Seismic activity triggered by the 1999 Izmit earthquake and its

F A S T - T R A C K P A P E R

Seismic activity triggered by the 1999 Izmit earthquake and itsimplications for the assessment of future seismic risk

Ali Pinar,1,* Yoshimori Honkura1 and Keiko Kuge2

1 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152–8551, Japan2 Department of Geophysics, Kyoto University, Kyoto 606–8502, Japan

Accepted 2001 April 24. Received 2001 April 20; in original form 2000 November 7

SUMMARY

A serious question has remained as to the location of the western end of the mainrupture zone associated with the 1999 Izmit, Turkey, earthquake. A clear answer to thisquestion is extremely important for the assessment of future seismic risk in the easternMarmara Sea region, Turkey. In this paper we show an effective approach to answeringthis important question, unifying different kinds of information such as seismic activity,focal mechanism solutions and stress changes caused by the main shock into a clearimage. We first point out that the major moment release is 1.6r1020 N m and coveredthe area between 29.7uE and 30.5uE and we then claim that the enhanced seismic activityafter the main shock in the eastern Marmara Sea region should be regarded as activitytriggered by the increase of stress, rather than as aftershock activity along the rupturedzone. We propose three fault segments with an average stress increase on each in thewestern extension of the main-shock rupture zone as potential sites for future large earth-quakes, namely (i) the 50 km long Yalova–Hersek segment (0.45 MPa), (ii) the NW–SE-trending right-lateral strike-slip fault known as the Princes Islands segment (0.18 MPa),and (iii) the Cinarcik–Yalova segment (0.09 MPa) characterized by normal faulting,which was subject to rupture in 1963.

Key words: faults, focal mechanisms, Izmit earthquake, Marmara Sea, seismic activity,stress change.

I N T R O D U C T I O N

The Mw=7.4 Izmit earthquake was a unique event in Turkey in

that it occurred in an area where several kinds of data are

available: far-field, near-field, strong-motion and weak-motion

seismograms and also GPS data. A clear source model for the

Izmit earthquake would be expected with these data sets, and

in fact some models have been proposed (Cemen et al. 2000;

Ito et al. 2000; Reilinger et al. 2000; Yagi & Kikuchi 2000) as

well as their implications for the seismic risk assessment for a

possible next earthquake (Parsons et al. 2000; Hubert-Ferrari

et al. 2000); the seismic risk tends to increase in a huge city,

i.e. Istanbul. The facts that the last two major events occurred

in 1766 and the recent slip-rate estimate is about 20 mm yrx1

(Straub et al. 1997; McClusky et al. 2000) indicate the possibility

of strain accumulation to a nearly critical level in the proximity

of Istanbul, unless creeping events have been occurring beneath

the Marmara Sea region.

In view of the nature of westward migration of large

earthquakes along the NAFZ (Stein et al. 1997), it is crucial

to understand in more detail how far to the west the main

rupture zone associated with the Izmit earthquake extended.

The estimation of rupture extension has relied on surface obser-

vations (Honkura et al. 2000) and the distribution of aftershocks

(Ito et al. 2000). Recent progress in studies of focal mechanisms

based on the seismic waveform inversion (Kikuchi & Kanamori

1991; Pinar et al. 1994, 1996; Pinar 1998) has made the estimation

possible without local information such as surface observations

and the distribution of aftershocks. Sometimes, however, the

different kinds of information are contradictory to each other,

as described below for the case of the Izmit earthquake.

Our basic standpoint from which to solve the contradiction

of results obtained from different kinds of data is as follows.

Seismic activity after the main shock is triggered by an increase

of Coulomb failure stress. The Coulomb failure stress estimates

depend on the rupture length, faulting geometry and slip distri-

bution, which are all determined by a source process model.

The Coulomb failure stress changes control seismic activity

including aftershocks on the ruptured fault plane, which in turn* Present address: Department of Geophysics, Istanbul University,

34850, Istanbul, Turkey. E-mail: [email protected]

Geophys. J. Int. (2001) 146, F1–F7

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affects source process models through the estimation of rupture

length and faulting geometry. In this sense, the seismic activity

after the main shock is a key factor for the relation between the

Coulomb failure stress and the rupture process. Overestimation

or underestimation of the rupture extent, for example, through

interpretations based on the aftershock distribution, will result

in disagreement between the increased Coulomb failure stress

area and the enhanced seismic activity.

