1. Izmit (Turkey) Earthquake of 17August 1999:
First Report
M. N. Toksoz, R. E. Reilinger, c. G. Doll
Massachusetts Institute ofTechnology
A. A. Barka
Istanbul Technical University
N. Yalcin
TUBITAK Gebze Research Institute
INTRODUCTION
The earthquake of 17 August 1999 was the largest and most
destructive earthquake to occur in Turkey since the 1939
Erzincan earthquake. The earthquake occurred on the west-
ern extension of the North Anatolian Fault, on a segment
that has been identified as a "seismic gap." The epicenter
(40.76°N, 29.9rE, Kandilli) was near Izmit (Figure 1), at
the eastern end of Izmit Bay, and the moment magnitude
was M w = 7.4 (USGS). The focal mechanism and the fault
surface rupture indicate an east-west striking fault with pure
right-lateral strike-slip motion.
The eastern shores of the Gulf of Izmit, where the epi-
center was, are densely populated, with extensive industrial
and residential complexes. The earthquake devastated towns
on the shores of Izmit Bay, Adapazari to the east, and a
neighborhood in the western suburb of Istanbul about 150
km west of the epicenter. In the towns ofIzmit and Golcuk,
the destruction was severe. More than 15,000 people died.
About 30,000 are still listed as missing, scores were injured,
and 300,000 people were left homeless. The economic
impact of the quake is estimated to be about 20 billion
dollars.
In this article we describe briefly the regional tectonics,
including the plate and block motions based on GPS survey
results. Then we review the rupture history along the North
Anatolian Fault, earthquake source mechanism, and effects
of the mainshock. Finally, we discuss the potential for future
earthquakes.
REGIONAL TECTONICS AND THE NORTH
ANATOLIAN FAULT
The tectonics of Anatolia and the surrounding regions are
controlled by the interactions of the Arabian and African
Plates with the Anatolian and Eurasian Plates. Figure 2
shows the regional tectonic map and the major faults in the
eastern Mediterranean. The North Anatolian Fault (NAF)
represents the boundary between the west-moving Anatolian
Plate and the relatively stable Eurasian Plate. The fault
extends more than 1400 km and resembles the San Andreas
in California. It has been studied extensively (Allen, 1969;
Ambraseys, 1970; Dewey, 1976;Toks6zetal., 1979; Kasapo-
glu and Toksoz, 1983; Barka et al., 1987; Jackson and McK-
enzie, 1988; Neugebauer, 1995; Barka, 1996).
The seismicity along the North Anatolian Fault is dom-
inated by intermediate to large magnitude events with rela-
tively few small events (i.e., b value is about 0.6). With few
exceptions, all earthquakes have right-lateral strike-slip
mechanisms (Figure 3). East of the Sea of Marmara, the fault
splays into two prominent branches, the southern branch
going though Iznik Lake and Gemlik Bay. The northern
branch goes through Sapanca Lake and Izmit Bay. Two fin-
ger-like gulfs at the eastern extension of the Sea of Marmara
are delineated by the two traces of the fault.
Further west the southern branch of the fault breaks
into a series of parallel southwest-trending strike-slip faults
with sizable north-south extensional components. The
northern branch of the fault, west ofIznik Lake, is under the
Sea of Marmara and is difficult to trace. Further west, the
fault trace is observable on land, along the 1912 earthquake
surface break. The fault zone extends into the northern
Aegean Sea, delineating the boundary between Aegea and
the Eurasian Plate.
To the east, the North Anatolian Fault meets with the
left-lateral, strike-slip East Anatolian Fault at the Karliova
Triple Junction. Further east, seismicity is associated with a
series ofsoutheast- and northeast-trending faults, most char-
acterized by strike-slip motions with some components of
north-south compression.
Seismological Research Letters Volume 70, Number 6 November/December 1999 669
3. 46
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42
40
38
36
34
32
30
20 25 30 35 40 45
-6000-5000-4000-3000-2500-2000-1500-1000 -500 0 250 500 750 1000 1500 2000 2500 3000 4000 5000 6000
... Figure 2. Tectonic map of Turkey and surrounding regions. The arrows indicate the relative rate and direction of tectonic plate movement with the Afri-
can and Arabian Plates moving north under the Eurasian Plate. Along the North Anatolian Fault the Anatolian Plate (south side of fault) ismoving laterally
westward relative tothe Eurasian Plate (north side of fault).
