The neglected Istanbul earthquakes in the North Anatolian Shear Zone: tectonic implications and broad-band ground motion simulations for a future ...
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Geophys. J. Int. (2023) 233, 700–723 https://doi.org/10.1093/gji/ggac477
Advance Access publication 2022 November 30
GJI Seismology
The neglected Istanbul earthquakes in the North Anatolian Shear
Zone: tectonic implications and broad-band ground motion
simulations for a future moderate event
Onur Tan ,1 Özlem Karagöz,2 Semih Ergintav3 and Kemal Duran4
1 Department of Geophysical Engineering, Faculty of Engineering, İstanbul University – Cerrahpaşa, 34320 Istanbul, Turkey. E-mail: onur.tan@iuc.edu.tr
2 Department of Geophysical Engineering, Faculty of Engineering, Çanakkale Onsekiz Mart University, 17100 Çanakkale, Turkey
3 Department of Geodesy, Kandilli Observatory and Earthquake Research Institute, Boğaziçi University, 34684 Istanbul, Turkey
4 Department of Soil and Earthquake Research, Istanbul Metropolitan Municipality, 34440 İstanbul, Turkey
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Accepted 2022 November 29. Received 2022 November 1; in original form 2022 June 7
SUMMARY
İstanbul (Marmara Region, NW Turkey) is one of the megacities in the world and suffered
from destructive earthquakes on the North Anatolian Fault, a member of the North Anatolian
Shear Zone, throughout history. The 1999 Kocaeli and Düzce earthquakes emphasize the
earthquake potential of the fault, crossing the Sea of Marmara, and the importance of seismic
hazards in the region. The studies in the last 20 yr have concentrated on the main fault
and its future destructive earthquake potential. In this study, unlike the previous ones, we
focus on the two main topics about the earthquakes not interested previously in İstanbul: (1)
Investigating recent earthquake activity masked by the blasts in the metropolitan area and its
tectonic implications, (2) revealing their effects in İstanbul utilizing numerical ground motion
simulations for a future moderate event (Mw 5). First, the 386 earthquakes from 2006 to 2016
are relocated with the double-difference method using the dense seismic network operated in
the same period. The source mechanisms of the events (ML ≥ 3), including the most recent
2021 Kartal–İstanbul earthquake (ML 4.1), are determined. In addition to the analysis of the
recent seismic activity, the location of the two moderate and pre-instrumental-period İstanbul
earthquakes, which occurred in 1923 (Mw 5.5) and 1929 (Mw 5.1), are revised. Using the
relocated epicentres outside of the principal deformation zone and the fault plane solutions,
the roles of the earthquakes in the stress regime of the Marmara region are explained. The
epicentres on the Cenozoic or Palaeozoic formation in the Istanbul–Zonguldak Zone are
interpreted as the re-activation of the palaeo-structures under the recent tectonic stresses,
and their fault plane solutions agree with the synthetic/antithetic shears of a transtensional
regime corresponding to the right lateral strike-slip system with mainly N–S extension in the
Marmara Region. In the second part, we investigate the effects of moderate scenario events
(Mw 5) considering the current earthquake epicentres in the İstanbul metropolitan area, using
characterized earthquake source model and 1-D velocity structure verified with the broad-band
(0.1–10 Hz) numerical ground motion simulation of the 2021 Kartal–İstanbul earthquake.
The simulated PGAs agree with the ground-motion prediction equations for short epicentral
distances (
Neglected İstanbul earthquakes 701
no Holocene active fault in the metropolitan area, the Phanerozoic
1 I N T RO D U C T I O N
units are cut bNy palaeofaults in various orientations and sizes,
The North Anatolian Fault (NAF), as one of the largest plate- especially on the Asian side. The previous studies also mention
bounding transform faults, separates the Anatolian and Eurasian two remarkable old fault zones on the European side. The West
plates and extends for ∼1600 km between eastern Anatolia and Black Sea Fault (WBSF) is a dextral transform fault zone covered
northern Aegean. Şengör et al. (2005) mentioned that the NAF is a by undeformed Eocene sediments, between the Strandja Massif and
member of the E–W North Anatolian Shear Zone (NASZ). Anato- İstanbul–Zonguldak Palaeozoic Zone, in the west of İstanbul (Okay
lia moves westward with respect to the collision zone between the et al. 1994). The recent geophysical observations also show a sharp
Eurasian and Arabian Plates at a rate of ∼25 mm yr–1 (Reilinger resistive-conductive boundary in the upper-most crust interpreted
et al. 2006), activating major strike-slip and also N–S extensional as the WBSF (Karcıoğlu et al. 2013). The NW–SE oriented dextral
normal faulting earthquakes south of the Marmara region (Am- Çatalca Fault is the other fault in the area that is the boundary
braseys 2009). The seismic activity is mainly on the NAF, the prin- between the Strandja Massif and Thrace Basin (Yılmaz et al. 1997).
cipal deformation zone (PDZ). A series of large earthquakes started The geodetical observations indicate slip partitioning between
in eastern Anatolia in 1939. They propagated westward, along the the northern and southern branches of the NAF (Armijo et al. 2002;
northern branch of NAF in the Marmara Sea, towards the Istanbul– Ergintav et al. 2014), and the deformation mainly occurs along
Marmara region in northwestern Turkey. This migration ended in the northern branch (Meade et al. 2002; Reilinger et al. 2006).