S O U R C E M O D E L B A S E D O N W A V E F O R MI N V E R S I O N

The distribution of earthquakes that occurred after the Izmit

earthquake main shock covers the 200 km long E–W-trending

area between 29uE and 31uE, as shown in Fig. 1 (Ito et al. 2000).

A series of surface ruptures with a total length of 120–130 km

were observed from field surveys (Cemen et al. 2000; Honkura

et al. 2000) between 29.7uE and 31uE. Our inversion results for

far-field seismograms suggest that the coseismic rupture extent

should not exceed 70–80 km and should cover only the area

between the 29.7uE and 30.5uE, with the implication that the

observed surface rupture beyond 30.5uE is associated either with

a very low rupture velocity so that no signals could be recorded

in the near-field accelerographs or with afterslip events. Recently,

Iio et al. (2000) suggested that a creeping process is likely to

have occurred in the area east of 30.6uE, based on their precise

determination of aftershock hypocentres. In this paper we try to

constrain the coseismic rupture extent associated with the Izmit

earthquake, with more emphasis placed on the relation between

the source process, the aftershock distribution, the Coulomb

failure stress changes and the seismic activity after the Izmit

earthquake.

Our rupture extent analysis is based on the inversion of

far-field complex body waves (Kikuchi & Kanamori 1991), in

which the source process is expressed by the spatio-temporal

distribution of subevents along with their centroid moment

tensor (CMT) parameters. For the Izmit earthquake main shock,

the best waveform fitting of the synthetic to the observed

seismograms was attained by two subevents: a major E–W-

striking subevent with a seismic moment of 1.6r1020 N m,

similar to the result of Yagi & Kikuchi (2000), and a minor

shallow-dipping normal faulting subevent at the eastern end of

the major subevent, where a scattered earthquake distribution

can be seen.

Figure 1. Digital elevation map produced by Tim Wright (Oxford University) and earthquakes, shown by yellow dots, that occurred between

August 17 and October 29 1999 (Ito et al. 2000). The thick red line extending between the east of Hersek and Sapanca indicates the coseismic rupture

extent derived from our teleseismic waveform inversion. The total rupture length is 80 km with an average slip of 4.0 m. The locations referred to in

the text are abbreviated as follows: PI=Princes Islands; Y=Yalova; C=Cinarcik; GB=Gemlik Bay; H=Hersek; G=Golcuk; S=Sapanca.

F2 A. Pinar, Y. Honkura and K. Kuge

# 2001 RAS, GJI 146, F1–F7


The rupture associated with the main subevent started at the

hypocentre determined precisely in Honkura et al. (2000) and

propagated 40 km westwards and also 40 km eastwards, and

then the second subevent occurred at the eastern end, covering

the area between the west of Golcuk and the east of Sapanca.

We also determined the moment rate function and the amount

of moment release along the ruptured fault. Assuming a rigidity

of 3r1010 Pa and a thickness of 15 km for the seismogenic fault

zone (Ito et al. 2000), we estimated the lateral slip variation

along the ruptured fault, which was then used as input for the

estimation of Coulomb failure stress. The highest slip was found

in areas close to Golcuk and Sapanca (G and S in Fig. 1); this is

consistent with the surface observations (Honkura et al. 2000).

It should be noted that there is a seismicity gap at the western

end of the rupture zone, as clearly seen in Fig. 1. As we show

later, seismic activity in the west of Hersek (H in Fig. 1) is likely

to be activity triggered by the main shock.

O N S O U R C E M O D E L S B A S E D O NG E O D E T I C S T U D I E S

Reilinger et al. (2000) used the data at five continuous GPS

stations that were operating during the main shock in the vicinity

of the source region in order to infer a coseismic deformation

model for the 1999 Izmit earthquake. Four of the stations

were located very close to the western side of the rupture zone,

that is, within the deformation area. The coseismic deformation

model of Reilinger et al. (2000) required no significant slip on

the Yalova–Hersek segment in the west of the Hersek peninsula

and gave an upper limit of 60 cm of slip. Michel & Avouac

(2000) measured coseismic displacements from SPOT images

and identified a sharp and very linear fault trace that extends

to 70 km between Golcuk and Akyazi. Comparing their result

with field observations, they concluded that very little slip

occurred off the main fault trace. Delouis et al. (2000) carried

out a joint inversion of SAR interferometry and teleseismic

data for the slip history of the Izmit earthquake. They found a

good correspondence between slip at depth and at the surface

from Golcuk to Sapanca, but further to the west and east of

this zone, the slip was confined to the near-surface. The field

observations support this conclusion for the eastern side of the

source region but not for the western side. Lindval et al. (2000)

