Düzce, Turkey: A City Hit by Two Major
Earthquakes in 1999 within Three Months
Haluk Sucuoğlu and Tolga Yılmaz
Middle East Technical University
Earthquake Engineering Research Center
Ankara 06531, Turkey
e-mail: sucuoglu@ce.metu.edu.tr
INTRODUCTION
Turkey experienced two major earthquakes in 1999 which occurred 86 days apart on the
North Anatolian Fault System. On 17 August 1999, Mw7.4 Kocaeli earthquake had broken the
140 km long western part of the 1200 km North Anatolian Fault in a multiple rupture process.
The fault rupture in the eastward direction propogated almost toward Düzce, and stopped 12
km away from the city. On 12 November 1999, Mw7.2 Düzce earthquake ruptured another 40
km segment of the same fault which was broken during the August event, toward further east.
The second rupture zone started from the termination of the 17 August rupture (see cover page
of SRL July/August 2000 issue) and passed 6 km from the south of Düzce.
Düzce is a provincial capital city with a population of 78 000 in Northwest Turkey.
The city has experienced severe near field strong motions from an unusual sequence of two
major earthquakes in less than three months. Unable to recover from the first event in such a
short time, Düzce is so caught by the second event with a significantly weakened building
stock, which eventually led to almost complete destruction. The number of casualties in
Düzce during the 17 August and 12 November events were 99 and 372, respectively. The
second figure does not reflect the actual devastation in the city since many residents had
already left Düzce after the first earthquake.
In this article, the effects of the two consequent major earthquakes on Düzce are
discussed in relation to the recorded strong motions and observed building damages. Main
emphasis is placed on the interpretation of strong motions and related damage accumulation,
that is based on detailed building damage survey in the city.
17 AUGUST 1999 KOCAELİ AND 12 NOVEMBER 1999 DÜZCE EARTHQUAKES
Kocaeli and Düzce earthquakes occurred on the western extention of the North Anatolian
Fault, along the northern branch between Bolu and İzmit Bay (Figure 1). Large earthquakes in
the last 60 years displayed a migration phenomena from east to west on the North Anatolian
Fault, starting with the 1939 Erzincan earthquake (Barka, 1996; Toksöz et al., 1979). The last
two in the series on the west were the 1957 Abant (M 7.1) and 1967 Mudurnu Valley (M 6.8)
earthquakes, which ruptured the southern branch between Akyazı and Bolu shown in Figure
1.
The Kocaeli earthquake (Mw7.4) had broken a 140 km segment of the North Anatolian
Fault on 17 August 1999 (Toksöz et al., 1999). Its epicenter is located at 40.76 oN and
29.97oE, the depth is at 16 km. During the Kocaeli earthquake, a multiple strike-slip rupture
process occurred in a propagating fashion along three consecutive segments of the fault. First
rupture was initiated at Gölcük with a bi-lateral propagation into the Gulf of İzmit on the west
and Sapanca Lake on the east. This rupture triggered further rupturing of the two segments on
the east, one between Sapanca Lake and Akyazı, and the other between Akyazı and Melen
Lake, which stopped 12 km away from Düzce (Yagi and Kikuchi, 1999). Since rupture in the
third segment was directed toward Düzce, the highest intensity ground motions were recorded
at the Düzce strong motion station during the 17 August event which were rich in high
velocity pulses, typical of near field accelerograms under forward directivity effects
(Anderson et al., 2000). GPS measurements along the 17 August surface trace are marked on
Figure 1.
Three months after the 17 August event, the 40 km unbroken segment of the northern
branch on the east of Melen Lake was broken with a Mw7.2 Düzce earthquake on 12
November 1999. A bi-lateral, symmetrical, predominantly strike-slip rupture occurred, with
the epicenter located at 40.76oN and 31.15oE, and the focal depth at 12 km.
The epicenter of the Düzce earthquake was at 6 km south of Düzce (Figure 1) where
the fault rupture propagated mainly in the upward direction toward Düzce along the highly
stressed region under the city (Figure 2, from Yagi and Kikuchi, online). GPS measurement
locations along the surface fault trace of the 12 November event are also marked on Figure 1,
with some overlap with the 17 August event in the vicinity of Melen Lake (SRL, 2000).
STRONG GROUND MOTIONS
Düzce strong motion station, operated by the Directorate of Disaster Affairs, recorded
accelerograms from both 17 August and 12 November events. The SMA-1 instrument is
located at the ground level of the Meteorology Station which is a simple, two strong building
at the central part of the city. It is unlikely that the station building shown in Figure 3 modifies
the character of seismic waves, hence the recorded accelerograms represent free field ground
motions.