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1999 with the devasting Kocaeli earthquake (Mw 7.4). West of the The rate of secular deformations of the northern branch of the
1999 Kocaeli rupture, a ‘seismic gap’ exists along a ∼100-km-long NAF is around 10–15 mm yr–1 in the southern part of the Istanbul
segment below the Sea of Marmara which connects the Mürefte province. After the devastating 1999 Kocaeli earthquake, İstanbul
(1912, Mw 7.3) and Kocaeli (1999, Mw 7.4) ruptures (Toksöz et al. was affected by its post-seismic deformations. Its order was cm-
1979; Altunel et al. 2004; Aksoy et al. 2010; Bohnhoff et al. 2013). level after immediately the earthquake (Ergintav et al. 2009) and
The region has very high seismic activity, and several destructive reduced to the mm-level (Diao et al. 2016) in time, as the logarithmic
events (Mw > 7) on the segments of the NAF are reported in both nature of the post-seismic deformations (Hearn et al. 2002; Ergintav
historical and instrumental period earthquake catalogues (Fig. 1). et al. 2007, 2009). Hence, as expected, the stress change increased
The instrumental period seismological observations indicate high on the active and passive fault systems in the eastern Marmara
seismic activity in a narrow zone in the Sea of Marmara with a Region. Correspondingly, the seismic hazard potential of İstanbul
seismic gap on the central part of the northern branch (Kumburgaz increases and hazard assessment studies for utilizing probabilistic
segment in Fig. 2). There are also seismological and geodetical and hybrid simulations based on a destructive earthquake on the
evidences of another seismic gap on the Princes’ Island segment NAF are performed (i.e. Sorensen et al. 2009; Yalcinkaya 2014;
(Bohnhoff et al. 2013; Ergintav et al. 2014; Wollin et al. 2018). Aochi & Ulrich 2015; Douglas & Aochi 2016).
These two seismic gaps are the potential destructive event source The last decade-earthquakes show that most of the buildings in
considering the recurrence of the destructive Marmara earthquakes. Turkey are still not resistant to a nearby moderate or distant large
In the western part of the Marmara Region, the Ganos segment rup- earthquake. For example, the significant 2011 Van-Erciş (eastern
tured by the 1912 Mürefte earthquake (Mw 7.3) accumulated strain Turkey, Mw 7.1) devastated more than 8000 buildings. On the other
due to an Mw 7 earthquake (Ergintav et al. 2014). The other branch hand, the moderate 2017 Çanakkale–Ayvacık earthquake sequence
of the NAF follows the southern part of the Marmara with scattered (western Turkey, Mw 5.3) damaged adobe and masonry buildings in
seismic activity, and geodetic data shows that they have lower strain Ayvacık city and the nearby villages. Similarly, buildings were dam-
accumulation with respect to the northern branch ( 5) in the west The effects of smaller events on buildings, such as the 2021
of İstanbul on 26 October 1923 and 10 October 1929 are reported İstanbul-Kartal earthquake (ML 4.1), are ignored due to their low
in the International Seismological Summary (ISS), and the magni- intensities. However, they should be considered because an unbro-
tudes were determined as 5.5 and 5.1, respectively, by Kondorskaya ken asperity at the same focal area may be capable of generating a
& Ulomov (1999). moderate event (Mw 5.0–5.5). In this case, the radiating waves from
The metropolitan area of İstanbul, the interest of this study, covers a seismic source close to a densely populated city, such as İstanbul,
both sides of the Bosporus and is mainly located on Palaeozoic may affect the residential and industrial structures with low seismic
sedimentary rocks which are intruded or overlain by Mesozoic and resistance.
Cenozoic magmatic and sedimentary deposits, which we simplified In this study, unlike the previous ones, we focus on the moderate
as pre-Miocene rocks and Miocene–Pliocene deposits in Fig. 2. The earthquakes beneath the İstanbul metropolitan area and their possi-
Quaternary alluvium is only observed in the northern part of the ble seismic hazard potential in the future. For this aim, the activity
province and the creek beds in the city centre. Although there is and source properties of the earthquakes in the period of 2006–2016,
702 O. Tan et al.
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Figure 1. Seismicity of the Marmara Region (NW Turkey) and study area. Seismology stations in the region are shown in the inset map. Circles are events
between 1900 and 2021 with equivalent moment magnitudes (Mw ∗ ≥ 3.0, from the TURHEC database by Tan 2021). Small stars are historical events before
1900 (Tan et al. 2008). The ISS epicentres of the 1923 and 1929 (Mw > 5) İstanbul earthquakes are also shown with a big star. Black solid lines are active
faults Emre et al. (2018). Dashed lines are the boundary of Istanbul province. NAF, North Anatolian Fault.
including the recent 19 June 2021 Kartal earthquake, are investi- & Çağnan 2010; B2014: Boore et al. 2014) and the 2018 Turk-
gated using the data from the available seismic networks. The foci ish Building Earthquake Code (TBEC), respectively. Consequently,
of the events are relocated with the double-difference algorithm uti- the possible effects of a moderate earthquake, depending on the
lizing the waveform similarity. The focal mechanism solutions with source and the site condition in the İstanbul metropolitan area, are
low uncertainties are determined by using numerous P-wave first- evaluated.
motion polarities. The 1923 (Mw 5.5) and 1929 (Mw 5.1) İstanbul
earthquakes, which are never discussed before, are also relocated
with the body wave arrival times reported in the bulletins and their
2 SEISMIC ACTIVITY IN THE
roles in the regional stress regime are estimated. In addition to the
I S TA N B U L M E T R O P O L I TA N A R E A
tectonic implication of the source mechanisms of the İstanbul earth-
quakes, we numerically simulate scenario earthquakes in regard to
2.1 Data analysis
ground motion prediction. The numerical simulation methodology
utilizing the discrete wave number method (DWNM) and including The parametric and waveform data of the earthquakes used in this
the effects of propagating source, path and the site amplification study are obtained from regional and national seismological net-
of shallow 1-D soil layers over the engineering bedrock are veri- works around the Marmara Sea. The regional weak motion networks
fied with the 2021 Kartal earthquake records. The fundamental soil for micro-earthquake observations are operated in the projects be-
frequencies at the AFAD strong motion stations are also estimated tween 2006 and 2016, such as MARsite (www.marsite.eu). Ad-
using the horizontal-to-vertical spectral ratio (HVSR, Nakamura ditional weak and strong motion data are retrieved from the na-
1989) method with at least 10 earthquake records to verify the pre- tional network stations of the Kandilli Observatory and Earthquake
viously reported values. After identifying the source locations for Research Center (KOERI 1971) and the Disaster and Emergency
possible future Mw 5 events in the metropolitan area, the scenario Management Authority (AFAD 1990).