conducted palaeoseismic trenching on the Hersek peninsula and

observed that the main fault trace crosses the peninsula but did

not experience major rupture in 1999; rather, it experienced minor

cracking that could be due to soil desiccation. Contrary to the

studies mentioned above, the slip model of Wright et al. (2000)

based on SAR interferometry data suggests 2 m of coseismic slip

on the Yalova–Hersek segment. We incorporated this amount

of slip in our Coulomb stress calculations shown in Fig. 4(b).

Studies dealing with different types of data, for example,

GPS, near-field strong motion, far-field body waves and field

observations, all yield the result that the major moment release

took place between Golcuk and Sapanca. The disagreement

between different slip models emerges from the slips derived or

observed on the faults located outside the main rupture zone

(off-faults) of the main source region. The seismic moment

release contribution by the off-faults (4–5r1018 N m, estimated

from the observed rupture lengths and offsets) in the east is

very small compared with the moment release between Golcuk

and Sapanca (1.6r1020 N m). These seismic moments suggest

that the amplitudes generated by the off-faults should be about

40 times smaller than the teleseismic body wave amplitudes

generated by the Golcuk–Sapanca segment. Therefore, it would

be very hard to distinguish their contribution in the teleseismic

records; consequently, the teleseismic waveform modelling could

not resolve the contribution of the fault ruptures observed in

the east of Sapanca. GPS data resolved the contribution of the

off-faults in the east of Sapanca (Reilinger et al. 2000).

E S T I M A T I O N O F R E G I O N A L S T R E S S

The regional stress tensor, which can be determined from the

focal mechanisms of many earthquakes, is also an important

parameter in estimating the Coulomb failure stress on the

optimally oriented fault planes. For this reason, we determined

the moment tensors of 36 earthquakes through waveform

modelling developed by Kuge (1999). These earthquakes,

recorded by a local broad-band seismic network operated by

Kandilli Observatory, are located in the western part of the

rupture zone and its western extension, as shown in Fig. 2.

Here several moderate-sized earthquakes followed the Izmit

earthquake main shock and we were able to derive their focal

mechanisms, as also shown in Fig. 2.

Using the focal mechanism parameters for the earthquakes

occurring outside the main rupture zone (events 1–16 and 24–36)

together with the mechanisms given in Kalafat (1998) and

Gurbuz et al. (2000) and the program given in Gephart (1990),

we determined the regional stress tensor. The result is as follows.

The azimuths and plunges are 302u and 7u, 180u and 77u, and

34u and 11u, respectively, for the maximum compression axis,

the intermediate stress axis and the minimum stress axis, as

shown in Fig. 3. The amplitude ratio expressed by R=(s2xs1)/

(s3xs1) was found to be 0.4, implying a transtensional tectonic

regime (transition from shear to extension).

S E I S M I C A C T I V I T Y T R I G G E R E D B YC O U L O M B F A I L U R E S T R E S S C H A N G E S

A comprehensive list of references on stress interactions can be

found in Harris (1998). In this paper, we calculate the static

stress changes using the program Coulomb 1.3 written for

Mac computers by S. Toda (Stein et al. 1992; King et al. 1994;

Okada 1994; Toda et al. 1998). It yields stress changes on a

specified or optimally oriented strike-slip or dip-slip fault caused

by slip on a source fault in an elastic half-space. The main para-

meters necessary to estimate the Coulomb failure stress changes

are a main-shock static slip model, an apparent coefficient

of friction including pore fluid pressure, the regional stress

field orientation and fault plane parameters of the source and

receiver faults. With the exception of the coefficient of friction,

which is assumed to be 0.4 in our study, we determined all of

these parameters and we were able to obtain the parameters

necessary to calculate Coulomb failure stress changes caused

by the 1999 Izmit earthquake.