Düzce is located on a large quaternary alluvial plane. Its name implies "flatland" and
topography is indeed uniform over the entire city and the surrounding farmlands. Young
alluvial deposits having thicknesses exceeding 70 meters consist of sand and gravel series.
The water table has seasonal variations between 2 and 5 meters. This apparent uniformity of
soil conditions over Düzce valley also implies uniformity of ground motion intensity
throughout the city during the two earthquakes.
The ground motion accelerograms from 17 August and 12 November events are
shown in Figure 4. In Table 1, the distance of the station from the fault and basic ground
motion intensity parameters for horizontal components are described, where FD is the fault
distance, PGA and PGV are the peak ground acceleration and velocity, respectively. Further,
the acceleration response spectra and energy equivalent velocity spectra of the horizontal
components are given in Figures 5 and 6.
Table 1. Characteristics of Strong Motions Recorded at the Düzce Station
COMP
FD(km)
PGA (cm/s2)
PGV(cm/s)
17 Aug 99
West
13
357
59
17 Aug 99
South
13
306
57
12 Nov 99
West
8
507
88
12 Nov 99
South
8
400
70
DATE
It is inferred from Table 1 that the ground motions recorded at Düzce have high
intensities, and they have to be considered as having significant damage potential. Directivity
of fault rupture toward Düzce in both events, and site amplification characteristics of the
Düzce Valley perhaps played primary roles on the high intensities of recorded ground
motions. Figure 4 compares linear elastic response spectra of the recorded accelerogramswith
the design spectrum in the Turkish Seismic Code (Ministry, 1997) specified for Seismic Zone
1 (most severe) and medium dense deep alluvial soils, having a 10% probability of
exceedence in 50 years. Considering that the building stock in the city consists of buildings
less than 6 stories, hence having approximate fundamental vibration periods of 0.1-0.6
seconds, this figure reveals that the spectral forces reach the code level during the 17 August
event, but exceed that that level by far during the 12 November event. Therefore building
damage is certainly expected in Düzce during both events. Further, considering that most of
the buildings in the city do not conform with the recent code requirements, damage
expectations must be even more. However, neither the accelerograms in Figure 4, nor the
acceleration response spectra in Figure 5 can exhibit the cumulative damaging effects of the
two earthquakes since they are not physically additive. An appropriate tool that can be
employed for this purpose is the input energy spectra expressed in the energy equivalent
velocity spectrum format, shown in Figure 6. Here, input energy spectrum defines the total
energy imparted into a single degree of freedom (SDOF) system along the strong motion
duration, and energy equivalent velocity is related to input energy through kinetic energy
formulation. Therefore spectral input energies can be added as scalar quantities for a SDOF
system, and consequently spectral equivalent velocities are added by using the square root of
the sum of squares approach for the two earthquake excitations in each direction. The spectral
results presented in Figure 6 indicate that, although the 12 November event imposes a higher
energy dissipation demand compared to the 17 August event, their compound effect results in
a much higher demand. In engineering terms, the buildings which were weakened and lost a
significant portion of their energy dissipation capacities after the first event, were subjected to
an even higher energy dissipation demand during the second event. This is in fact a life size,
live field testing of a large number of prototype building specimens, which is indeed an
expensive experiment.
DAMAGE DISTRIBUTION IN TWO EARTHQUAKES
Official damage survey in Turkey is based on the households, not on individual buildings.
Therefore these numbers do not reflect the damage distribution with respect to the structural
properties of buildings. The results of the official damage surveys are given in Table 2 in
percentages, where the numbers for the 12 November earthquake reflect accumulated damage
values after two earthquakes. Each household is rated a damage grade of none, light, moderate
or severe/collapsed by damage survey teams shortly after each earthquake. The total number
of households is 19015 in Düzce, in a total number of 9776 buildings from one to six stories.