In the past ten years a major GPS campaign was under-
taken by the Massachusetts Institute ofTechnology (MIT) in
collaboration with a number of other institutions from the
United States, Europe, and Turkey (Reilinger et al., 1997). A
summary figure ofthe GPS horizontal velocities in a Eurasia-
fixed reference frame is shown in Figure 4 (McClusky et al.,
1999). The velocity vectors clearly show counterclockwise
rotation of the Anatolian and Aegean Plates. Along central
Anatolia the velocities are about 25 mm/yr, paralleling the
North Anatolian Fault trace. The strain accumulation along
the fault is due to the 25 mm/yr relative motion between the
northern (Eurasian) and southern (Anatolian) plates.
In western Anatolia and Aegea the GPS-derived veloci-
ties increase, reaching values ofabout 35 mm/yr in southern
Aegea relative to Eurasia. The velocity vectors define a north-
south extension in western Turkey, Aegea, and Greece, con-
sistent with the predominance of normal faulting, geology,
and earthquake focal mechanisms shown in Figure 3.
MIGRATION OF EARTHQUAKES AND ASEISMIC
GAP
The Izmit earthquake of 17 August 1999 occurred on a pre-
viously identified seismic gap (Toksoz et al., 1979). Between
1939 and 1967 a series of earthquakes. moving from east to
west, ruptured more than 800 km of the North Anatolian
Fault, as shown in Figure 5 (Allen, 1969; Dewey, 1976; Tok-
soz, 1979; Barka et al., 1987; Jackson and McKenzie, 1988;
Barka, 1996; Stein et al., 1997). In 1912, another earthquake
in the west, between Tekirdag and the GulfofSaros, ruptured
more than 80 km of the Ganos Fault, the extension of the
northern branch of the North Anatolian Fault (Figure 6) .
Other earthquakes to the south of the Marmara Sea further
highlighted the unruptured parts of the fault. Toksoz et aL
(1979) analyzed the seismicity and rupture patterns and
identified a 100 km segment extending from the Gulf of
Izmit to the westernmost point of the 1967 earthquake
Seismological Research Letters Volume 70, Number 6 November/December 1999 671
4. 35
c
3025
400 600
km
200
20
32
30
40
=-~=-=-'='-= - :-- ""'7Vl
,!,,-,
,
34
36
42
38
Earthquake Focal Mechanisms
• Figure 3. Map of epicenters and focal mechanisms of intermediate tolarge magnitude earthquakes for the same region shown in Figure 2. The earth-
quakes along the North Anatolian Fault have mostly right-lateral, strike-slip focal mechanisms. (Modified from McClusky etaI., 1999)
42
40
38
36
34
32
20
km
200 400
- .----,
25 35
r--
40 45
• Figure 4. Map ofGPS horizontal plate velocities (arrows) in aEurasia-fixed reference frame for the same region shown in Figure 2. The velocity vectors
indicate acounterclockwise rotation ofthe Anatolian and Aegean Plates. (Modified from McClusky etal., 1999)
672 Seismological Research Letters Volume 70, Number 6 November/December 1999
6. .,,,
..'
.i
)
.._....'...-_.
I
T
• Bohk.sir
£. Figure 6. Map ofepicentral regions ofintermediate tolarge magnitude historic earthquakes inwestern Turkey. Aseismic gap was identified by Tokstiz
etal. (1979) between the 1912 rupture and the western end ofthe 1967 rupture along the NAF. The recent earthquake was caused by rupturing along aseg-
ment ofthe NAF just tothe west ofthe 1967 rupture inthis seismic gap. (From Sarka efaI., 19a7)
rupture as being a seismic gap. Since 1980 a series of mea-
surements were carried out on the "gap" by a large number of
investigators to determine whether the fault was locked or
creeping and to look for physical precursors ofa major earth-
quake. A creepmeter installed by MIT detected no creep.
Other precursor searches were negative or inconclusive.
The most convincing evidence that the fault was locked
and strain was accumulating along the northern branch
between Izrnit and Adapazan came from the dense GPS sta-
tion network established in the region (Straub and Kahle,
1995; Reilinger et al., 1997; McClusky et al., 1999). Based on
that information, a major project was funded. An intensive
study with a continuous GPS monitoring system, a densified
seismic network, and other supporting measurements was
getting underway in the region at the time of the earthquake.
THE EARTHQUAKE MECHANISM
The earthquake occurred on 17 August 1999 at 00:01:37
VT (3:01:37 AM local time). The epicenter was at 40.76°N
and 29.9JOE (Kandilli Observatory) at the eastern end ofthe
GulfofIzmit. The maximum damage occurred in the towns
of Izmit and Golciik, close to the epicenter. The source
mechanism is pure strike-slip, with an east-west strike and a
nearly vertical fault plane. The source parameters are sum-
marized in Table 1. The source depth is not well constrained.