Fig. 4(a) shows the distribution of Coulomb failure stress

changes derived from the rupture model shown in Fig. 1, and

Fig. 4(b) shows the result for the case in which the rupture zone

is extended further west to a longitude of 29.1uE. We can

clearly see that seismicity is high in the stress increase areas in

case (a).

The SAR interferometry model of Wright et al. (2000) suggests

2 m of slip on the Yalova–Hersek segment. This corresponds

to a seismic moment of M0=0.45r1020 N m released on the

Seismic activity triggered by the 1999 Izmit earthquake F3

# 2001 RAS, GJI 146, F1–F7


50 km long Yalova–Hersek segment. Taking into account

the relation between the seismic moment and the stress drop

(Dd=2.5 M0Ax1.5, where A is the ruptured fault area) and the

15 km thickness of the seismogenic zone, we can estimate a

stress drop of 5 MPa for the Yalova–Hersek segment and

10 MPa for the Golcuk–Sapanca segment. Fig. 4(b) shows the

Coulomb stress changes derived from the average slip of 2 m on

the Yalova–Hersek segment and 4 m on the Golcuk–Sapanca

Figure 3. Results of regional stress tensor analyses for the focal mechanisms given in Fig. 2 (except for events 17–23) and in Gurbuz et al. 2000) and

Kalafat (1998). (a) Histogram of R-values; (b) distribution of estimated principal stress axes; (c) distribution of the observed P- and T-axes. The best fit

was attained for R=0.4 and for azimuth and plunge pairs of (302u, 7u) for s1, (180u, 77u) for s2 and (34u, 11u) for s3. In (b), red dots show the azimuth

and plunge of the maximum compression axis, s1, blue circles those of the minimum stress axis, s3, and green triangles those of the intermediate stress

axis, s2. In (c), red dots show the P-axes and blue circles the T-axes. Black symbols denote the axes for the best stress model.

Figure 2. Epicentres (Ito et al. 2000) and focal mechanisms of earthquakes for which moment tensor inversions were carried out. The data sources are

the three-component seismograms recorded at local broad-band stations operated by the Kandilli Observatory of Bogazici University. The method of

moment tensor inversion was developed by Kuge (1999). The synthetic seismograms were calculated following Kohketsu (1985). The size of focal

mechanism diagram is proportional to the moment magnitude, Mw, of the earthquake. The magnitude range of the earthquakes shown is 3.1–5.2.


# 2001 RAS, GJI 146, F1–F7


Figure 4. Coulomb failure stress changes on optimally oriented strike-slip faults caused (a) by the major subevent that ruptured the fault between

Hersek and Sapanca, as shown in Fig. 1, and (b) by another rupture model in which the rupture is assumed to have extended further west to 29.1uE.

The stress changes were calculated at a depth of 7.5 km, assuming an apparent coefficient of friction, including the fluid pore pressure, of 0.4. The

deviatoric tectonic stress is assumed to amount to 10 MPa as in King et al. (1994), with the compression axis at an azimuth of 302u, as obtained in this

study; this azimuth results in an optimum right-lateral fault in a nearly E–W direction. We made calculations for several other orientations of principal

compression axis and found that the above azimuth yields the best correlation between the stress increase areas and the active seismic areas after the

main shock. The circles show the epicentres of the earthquakes that occurred during the period August 17–October 29 1999 (Ito et al. 2000). Circle

sizes are proportional to magnitude.


# 2001 RAS, GJI 146, F1–F7


segment. This figure clearly shows that the Yalova–Hersek seg-

ment is located in the stress-reduced region where aftershocks

should be rare, which is obviously not the case. The model of

Wright et al. (2000) thus contradicts our stress modelling as

well as the results of Reilinger et al. (2000) and Lindval et al.

(2000). Therefore, a shorter fault rupture in the west seems to

be more plausible.

The seismic gap at the western end of the rupture zone seems

to be well explained by the model in case (a). In fact, if we

assume that the fault between the 29.6uE and 29.1uE has the

same geometry of E–W strike-slip faulting as the main rupture

zone, the Coulomb failure stress decreases locally at the eastern

end of this assumed fault. Alternatively, one may claim that the

gap area is more likely to represent a barrier in the fault zone

(Aki 1979). The essential point in this argument, though, is that

the rupture did not propagate further west beyond 29.6uE, and

the aligned earthquakes between the 29.6uE and 29.1uE can be

interpreted more reasonably as the activity triggered by Coulomb

failure stress increase. We call this portion the Yalova–Hersek

(Y and H in Fig. 1) segment, and according to our interpretation,

this segment was not subject to major rupture in 1999.