Table 2. Ratio of Damaged Households in Two Earthquakes (percentage)
Earthquake
Severe/Collapsed
Moderate
Light
None
17 August 1999
6.9
13.8
9.2
70.1
12 November 1999
29.2
28.7
19.6
22.5
The progress of damage is clearly evident from Table 2. Only a small ratio of
buildings survived the two earthquakes without damage. A main street view after 12
November 1999, shown in Figure 7, verifies the extent of destruction in the city. Many
buildings that were moderately damaged after the first event collapsed during the second
event, such as the Düzce High School building in Figure 8. In Figure 9, the empty space of a
demolished building after the 17 August earthquake is shown in the foreground, and the two
multistorey buildings in the picture were moderately damaged after the 17 August event. The
one on the right was collapsed, and the other on the left was severely damaged in the 12
November earthquake as a result of accumulated seismic demands. A significant number of
buildings which were rated as undamaged or lightly damaged without proper safety
assessment after the first earthquake were permitted for occupancy. Some of them collapsed
during the second earthquake, leading to the loss of 372 lives. A number of damaged
buildings that were improperly repaired, but not strengthened after the first earthquake also
collapsed and killed their occupants in the second earthquake. These repair works mainly
consisted of resin epoxy injection into the cracks of reinforced concrete members.
Düzce Municipal Government conducted a building damage survey after the second
earthquake which reveals valuable information on relating damage grade to building height.
These results are presented in Table 3. It is evident that building
Table 3. Building Damage Distribution in Düzce After Two Earthquakes
Number of Stories
1
2
3
4
5
6
Total number of buildings
3207 3612
1278
810
765
13
Number of collapsed/severe damage
370
242
319
518
4
445
Number of moderate damage
477
787
421
381
227
9
Number of none/light damage
2360 2380
615
110
20
0
vulnerability remarkably increases with the number of stories. This trend can be observed
more clearly from Figure 10 where the percentages of damage grades are given for each total
story
number seperately. Especially four and five story buildings appear to be more
vulnerable than the others. Although majority of buildings in Düzce do not comply with the
recent seismic code requirements, six story buildings are newer, hence an increased
consideration of seismic design can be expected from these buildings. Four and five story
buildings having fundamental vibration periods between 0.3-0.5 seconds were subjected to
much higher seismic demands than the fewer story buildings during the two earthquakes, as
indicated in Figures 5 and 6. Moreover, one or two story buildings possess an excess lateral
load capacity against seismic actions since their design is governed more by gravity loads than
lateral loads.
BOLU FREEWAY VIADUCT
Another significant aspect of the 12 November Düzce earthquake was a major freeway
viaduct crossed by the ruptured fault at the eastern terminations of the fault rupture shown in
Figure 2. The Bolu viaduct was completed weeks before the November 12 earthquake. It is a
2.5 km long structure composed of a pair of independent paralel decks, each of 17.5 meter
width for two traffic directions. It consists of sixty 40 meter spans simply supported by single
piers with maximum height of 49 meters. The viaduct was designed for the code acceleration
response spectrum shown in Figure 5, however relative transverse displacement of piers
during fault displacement was not accounted for properly. In fact, a relative displacement of
adjacent piers by 1.50 meters as measured at the fault crossing is very difficult to
accommodate. Seismic dampers were mounted between pier caps and the end diaphragms of
the decks to supply passive energy dissipation, however they were completely destroyed
during the earthquake. The 500 Million USD viaduct sustained significant damage at the deck
structure during the November 12 earthquake, as shown in Figures 11 and 12. It requires
significant and costly repairs, possibly replacement of the deck. This uunique case is a clear
indication that crossing an active fault with a long viaduct may not be a sound engineering
decision.
CONCLUSIONS
Severe strong ground motions occurred in Düzce during the 17 August 1999 Kocaeli and 12
November 1999 Düzce earthquakes, primarly due to the fault rupture directivities toward
Düzce in both earthquakes, and site amplification characteristics of the Düzce Valley.
Intensities of ground motions recorded in Düzce during both earthquakes exceeded the
intensities specified in the Turkish Seismic Design Code. The building structures in Düzce
that eperienced two consecutive strong ground motions in three months provide a valuable
case for understanding damage accumulation under intensive strong motion sequences. The
Düzce earthquake further proved that it is difficult to accomodate fault displacements in a
long, multispan bridge structure crossing the fault during a major earthquake.
ACKNOWLEDGEEMENTS
We gratefully acknowledge the discussions with Professors M. Erdik and J.G. Anderson in
preparation of the manuscript. METU Research Assistants A. Erberik, S. Bayılı, A. Peköz and
A.C. Pay contributed to the preparations of Figures 5, 6 and 10. The GPS measurements of
fault locations in Figure 1 are obtained from R. Hartleb and J. Dolan.
REFERENCES
Anderson, J.G, H. Sucuoğlu, A. Erberik, T. Yılmaz, E. İnan, E. Durukal, M. Erdik, R.