earthquakes with different source models are numerically simulated The Hypocentre location algorithm (Lienert & Havskov 1995)
at the strong motion stations. Then the simulated peak ground ac- is used to determine the parameters of the events in the Istanbul
celerations (PGAs) and pseudo-acceleration response spectra (Sa) metropolitan area with the velocity model proposed by Karabu-
are compared with two attenuation relationships (AC2010: Akkar lut et al. (2011). The total 386 events between 2006 and 2016 in
Neglected İstanbul earthquakes 703
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Figure 2. Geological map of İstanbul (compiled from Yilmaz et al. 1997; Okay et al. 2000; Özgül 2012; Lom et al. 2016). Dashed lines are faults in the
Palaeozoic zone. Segments of the NAF are shown with solid lines. BçL, Büyükçekmece Lake; KçL, Küçükçekmece Lake.
the study area shown in Fig. 1 are located using the 3038 P- and Fig. 3 shows two examples of the waveform similarities of event
2581 S-arrivals. The minimum and the average number of stations pairs in the study area. The average coherency in the first example
used in the relocation procedure are four and eight, respectively. is low (Cxy 2 = 0.65) for two neighbouring events (07.05.2010 02:41
The average uncertainties of the absolute event hypocentres are ML 1.7, 11.05.2010 22:40 ML 1.5) in Fig. 3(a). The CC function
about ±2 km. indicates the P waves’ arrival time difference (Tcc ) of 0.03 s. The
The hypocentres are improved with double-difference (DD) in- second event’s waveform is shifted according to the Tcc value to
version, one of the methods to reduce location uncertainties. For show the waveform similarity in the bottom row of the figure. A
this aim, we re-analyse the events utilizing the hypoDD algorithm high coherency example (Cxy 2 = 0.92) of P-waveforms is given
developed by Waldhauser & Ellsworth (2000). The algorithm as- in Fig. 3(b) for the neighbouring events with ML 1.3 (20.09.2009
sumes that the hypocentral separation between two earthquakes is 22:22) and ML 2.0 (08.05.2011 04:49). The spiky CC function with
small compared to the event-station distance and the scale length of 0.07 s time-lag indicates a good waveform match of the two events.
velocity heterogeneity, so the ray paths are similar. The traveltime In the hypoDD inversion scheme, the Cxy 2 values of the waveforms
difference between two events observed at one station can be ac- are used for weighting. The location uncertainties of the relocated
curately attributed to the spatial offset between the events (Fréchet events are estimated using the statistical approach described by
1985; Got et al. 1994; Waldhauser & Ellsworth 2000). We use ab- Tan et al. (2010) and Tan (2013). The random numbers (distances)
solute location parameters of the events from the catalogue and between −3.0 and + 3.0 km that agreed with the average hypocen-
the P/S traveltime differences between the event pairs as the in- tral uncertainties (±2 km) were added to the initial locations of the
version inputs. P/S-wave cross-correlation (CC) data is also used events in the X, Y and Z directions. The randomly shifted event pairs
to obtain traveltime lag with a 0.01 s resolution. The differential are relocated with repeated ∼500 well-conditioned inversions. The
traveltime data, with a relative timing precision of approximately outliers in the data set are removed using the interquartile range
tens of milliseconds, allows for calculating the relative location be- (IQR) method. The average horizontal and vertical location uncer-
tween earthquakes with errors of only a few hundred metres. A tainties for the earthquakes in Istanbul city are ±400 and ±1200 m,
10 km-search radius is chosen to select neighbouring earthquakes, respectively.
and a minimum of 8 P/S arrival times, the lower limit used to solve
unknown parameters of pairs (6 for space and 2 for time), are cho-
sen as a threshold for an event pair. The waveform similarities of
the events are determined with the coherence algorithm in the MT- 2.1.1 ML magnitudes
SPEC package of Prieto et al. (2009). Two waveforms recorded at a To standardize the calculation of the magnitudes from the differ-
common station are considered similar when all squared coherency ent instruments, we use local magnitudes (ML ). A methodology
values (Cxy 2 ) exceed 0.5 in the frequency range from 1 to 10 Hz. was introduced into the Seismic Analysis Code (Goldstein et al.
704 O. Tan et al.
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Figure 3. Low (a) and high (b) P-waveform similarities of the two neighbouring events in İstanbul at a common station. The waveform amplitudes are
normalized for display only. The travel times of the recorded waves of the two events are aligned according to the time differences (Tcc ) calculated from the
cross-correlation function.
2003) to calculate the ML magnitudes of an event. First, the sen- The blast catalogue of KOERI (2022) indicates that the majority of
sor and digitizer responses are removed from the velocity records, the blasts have a local magnitude between 1.0 and 2.0 (Fig. 4b). The
and a bandpass Butterworth filter between 0.1 and 20 Hz is used. magnitudes of blasts can reach up to ML 2.0–2.5, and the records
Then, each waveform is convolved with the Wood–Anderson seis- at the distant stations are very similar to earthquake waveforms.
mometer response to generate a displacement record in units of The blast activities generally begin in the early hours and show
nanometers. Next, the maximum zero-to-peak amplitude is selected two picks in the daytime around the daily lunch break (Fig. 4c).
from the three components recorded at each station, and ML is cal- Although there are a few studies about blast identification (i.e. Ho-
culated using the equation given by Hutton & Boore (1987). Finally, rasan et al. 2009; Yıldırım et al. 2011), which are applied to limited
station-related low and high magnitude values (larger than one stan- parts of the catalogues, and a published quarry-free catalogue is not
dard deviation) of the events are removed, and then the remaining available for a long period. Hence, international (i.e. ISC, EMSC)
magnitudes are averaged for that event. and Turkish national earthquake catalogues are contaminated by ar-
tificial seismic events. Unfortunately, it is impossible to view a clear
long-term earthquake activity for İstanbul. Therefore, following our
2.1.2 Fault plane solutions range estimation in Fig. 4(b), applying a magnitude threshold of 3.0
for the small events in the land area is an effective way to clean the
The earthquake fault plane solutions (FPS) are determined from
catalogues and interpret the Istanbul seismicity.
