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

AA

21IndexColumn (2)

∑+

=floor

walls101

wallsRC

AA

Index Wall 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%.

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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.

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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

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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

PRIO

RIT

Y IN

DEX

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

Popu

latio

n

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 7NUMBER OF STORIES

PR

IOR

ITY

IND

EX

99%

Figure 11. Zeytinburnu number of stories versus priority index distribution.

16

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