From other earthquake studies we assume that the focal
depth was about 15 km. Surface rupture was observed for
about 120 km, in three segments, extending east from the
Gulf of Izmit to near Duzce, with some discontinuities in
surface breaks (IPGP and lTV, 1999, Web sites). All along
the fault trace, surface displacement was right-lateral strike-
slip. Average surface displacement was about 2.5 m along the
central part of the fault with the western segment (near the
epicenter) showing offsets of as much as 5 m and the eastern
segments about 1.5 m. Figures 7 and 8 show examples of
observed fault ruptures.
A preliminary estimate of the fault length can be
obtained from P-wave durations and calculated source time
functions. A set of seismograms is shown in Figure 9. The
average P-wave duration is about 40 seconds. The Harvard
CMT solution has a halfduration of21 sec. Taking a rupture
velocity of2.5 km/sec and a total rupture time of40 seconds,
we find a fault length of about 100 km. Given that theepi-
674 Seismological Research Letters Volume 70, Number 6 November/December 1999
7. TABLE1
HypecentralParametersof 08/17/1999 TurkeyEarthquake
Fault Plane Parameters
Lat Lon Depth Magnitude Momenta
(~ (~ (km) (Mw) (Me) Strike Dip Slip
Date Time
Network (DaMoYr) (HrMnSec)
KOERIb 170899 000137.6
USGS 170899 000138.6
HRV 170899 000138.6
40.76 29.97 18.0
40.64 29.83 15.0 7.4 1.4 x 1020 91 76 179
40.81 30.08 16.6 7.5 2.1 x 102o 268 84 180
a, UnitsNm
b. KandilliObservatoryand EarthquakeResearchInstitute
9 Figure 7. Surfacerupturing from the recentearthquakein Turkey.The
rupturehas beenobservedto beabout120 km in length.(ITU,1999,pers.
comm.)
center was near the western end of the observed faulting, the
rupture had propagated mostly from west to east. The fault
rupture process was complex. The P-wave trains shown in
Figure 9 are consistent with a complicated rupture propagat-
ing mostly eastward. Using the observed fault length of 120
km, width of 15 km, average displacement of 2.5 m, and
shear modulus of 3 • 101~N/m 2, we obtain a static moment
of 1.35 x 1020Nm. This value is comparable to but smaller
than dynamic moments obtained from CMT solutions (see
Table 1). Given that preliminary figures of observed field data
were used for fault length and displacement, comparisons of
9 Figure 8. Another observationof rupture at the surface with arrows
drawn to illustrate the pure right-lateral, strike-slip motion. (ITU, 1999,
pers.comm.)
static and dynamic moments are acceptable. A longer fault
length, as indicated by aftershocks and larger displacements at
depth, could increase the value of the static seismic moment.
The damage caused by the earthquake was extensive.
This was due to the magnitude of the earthquake, strong
ground motions, long fault length, high population density,
and the quality of construction. The northern shore of the
Gulfoflzmit is a major industrial area, in addition to being
a significant population center. South of Izmit Bay is pre-
dominantly residential, including extensive seaside develop-
ment for summer resorts. At the time of the quake there
might have been more than two million people inhabiting
the area. The center of Istanbul, a megalopolis with 12 mil-
lion people, is about 100 km to the west-northwest of the
epicenter. There was extensive damage in Avcilar, a western
suburb oflstanbul. This was most likely due to local geology
and soil conditions.
Figures 10, 11, and 12 show examples of damage done
to residential apartment buildings and railroads. The multi-
story brick and concrete buildings, collapsing like accordi-
ons, were responsible for most casualties. Some buildings
fared better than neighboring buildings. This may be attrib-
Seismological ResearchLetters Volume 70, Number6 November/December 1999 675
9. 9 Figure 10. Pictureshowingmajor deformationofarail linecausedbyfault displacementduringthe earthquake.Thedisplacementaveragedabout 2.5
m alongthe fault ruptureatthe surface.
9 Figure 11. Building damageproducedbytheearthquake.Threeresidentialapartmentbuildings collapsedsimilarly in roughly the samedirection.
Seismological ResearchLetters Volume70, Number6 November/December 1999 677
10. ~ Figure 12. More building damage from the recent earthquake. Some apartment buildings collapsed while nearby structures remained standing. Dif-
ferences inthe quality ofconstruction and local foundation soil, along with variability of ground motion, may explain this observation.
utable to the quality of construction, to local soil effects, and
to the variability of ground motion. Even within a dense
strong motion array established 40 km west of the epicentet,
peak ground acceleration varied by a factor oftwo (M. Erdik,
pers. comm., 1999).
