seismic activity triggered by the 1999 izmit earthquake and its
TRANSCRIPT
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.
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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
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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.
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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.
Seismic activity triggered by the 1999 Izmit earthquake F5
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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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Seismic activity triggered by the 1999 Izmit earthquake F7
# 2001 RAS, GJI 146, F1–F7
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