the P-wave first motion (FM) polarities using the focmec algorithm
(Snoke et al. 2003). All available polarities at the local and regional
weak and strong motion stations are read to constrain the nodal
planes. The P-wave incidence angles for each station are calculated 2.3 Recent earthquake activity in Istanbul
using the focal depth and 1-D velocity model. If the incidence angle
The earthquake activity in the İstanbul metropolitan area is very
is larger than 90◦ for a local station in İstanbul, the polarity is located
low compared to the Marmara Sea. There are eight events with Mw ∗
to the antipode on the focal sphere. Therefore, the stations close to
≥ 3.0 on both sides of İstanbul between Büyükçekmece Lake and
the epicentre are shown in the opposite azimuthal direction on the
Tuzla in the years 2006–2016 (Fig. 5). The largest two events in the
lower hemisphere projection.
land area are on the coastline between Kartal and Tuzla districts in
eastern İstanbul. The 29 September 1999 Tuzla earthquake was the
largest event (Mw 5.2) and occurred in Tuzla–İçmeler geothermal
2.2 Blasts
area, shown in Fig. 2. Its Global Centroid Moment Tensor (GCMT)
While studying the seismicity of İstanbul, the artificial seismic solution indicates oblique normal faulting (Fig. 5, Table 1). We
events in the city, masking the earthquakes, must be classified. cannot analyse this event due to the lack of nearby seismic stations
There are several large quarry areas and stripe coal mines on both in the region. The second event (Mw 4.5) occurred on 7 July 2000,
Asian and European sides, shown with white diamonds in Fig. 4(a). northwest of the previous. There are no reported source parameters
Neglected İstanbul earthquakes 705
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Figure 4. (a) Events catalogued as blasts in İstanbul between 2012 and 2021 (KOERI). There are about 3070 events in the selected area. White diamonds are
large quarry and coal mines identified from satellite images. Dense settlement areas are bounded by dotted lines. (b) Local magnitude (ML ) histogram of the
events. (c) Hourly (local time) distribution of the events.
for the event. Both events occur after the 1999 Kocaeli main shock polarity changes at the NNW and NE stations constrain both nodal
(Mw 7.4) and are in the positive lobe of Coulomb stress change by planes. The NNW–SSE (strike 335◦ ) nodal plane indicates left-
Çakır et al. (2003). lateral strike-slip faulting with a dip angle of 85◦ . The second
A few earthquake clusters were on the European side of Istanbul event with strike-slip faulting occurred on 5 February 2014 (ML
from 2006 to 2016 (Fig. 5). The micro-earthquake cluster labelled 3.9) on the Black Sea coastline, northwest of the new İstanbul
with c1 is observed on the shelf area in east Silivri. A total of International Airport. The nodal planes are controlled by the 45
22 events are identified in the cluster on 27–28 March 2014. The first motion polarities around the epicentre. Both 2012 and 2014
focal depths are between 10 and 15 km, and their local magnitudes events have a focal depth of about 10 km. The third event (ML
(ML ) range from 1.5 to 2.8. The offshore cluster c2, in Selimpaşa 3.2) on the European side occurred in 2008 in the Sultangazi dis-
town, contains 15 events occurring at a depth of ∼10 km in different trict, with a population of ∼540 000. Its epicentre is very close
years. The other cluster (c3) in the westernmost of the study area has to the coal mines in the region. Although its origin time is 19:57
events with ML between 2.0 and 2.5. There is insufficient P-wave (local) and the focal depth is 5.9 km, it is catalogued as a quarry
first motion polarity to obtain reliable focal mechanism solutions blast by KOERI. The revised focal depth of the event is 12 km,
for these three clusters. Besides, the cluster c4 is observed on the and the P-wave polarities indicate normal faulting with a strike-slip
shelf area between the lakes of Büyükçekmece and Küçükçekmece. component. The magnitudes, focal depths and fault plane solu-
The largest event (ML 3.0) in the cluster occurred on 19 January tions prove that the three events (ML > 3) are not blasts. How-
2015 at a depth of 10 km. There are 26 reliable polarity readings ever, there is no event to interpret as subsequent aftershocks 1 or
in the three quadrants on the focal sphere (Fig. 6). The best nodal 2 yr later.
planes that divide the quadrants show a strike-slip mechanism with The earthquakes are clustered in two localities on the Asian side.
a thrust component. The most active cluster is in the Tuzla district (pop. 273 000), and its
The two events with a high number of first motion polarities distance to Sabiha Gökçen International Airport is about 5 km. Af-
show strike-slip events on the southern and northern coastline of ter the 1999 Tuzla earthquake (Mw 5.2), the seismic activity in Tuzla
the European side. The 19 October 2012 earthquake (ML 3.6) becomes significant. The epicentral coordinates and focal depths of
occurred beneath the Esenyurt district (pop. ∼960 000) between the two events in 2009 (ML 2.5) and 2010 (ML 3.6) are the same
the Büyükçekmece and Küçükçekmece lakes and has 39 polar- (Fig. 5, Table 1). Their first motion polarities are in good agree-
ity readings with good azimuthal coverage (Fig. 6, Table 1). The ment, and their joint solution shows a strike-slip fault with a normal
706 O. Tan et al.
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Figure 5. Earthquake activity (2006–2016, ML ≥ 1.0) in the Istanbul metropolitan area (white dashed polygon). Focal mechanism parameters are given in
Table 1. c1–c4 are the micro-earthquake clusters. The epicentre location used in the simulation models is shown with white upper letters (A–E). Stars are
relocated epicentres of the 1923 and 1929 İstanbul earthquakes. White and grey triangles are national and local weak-motion stations, respectively. Inverted
triangles with station codes are AFAD strong motion stations. Faults are from Armijo et al. (2005). Dense settlement areas are bounded by dotted lines. BçL,
Büyükçekmece Lake; ÇB, Çınarcık Basin; CH, Central High; KB, Kumburgaz Basin; KçL, Küçükçekmece Lake; Av, Avcılar; Es, Esenyurt; Bd, Beylikdüzü
districts.
component. The 9 May 2011 event (ML 3.4) also occurred at the have occurred in the inactivate zone of the WBSF. The moment
same location, and its source has similar faulting parameters. These magnitudes (Mw ) were calculated as 5.5 and 5.1, respectively, by
three events originate at 6–7 km depth on the same fault surface. A Kondorskaya & Ulomov (1999) using the observed amplitudes in
detailed map and the cross-section of the Tuzla cluster are presented the stations’ periodic bulletins. Because the original records of both
in Fig. 7. The events dip to the north–northeast and agree with the dip events are unavailable, the reported observations in the ISS cata-
angles of the fault plane solutions (62–67◦ ). This consistency may logues are used for relocation (Fig. A1). For this aim, the scanned
indicate a mainly E–W-oriented right-lateral fault plane in Tuzla. version of the periodic ISS bulletins by Villasenor et al. (1997)
The micro-earthquake activity extends to the northeastern Marmara is obtained from the ISC Seismological Dataset Repository (ISC
shelf and joints with seismicity of the NAF in the Çınarcık Basin 2021a).