POTENTIAL FOR FUTURE EARTHQUAKES
After a major earthquake, a question most frequently asked
is whether the event might trigger other damaging earth-
quakes. A large earthquake affects other faults immediately
by dynamic vibrations induced by body and surface waves,
and by static stress changes. Major earthquakes can influence
the timing and locations of subsequent earthquakes. There
are examples where "induced" or "triggered" earthquakes
may have occurred. The topic has been studied by a wide
spectrum of researchers. We refer the reader to a special issue
of the journal ofGeophysical Research (Vol. 103, B10, Octo-
ber 10, 1998) and the introductory lead article by Harris
(1998) in the same issue.
There are some special points that need to be discussed
on earthquake triggering in conjunction with the 17 August
earthquake. Given the large number of active faults near the
earthquake epicenter, an immediate emergency response
question was raised as to whether other earthquakes of com-
parable magnitude could occur. The history of earthquakes
along the North Anatolian Fault zone has not shown trig-
gered events occurring within hours or days of a large earth-
quake, and at present this assessment is still held to be true.
A second question that has been asked is whether the
M w = 5.9 earthquake in Athens, Greece (7 September 1999,
11:56:50 UT, 38.1ON, 23.6°E) may have been triggered by
the Turkish earthquake. In our judgment, this possibility
cannot be dismissed. However, given the large distance
between the two events, the probability that the Athens
earthquake was triggered is small.
The third issue is what may happen on the unruptured
segments of the North Anatolian Fault. The southern branch
of the North Anatolian Fault that traverses Lake Iznik and
continues into Gemlik Bay (see Figure 6) is of interest. GPS
results show about 5 mrn/yr deformation is occurring on this
fault. Given the fact that this fault segment was not ruptured
by a major earthquake during the 20th century, it warrants
watching.
The most significant question deals with the westward
extension of the fault from the Gulf of Izmit toward
Tekirdag, along the northern half of the Sea of Marmara. If
the westward extension of the fault that ruptured on 17
August 1999 continues under the sea and connects to the
segment (referred to as the Ganas Fault) that ruptured in
1912, then there is a seismic gap about 170 km long extend-
ing from east to west between Izmit and Tekirdag (Figure 6).
Should an earthquake of similar magnitude occur on this
gap, it would have an unmeasureable impact on Istanbul and
surrounding areas with a population of nearly 15 million.
This would approach in size the 1857 earthquake on the
southern San Andreas Fault in California, except the fault
trace would be closer to Istanbul and other population cen-
678 Seismological Research Letters Volume 70, Number 6 November/December 1999
11. ters on the northern shores of the Sea of Marmara than the
San Andreas is to the Los Angeles area.
Historic earthquakes of intensity IX and X occurred on
this fault segment in 1754, 1766, and 1894 (Figure 6). If
GPS data indicated a high rate of deformation in this north-
ern part of the Sea of Marmara, it would be essential to con-
sider this region as a seismic gap with great potential for a
major earthquake.
CONCLUSIONS
The 17 August 1999 Izmit earthquake has provided vast
amounts of data that could contribute to improvements
both to the science of earthquakes and to engineering prac-
tices. It demonstrated how significant the impact of a single
natural event could be on humanity. This earthquake should
motivate all people subject to earthquake hazards to intensify
plans toward better understanding of earthquake phenom-
ena and to develop engineering and societal response pro-
grams to abate earthquake losses, m
ACKNOWLEDGMENTS
Many individuals and institutions in Turkey contributed
data and information for this paper. We gratefully acknowl-
edge the contributions of the staff of the Kandilli Observa-
tory of Bosphorous University and thank Prof. A. M. Isikara,
the Director, and Prof. M. Erdik, Head of Earthquake Engi-
neering. Istanbul Technical University staff provided much
of the information on surface faulting. The Earth Sciences
Division of the Gebze Institute of TUBITAK, only 50 km
from the epicenter, overcame the physical damage to its facil-
ities and deployed GPS instruments promptly following the
earthquake. Web sites of many institutions in Turkey, the
USA, Europe (IPGP), and elsewhere provided essential data.
Dr. S. McClusky, Prof. V. Cormier, and Ms. M. Edie pro-
vided valuable assistance to enable the writing of this early
report.
This research was supported by a joint Turkish-United
States project titled "Continuously Recording GPS Network
in the Marmara Region", funded by the World Bank, and by
NSF Grants EAR-9304554 and NSF-9724114 at MIT.
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Earth Resources Laboratory
Department of Earth, Atmospheric, and Planetary Sciences
Massachusetts Institute of Technology
(M.N.T., R.E.R., C.G.D.)
Department of Geology
Istanbul Technical University
(A.A.B.)
Gebze Research Institute
TUBITAK
(N.Y.)
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