanbul Metropolitan Municipality (IMM), 2005, Census Results for Istanbul, http://www.ibb.gov.tr. Japan International Cooperation Agency (JICA), 2002, The Study on Disaster Prevention/Mitigation Basic Plan in Istanbul including Seismic Microzonation in the Republic of Turkey: Final Report, vol 2. Le Pichon, X., Şengör, A.M.C., Demirbağ, E., Rangin, C., İmren, C., Armijo, R., Görür, N., Çağatay, N., Mercier de Lepinay, B., Meyer, B., Saatçılar, R., and Tok, B., 2001, The active Main Marmara fault, Earth and Planetary Science Letters, 192, 595-616. NASA, 2004. Istanbul, Turkey: The Crossroads of Europe and Asia. The National Aeronautics and Space Administration Earth Observatory website: http://earthobservatory.nasa.gov. Ozcebe, G., H. Sucuoglu, M. S. Yucemen, A. Yakut, and J. Kubin, 2006. Seismic Risk Assessment of Existing Building Stock in Istanbul: A Pilot Application in Zeytinburnu Districti. Proceedings of
    • 17 the 8th U.S. National Conference on Earthquake Engineering, Paper No. 1737, April 2006, San Francisco, CA. Ozturk, B.M., 2003. Seismic drift response of building structures in seismically active and near-fault regions. Ph.D. Thesis. Purdue University, West Lafayette, Indiana. Parsons, T., 2004, Recalculated probability of M>=7 earthquakes beneath the Sea of Marmara, Turkey, Journal of Geophysical Research, Vol. 109, B05304. Parsons, T., S. Toda, R.S. Stein, and J.H. Dieterich, 2000a. Influence of the 17 August 1999 Izmit earthquake on seismic hazards in Istanbul, in A. Barka, O. Kozaci, S. Akyuz and E. Altunel (Eds.), The 1999 Izmit and Düzce Earthquakes: Preliminary results, pp. 295-310. Parsons, T., S. Toda, R.S. Stein, A. Barka, and J.H. Dieterich, 2000b, Heightened odds of large earthquakes near Istanbul: an interaction-based probability calculation, Science, 288, 661-665.