(Fig. 5). The P- and S-wave arrival times are preferred and used in the
The latest earthquake in the metropolitan area (Kartal district, hypocentre location algorithm. The doubtful phases labelled with
pop. 474 000) occurred on 19 July 2021 and was felt around the letters such as ‘?S’ and ‘?L’ are ignored to reduce the complexity
city. The local magnitude of the event is calculated as 4.1. Clear 32 of the location problem (Fig. A1). In addition, the event depths
P wave first motion polarities are grouped in all quadrants on the are fixed at 10 km to reduce the unknown parameters due to the
focal lower hemisphere (Fig. 6). The nodal planes are well bounded limited observations. Then, the origin time is estimated by fixing
by the local stations around the epicentre. The fault plane solution the reported epicentre. After that, the epicentre coordinates are
shows oblique faulting with both strike-slip and thrust components. determined using the new origin time. Finally, both parameters
are obtained freely using the previous estimations as initial model
parameters.
We determined that the origin time of the 1923 earthquake is
2.4 Relocation of the 1923 and 1929 İstanbul earthquakes 12:13:27, using the station arrivals in Table 2, and the epicentre
The 26 October 1923 and 10 October 1929 earthquakes that oc- is close to the Black Sea coastline (41.328◦ N 28.517◦ E, ±30 km).
curred at the beginning of the instrumental seismology period are The 1929 earthquake origin time is 23:01:16, and the epicentre
not mentioned in the previous studies. However, their locations and (41.093◦ N 28.583◦ E, ±20 km) is relocated 12 km south of the pre-
sizes are important to understand the seismicity of İstanbul. They viously reported location. The new parameters of both earthquakes
Neglected İstanbul earthquakes 707
Table 1. Source parameters of the Istanbul earthquakes. S/D/R: Strike/dip/rake angles with uncertainties (◦ ). Azimuth and plunge of the
P-axis are in degrees. Fault plane solution (FPS) for lower hemisphere projection is given in the last column.
Date UTC time Lat. Lon. h S/D/R
P-axis
(d.m.y) (hh:mm) (◦ ) (◦ ) (km) ML MW ∗ (◦ ) FPS
Az./Pl.
This study
135/70/−60
06.06.2008 22:57 41.123 28.852 12 3.2 3.2 83/55
±5/±10/±15
13.08.2009 03:30 40.847 29.317 6 2.5 2.5 272/62/−139
129/48
3.7 ±5/±5/±20
02.01.2010 04:14 40.859 29.307 7 3.6
264/67/−158
09.05.2011 03:01 40.857 29.299 6 3.4 3.5 124/31
±5/±5/±10
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335/85/15
19.10.2012 08:17 41.038 28.638 10 3.6 3.7 108/7
±5/±20/±15
160/70/−10
05.02.2014 01:56 41.346 28.633 11 3.9 4.0 118/21
±5/±10/±10
316/50/173
19.01.2015 11:10 40.922 28.689 10 3.0 3.0 176/23
±5/±10/±10
238/83/135
19.06.2021 12:07 40.942 29.251 9 4.1 4.2 295/24
±5/±5/±10
GCMT
29.09.1999 00:13 40.799 29.377 15 5.2 (Mw ) 66/48/−171 279/34
Figure 6. Lower hemisphere equal-area projections of the P-wave first motion polarities of the Istanbul earthquakes. The solution of the 02.01.2010 event also
contains the P-wave polarities of the 13.08.2009 (ML 2.5) event. Compression (up) and dilatation (down) polarities are shown with black and white circles,
respectively. Black and white diamonds are P (pressure) and T (tension) axes, respectively.
are given in Table 3. The updated 1923 and 1929 earthquake epicen- side of İstanbul and are away from the NAF, which is the only ac-
tres shown with stars in Fig. 5 are close to the recent events in 2012 tive fault in the study area. Consequently, the epicentres are most
(near Durusu Lake in the north) and 2014 (near Büyükçekmece likely in the tectonic boundary between the Strandja and İstanbul–
Lake in the south), respectively. The revised locations, with their Zonguldak Zone and can be correlated with inactive fault zones
inherently high uncertainties, disclose that they are on the European such as the WBSF (Fig. 2).
708 O. Tan et al.
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Figure 7. Seismic activity of the Tuzla cluster and SSW-NNE profile (A–A’). The cross-section is drawn at 1:1 scale, and the possible NNE dipping fault
surface is shown with a dashed line. İçmeler is the geothermal area of İstanbul.
Table 2. P- and S-wave arrival time residuals (seconds) for the 1923 and 1929 İstanbul
earthquakes.
26 October 1923 10 October 1929
Station Tres (s) Station Tres (s)
ATH—Athens P: −3 S: +3 SEV—Sevastopol P: +1
BYG—Belgrade P: +2 YAL—Yalta S: +4 (not P)
VIE—Vienna P: +1 SIM—Simferopol P: −6
TIF—Tbilisi P: −18 FEO Theodosia P: +17
SVE—Ekaterinburg S: +5 KSA—Ksara S: +7
PUL—Pulkovo P: −14 S: +6
Table 3. Updated parameters of relocated 1923 and 1929 İstanbul earthquakes. Focal
depths are fixed (f). Magnitudes are from Kondorskaya & Ulomov (1999). Herr: Horizontal
error for epicentre location.