In addition to the Yalova–Hersek segment, we can identify

two more segments where triggered seismicity can be seen (Fig. 1)

in the eastern Marmara Sea region. In Fig. 2 there is a clear

nodal plane oriented NW–SE for events 5–11. We regard this

nodal plane as a fault plane, taking into account the alignment

of epicentres in the NW–SE direction. We call it the Princes

Islands segment (PI in Fig. 1).

Okay et al. (2000) interpreted several seismic cross-sections

constructed from multichannel seismic reflection profiles acquired

in the proximity of PI and came to the conclusion that the

North Anatolian fault extends into the Marmara sea along

the PI segment, which they call the North Boundary fault seg-

ment. Its location and sense of motion are similar to those that

we derived from the focal mechanisms. Another piece of

evidence for dextral strike-slip motion on the PI segment comes

from the GPS study of Straub et al. (1997). However, there

are also contradicting studies concluding that the sense of

motion in the proximity of PI and several other segments in the

Marmara sea is predominantly normal faulting (Smith et al.

1995; Wong et al. 1995; Parke et al. 1999).

Events 27x36 in Fig. 2 show normal faulting in the proximity

of Cinarcik and Yalova (C and Y in Fig. 1). It is noteworthy

that the mechanism of the 1963 September 18 Cinarcik earth-

quake is also of normal faulting type (Eyidogan et al. 1991).

Also, according to the macroseismic information given in

Eyidogan et al. (1991), the maximum intensity associated with

this event was localized in the cities of Cinarcik, Yalova and

around Gemlik Bay (C, Y and GB in Fig. 1), whereas Istanbul

was not greatly affected. We conclude, therefore, that a seg-

ment characterized by normal faulting exists near Cinarcik and

Yalova and that it was subject to rupture in 1963. We call this

segment the Cinarcik–Yalova segment.

I M P L I C A T I O N S F O R T H E A S S E S S M E N TO F F U T U R E S E I S M I C R I S K

Using the focal mechanisms for the three segments defined

above, we estimated the static stress changes on these segments

caused by the Izmit earthquake and found Coulomb failure

stress increases of 0.45 MPa on the Yalova–Hersek strike-

slip fault segment, 0.18 MPa on the Princes Islands strike-slip

fault segment and 0.09 MPa on the Cinarcik–Yalova normal

fault segment. The rupture time for the Yalova–Hersek segment

is hastened by about a decade, taking into account the regional

stress loading rate of 0.04 MPa yrx1, estimated from a com-

bination of the stress drop associated with the Izmit earthquake

(about 10 MPa) and the time since the previous major event.

We infer this result from the following calculations.

Our teleseismic modelling yields a seismic moment of

M0=1.6r1020 N m for the Golcuk–Sapanca segment. Using

this moment value and the relation between the stress drop

and the seismic moment, we derived a stress drop of 10 MPa.

The studies of Ambraseys & Jackson (2000) and Klinger et al.

(2000) suggest that the previous major rupture in Izmit took

place in 1719, i.e. 280 yr ago. Taking into account the 10 MPa

stress drop estimate and this time span, we can derive a

tectonic stress loading rate of about 0.04 MPa yrx1. Thus,

the 0.45 MPa stress loading on the Yalova–Hersek segment

corresponds to stress loading spanning about a decade.

Information on seismic activity in the area covering these

three segments is important and the IZINET project to increase

the number of stations in the seismic network (Ito et al. 2000;

Honkura et al. 2000; Ucer et al. 2000) is now under way.

A C K N O W L E D G M E N T S

Seismic data obtained from the IZINET seismic network were

essential for our study. We thank Akihiko Ito and Balamir

Ucer, who have been the principal contributors to IZINET, and

their colleagues. We also thank Haruo Horikawa for providing

us with the ASCII code for the fmsi program. The comments

raised by two referees were helpful in revising the manuscript.

This work was supported by the Research Fund of the University

of Istanbul through project number B-797/02112000 and also

by the Association of International Education, Japan.

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# 2001 RAS, GJI 146, F1–F7


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Seismic activity triggered by the 1999 Izmit earthquake and its