Anooshehpoor, J.N. Brune and S. D. Ni (2000). Strong ground motions from the Kocaeli and
Düzce, Turkey, earthquakes, and possible implications for seismic hazard analysis.
Earthquake Spectra, in press.
Barka, A (1999). The 17 August 1999 İzmit Earthquake. Science, Vol.285, No.5435, 1858.
Ministry of Reconstruction and Settlement (1997). Regulations for buildings in seismic zones,
Ankara, Turkey.
Seismological Society of America (2000). Cover page, Seismological Research Letters,
Vol.71, No.4.
Toksöz, M.N. A.F. Shakal and J. Michael (1979). Space-time migration of earthquakes along
the North Anatolian fault zone and seismic gaps. Pageoph, Vol.117, 1258-1270.
Toksöz, M.N., R.E. Reilinger, C.G.Doll, A.A. Barka, and N. Yalçın (1999). İzmit (Turkey)
earthquake of 17 August 1999: First report. Seismological Research Letters, Vol.6, No.6, 669679.
Yagi, Y. and M. Kikuchi (1999). Source
Rupture Proccess of the Kocaeli, Turkey,
Earthquake of August 17, 1999, Obtained by Joint Inversion of Near-field Data and
Teleseismic Data. Earthquake Research Institute, The University of Tokyo.
Yagi, Y. and Kikuchi, M. (Online), The 1999 Turkey Earthquake, ERI, University of Tokyo
Japan. Available: http://www.eic.eri.u-tokyo.ac.jp/yuji/trk2.
Figure 1. 17 August Kocaeli and 12 November Düzce earthquakes, mainshock and aftershock epicenters, surface faulting.
Figure 2. 12 November 1999 Düzce earthquake source properties, from Yagi and
Kikuchi
(online).
10
Figure 3. Düzce strong motion station.
11
500
WEST, PGA=356.5 cm/s 2
0
Acceleration (cm/s 2)
SOUTH, PGA=305.8 cm/s 2
UP, PGA=470.8 cm/s 2
0
5
10
15
Time (s)
20
25
30
a) 17 August 1999
WEST, PGA=507.1 cm/s 2
500
0
Acceleration (cm/s 2)
SOUTH, PGA=399.6 cm/s 2
UP, PGA=333.2 cm/s 2
0
5
10
15
Time (s)
b) 12 November 1999
12
20
25
30
Figure 4. Strong motion accelerograms recorded at the Düzce station
during
Kocaeli and Düzce earthquakes.
13
2.0
Düzce (17 Aug.) West
Düzce (17 Aug.) South
1.5
Düzce (12 Nov.) West
Sa (g)
Düzce (12 Nov.) South
Code Spectrum
1.0
0.5
0.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Period (s)
Figure 5. Acceleration response spectra of horizontal components recorded at
Düzce
station during 17 August and 12 November events.
14
4.0
400
Düzce 17 Aug. South
Düzce 12 Nov. South
Veq (cm/s)
300
Combined South
200
100
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Period (s)
Figure 6.a. Energy equivalent velocity spectra of the North-South
accelerograms from
17 August and 12 November events.
400
Düzce 12 Nov. West
Düzce 17 Aug. West
Combined West
Veq (cm/s)
300
200
100
0
0.0
0.5
1.0
1.5
Period (s)
15
2.0
2.5
3.0
Figure 6.b. Energy equivalent velocity spectra of the East-West accelerograms
from 17
August and 12 November events.
16
Figure 7. A main street view from Düzce after 12 November 1999 earthquake.
17
18
Figure 8. Düzce High School building was damaged, and evacuated after the 17
August 1999
Kocaeli earthquake. The building completely collapsed during the 12
November
1999 Düzce earthquake.
19
Figure 9. The empty space behind the parked cars belongs to a building collapsed
during the
17 August earthquake. Two buildings, one collapsed and the other
severely
damaged, during the second event on 12 November, were moderately
damaged after
the first event on 17 August.
20
80
Light / None
Percentage of damaged buildings
70
Moderate
Severe / Collapsed
60
50
40
30
20
10
0
1
2
3
4
5
6
Number of stories
Figure 10. Distribution of building damage with the number of stories after two
events in
Düzce.
21
Figure 11.a. Düzce Fault crossing the Bolu viaduct. The pier which the fault is
running
through has rotated by 10 degrees and its cap popped up.
22
Figure 11.b. Permanent displacement of the deck over the pier both longitudinally
and
transversally. The support ports are completely destroyed, and the deck
is damaged.
23