Time Lat. Lon. Depth Herr
Date (UTC) (◦ ) (◦ ) (km) (km) Mw
26.10.1923 12:13:27 41.328 28.517 10f ± 30 5.5
10.10.1929 23:01:16 41.093 28.583 10f ± 20 5.1
2.5 The tectonic implication of the fault plane solutions The secondary structures such as Riedel, anti-Riedel, P shears
and normal/thrust faults are well oriented according to the prin-
The earthquakes in the İstanbul metropolitan area occurred out-
cipal stresses of this dextral shear system. The findings (Fig. 5,
side the PDZ (NAF) and are evidence of the crustal deformation
Table 1) show that the E–W right-lateral nodal planes with nor-
in the İstanbul–Zonguldak Zone. One of the possible explanations
mal components of the 2010 and 2011 events are compatible with
for the focal mechanism solutions mentioned above is that the Mar-
the synthetic Riedel shears (R) of the transtensional character of
mara Sea and its surroundings are under the control of the NASZ,
the Marmara. The 29 September 1999 Tuzla event occurred in the
which significantly widens in the Marmara Region (Şengör et al.
same region after the 1999 Kocaeli earthquake (Mw 7.4) and has a
2005, 2014). Therefore, the Marmara Region is not a pure dex-
SW–NE nodal plane with a right-lateral motion like a P shear. The
tral strike-slip regime and is characterized by a transtensional sys-
high-resolution foci of the micro-earthquakes in the Tuzla cluster
tem. The sketch in Fig. 8 shows the structures associated with the
in Fig. 7 support the NNE dipping fault surface. The Tuzla cluster
transtensional regime and corresponding focal mechanism solutions
is under the Tuzla–İçmeler geothermal area used for medical treat-
obtained in this study. In pure dextral strike-slip deformation, the
ments. Therefore, this buried fault surface might be the source of
maximum principal axis (σ 1 , pressure) has an azimuthal direction
thermal water.
of 135◦ from the north for an ideal case. If the region has a transten-
The NW–SE nodal plane with the right-lateral strike-slip mo-
sional characteristic, the azimuth of σ 1 decreases to ∼120◦ given in
tion of the 19 January 2015 earthquake on the northern shelf of
Fig. 8(a) (Şengör et al. 2014). Wollin et al. (2019) also presented a
the Marmara Sea agrees with the NW–SE lineaments between
similar σ 1 direction of ∼125◦ analysing the Marmara earthquakes.
Büyükçekmece and Küçükçekmece lakes (Gökaşan et al. 2003).
The P-axis azimuth angles of the recent İstanbul earthquake in
Ergintav et al. (2011) interpret these lineaments as a series of right-
Fig. 8(b) indicate that the earthquakes occur under the WNW–ESE
lateral faults using high-resolution seismic data and local GPS cam-
compressional force. The average azimuth of P-axes is 110◦ , which
paigns. Although these faults extend to the NAF at an angle of ∼70◦ ,
is compatible with σ 1 of a transtensional regime.
they are not related to an anti-Riedel (R’) shear. They may relate
Neglected İstanbul earthquakes 709
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Figure 8. (a) Comparison of different fault structures in a transtensional region and the focal mechanism solutions of the İstanbul earthquakes. The FPSs are
not plotted according to the event locations. PDZ: Principal deformation zone (b) Orientation of the P axes. The plunge angle is 0◦ on the circumference and
90◦ at the centre of the circle. The average compression direction (Paz ) is N110◦ E. The 2015 event (open circle) was excluded from the average.
to the activation of the old fault systems, which represents the pre- 3 B ROA D - B A N D G RO U N D M O T I O N
times of initiations of the NAF in the region. On the other hand, S I M U L AT I O N S F O R A M O D E R AT E
the NNW-SSE left-lateral mechanisms of the 2012, 2014 and most E A RT H Q UA K E I N I S TA N B U L
recent 2021 Kartal earthquakes are likely on R’ shear. The clear
Based on the past and present activity levels in the metropolitan
normal fault solution of the 2008 event and its NW–SE nodal plane
area (Fig. 1, Fig. 5), ground motions of a moderate scenario earth-
agrees with the orientation of a tensional structure.
quake and their effect on weak constructions can be estimated.
The 1923, 1929, 2012 and 2014 earthquake epicentres are in the
For this aim, we conduct broad-band ground motion simulations to
zone of the buried right-lateral West Black Sea Fault between the
generate synthetic seismograms in the metropolitan area, including
Strandja Massif and İstanbul-Zonguldak Palaeozoic (Fig. 2, Fig. 5).
the source, path, and site effects. A hypothetical event source in the
The NW–SE left-lateral nodal planes of the recent two earthquakes
simulation is characterized by faulting parameters and rupture prop-
also agree with the orientation of the fault. These four events occur
agation. The path effect is defined with the appropriate 1-D crustal
in the deformation zone of the palaeo-transform fault that also has
velocity structure of the region utilizing the previous studies. The
the same orientation as anti-Riedel shears (R’). On the Asian side,
amplification of the subsurface soil calculated from the 1-D shallow
the possible fault plane strikes (Table 1) are similar to the faults be-
S-wave velocity structure is also included as the site effect.
tween the Palaeozoic units in the Kartal–Tuzla region. Considering
We use the epicentre locations in Fig. 5 for the hypothetic source
the orientation consistency between the possible Riedel/anti-Riedel
areas (A–E). Two of them are on the Asian side: Kartal (A) and
shears of the current transtensional regime in Marmara and the
Pendik (B). The other three sources are on the European side of the
Palaeozoic faults in İstanbul, we can interpret the earthquakes men-
city: Durusu Lake (C), Esenyurt (D) and Sultangazi (E). Simulating
tioned above may occur on the old fault surface under the current
earthquake ruptures for these five source areas at 11 strong motion
stress loading.
stations (Fig. 5, Table 4) allows us to overview the impact of a
Thus far, we evaluate the neglected seismic activity and seismo-
possible medium-sized earthquake in the metropolitan area.
tectonics in the dense settlement areas in İstanbul by conducting
current transtensional tectonics and Palaeozoic faults. The continu-
ous stress loading due to the westward motion of Anatolia may cause
3.1 Determining 1-D shallow S-wave velocity structures
an earthquake with a higher magnitude than the previous ones in
and site amplifications
the pre-existing weak zones in the study area. Such a moderate
event (Mw ∼5) can damage the old and won multistory buildings Defining a proper subsurface velocity model, which controls soil
in the city. The epicentre, depth and source mechanism of such a amplification, is a key point in obtaining a reliable synthetic wave-
future event in the populated area cannot be estimated. However, the form after deterministic numerical simulation on the bedrock. The
seismological and geological evidence discussed above allows us to 1-D shallow soil velocity structures beneath the strong motion sites
foresee the seismic hazard/risk of the metropolitan area. Apart from in the Istanbul metropolitan area are retrieved from the AFAD
the previous studies on a destructive (Mw ≥ 7) earthquake on the station reports (tadas.afad.gov.tr) based on MASW (Multichannel
NAF beneath the Marmara Sea (i.e. Sorensen et al. 2006; Douglas Analysis of Surface Waves), REMI (Refraction Microtremor) and
& Aochi 2016; Aochi et al. 2017), the effects of a future moderate single station microtremor measurements. The stations’ informa-
earthquake in the İstanbul metropolitan area must be investigated in tion and 1-D shallow soil Vs structures used in this study are given
light of the newly presented data. For this aim, we perform and dis- in Table 4 and Fig. 9, respectively. The MASW observation depths
cuss numerical ground motion simulations of an Mw 5 earthquake are limited in the reports, and there is no velocity information of
utilizing the source and 1-D velocity model properties in the next the soil layers deeper than 20 m. Therefore, the REMI observation
section. results that contain information down to engineering bedrock with710 O. Tan et al.
Table 4. Site information of the AFAD strong-motion stations. See the text for comment details. VS30 values are from AFAD (A ) or nearby measurements in
the İBB reports (I ). HVSReq is the HVSR parameters from earthquake data. hEB : Depth of engineering bedrock. TBEC site classifications: ZA (hard rock):
VS30 > 1500 m s–1 , ZB (rock): VS30 = 760–1500 m s–1 , ZC (dense soil/soft rock): VS30 = 360–760 m s–1 , ZD (stiff soil): VS30 = 180–360 m s–1 . Sedimentary
rocks according to the MTA Geology map: Al, Alluvium; CCl, Continental clastic rocks; CaCl, Carbonate and clastic rocks; ClCa, Clastic and carbonate rocks.
Rock ages: Q, Quaternary; M, Miocene; Ol, Oligocene; C, Carboniferous; D, Devonian; S, Silurian; O, Ordovician.
AFAD
Station Lat. Lon. VS30 site AFAD HVSReq hEB Comments on
code District (◦ N) (◦ E) Geology ( m s–1 ) class f0 /H0 f0 /H0 (m) AFAD site report
Asian Side
3405 Kartal 40.9111 29.1567 CCl (O) 1862A ZC 3.6/1.7 11.2/4.2 - Conflicts in results. Site class should be ZA.
3406 Ümraniye 41.0226 29.1588 ClCa (S) 436A ZC 3.9/3.6 3.0/5.0 31 2nd peak: f0 = 1.5 Hz, H0 = 5.0
3417 Sultanbeyli 40.9547 29.2563 CCl (O) 1747A ZA 7.3/1.8 7.6/2.4 - HVSR flat, H0 < 2
3418 Tuzla 40.8146 29.2755 CaCl (D) 1182A ZB 7.2/1.5 2.8/2.3 - HVSR flat, H0 < 2
3427 Üsküdar 41.0076 29.0671 CaCl (D) 400I ZC N/A 5.8/2.9 25 No report
European Side
3411 Fatih 41.0119 28.9761 CCl (M) 323A ZD 2.1/1.5 6.7/4.0 177 2nd peak: f0 = 7 Hz, H0 = 3.2
3412 Büyükçekmece 41.0206 28.5782 Al (Q) 247A ZD 1.0/4.2 0.6/6.0 51 Clear peak
3413 Eyüp 41.0943 28.9482 CaCl (C) 452A ZC 4.4/1.6 4.4/3.0 30 HVSR flat, H0 < 2
3415 Küçükçekmece 41.0273 28.7585 CCl (Ol) 283A ZD N/A 1.2/4.5 65 No report
3416 Yeşilköy 40.9747 28.8364 CCl (M) 420A ZC 6.4/0.9 0.7/4.6 108 HVSR flat, H0 < 2
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3428 Avcılar 40.9846 28.7296 CCl (M) 227I ZD N/A 0.3/5.3 192 No report
Figure 9. 1-D shallow Vs structures (bottom panels) and their calculated soil amplifications (top panels) at the AFAD strong motion stations in İstanbul. Sites
are grouped according to the soil classification.
Vs of 700–800 m s–1 are preferred. Because there is no site report for previous studies (Birgören et al. 2009; Karabulut & Özel 2018;
stations 3415, 3427 and 3428 in Fig. 5, the soil velocity models for Karagoz et al. 2019).
these three sites are obtained from the nearby microtremor study re- Because the amplification of the subsurface soil layers is hard
sults given in the İBB microzonation reports (İBB 2007, 2009). On to determine, it is assumed that SH wave is incident vertically to
the other hand, there is no engineering bedrock observation at the the horizontal layers on the engineering bedrock, and linear site
sites 3411, 3412, 3415 and 3428 in Fig. 5. Because an approximate amplification for S wave is calculated using Haskell’s (1960) 1-
depth of the engineering bedrock at these sites is needed to calculate D multiple reflections. The amplitude ratio between the waves at
soil amplification, we utilize the empirical relation by Karabulut & the surface and the incident on the engineering bedrock is used as
Özel (2018) for the study area. The engineering bedrock veloc- a soil amplification factor. Kramer (1996) indicates that the inci-
ity is used as 780 m s–1 , considering the AFAD site reports and dent motion amplitude is half of the surface motion amplitude dueNeglected İstanbul earthquakes 711
Figure 10. HVSR analyses at the four strong motion stations presented in the AFAD site reports. Note that the amplitudes at the reported fundamental
frequencies (grey bars) at the three sites are below the ratio of 2 (no amplification). Only 3412 has a clear HVSR peak. Solid and dashed lines are the average
and standard deviation of the HVSR.
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to the free stress condition. Therefore, the amplification factor is show a good correlation with the site amplifications in Fig. 9 (see
calculated as two at low frequencies (Fig. 9, top row). The linear Fig. A2 for goodness-of-fit scores). The stations with site class ZA
site amplification factor for the sites in İstanbul is presumed as the and ZB have no amplification. Only station 3417 has high HVSR
effects of the soil layers over the engineering bedrock. The quality values (∼5) between the frequency 0.2 and 2.0 Hz for the 19 June
factors (Q) of the layers are assumed to be constant at 1/15 of Vs 2021 Kartal earthquake that occurred beneath the site. Also, the es-
(Q = Vs/15) in this study (Iida et al. 2005; Karagoz et al. 2015). timated theoretical amplification of the defined 1-D shallow soil Vs
The rock sites (ZA, ZB) show no amplification at low frequencies model has a good agreement with the HVSR from the earthquakes
(712 O. Tan et al.
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Figure 11. HVSR spectral ratios of at least 10 earthquakes (grey lines) and their averages (black lines) at AFAD strong motion sites in Istanbul. The sites are
grouped according to TBEC site classifications (ZA-ZD).
(Fig. 12). The ASP subfault size related to rupture velocity and we numerically generate the seismograms utilizing the CSM of a
waveform sampling (smax = Vr × t = 2700×0.05 = 135 m) moderate event with Mw 5.0. According to the previous earthquake
ensures the limit of the largest subfault size proposed by Panza self-similarity studies (Wells & Coppersmith 1994; Mai & Beroza
& Suhadolc (1987). To generate a realistic rupture front and in- 2002; Tan & Taymaz 2005), the model event can rupture about 4 × 4
crease the high-frequency content of seismograms, incoherent ran- km2 area in the upper crust. The ASP area is 1.9 × 1.9 km2 at the
dom fluctuation is applied to the theoretical rupture time of each centre of the BG, agreeing with Irikura & Miyake’s (2011) recipe.
subfault (see Karagoz et al. 2018; Karagöz 2022, for details). Each The rupture propagation with random fluctuation of the CSM used
subfault is assumed to be a point source and has the same faulting in the simulations is given in Fig. 12. A circular rupture stars at
parameters (strike, dip and rake). the centre of both areas, and there is no time delay between them.
The velocity seismogram of each subfault on the bedrock is calcu- The different rupture propagation geometries and starting points
lated by the DWNM algorithm developed by Takeo (1985). Because are not tested as in larger earthquake (Mw ≥ 6) simulations because
of the maximum and minimum wave number related to the seismic the CSM has a smaller area. The smoothed-ramp type source time–
velocities in the velocity model, the maximum wavelength criteria function is used for the subfaults. The rise time (tr ) is 2.5 s for BG
by Bouchon (1981) are considered in the simulations. The calcu- to generate low-frequency content around 0.4 Hz and 0.2 s for ASP
lated seismograms of the subfaults are summed according to their to obtain high frequency components up to fmax = 5 Hz.
rupture start times utilizing the point source summation technique
by Spudich & Archuleta (1987) to obtain the overall synthetic ve-
locity time-series. Then, the surface ground motions are calculated
3.2.2 Crustal velocity model for ground motion simulation
using 1-D linear amplifications of the shallow soils in the frequency
domain. The amplitude spectra of bandpass filtered (0.1–10 Hz) The 1-D horizontal-layered crust model from the engineering
bedrock motions are divided by two to remove the free-stress effect bedrock (Vs 0.78 km s–1 ) to Moho is defined using the previous stud-
and multiplied by the site amplifications in the frequency domain. ies in Marmara (Karagoz et al. 2015, 2018, 2019; Karagöz 2022),
The surface motion time-series are obtained with the inverse Fourier and it is used in the numerical ground motion simulations to include
transform in the final stage. the path effects (Table 5). The velocity profile contains moderately
Besides the felt event magnitudes in the İstanbul metropolitan hard rock (Vs 1.4 km s–1 ) and seismic bedrock (Vs 2.2 km s–1 ) in
area ranging from 3.0 to 5.0 (Table 1), there is a possibility of a the uppermost crust. The seismic bedrock depth beneath İstanbul
moderate earthquake at the same locations in the future. Therefore, is between ∼50 and ∼400 m (Birgören et al. 2009). Therefore,Neglected İstanbul earthquakes 713
Figure 12. Characterized source model of a hypothetic source for the Mw 5.0 event used in numerical simulations. Dashed square is the asperity area (ASP)
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in the background area (BG). White star indicates the rupture starting point. Black contours show fluctuating rupture start times with 0.2 s and 0.1 s intervals
for BG and ASP, respectively.
Table 5. Crustal velocity model was used in the ground motion simulations.
Vp Vs Density Depth
(km s–1 ) (km s–1 ) (g cm–3 ) (km) Qp Qs Notes
2.16 0.78 2.1 0.00 300 150 Eng. Bedrock
2.84 1.40 2.2 0.06 400 200 Moderate Hard Rock
3.73 2.20 2.3 0.20 600 300 Seis. Bedrock (Palaeozoic)
5.90 3.40 2.5 4.00 600 300
6.40 3.70 2.7 20.00 800 400
6.75 3.90 2.8 24.00 1000 500
8.00 4.60 3.3 32.00 1000 500 Moho
the common depth of the seismic bedrock is assumed to be 200 m 3.2.3 Estimation of the broad-band ground motions for scenario
which also agrees with the depth of the Istanbul Palaeozoic by Pi- earthquakes
cozzi et al. (2009). The Q values of P and S waves (Qs = Qp/2) are
We construct different faulting parameters at five locations, con-
obtained from previous studies in the Marmara Region (Mindevalli
sidering the results in the previous section, to investigate peak
& Mitchell 1989; Sekiguchi & Iwata 2002; Karagoz et al. 2019).
ground motions and spectral accelerations for an event in the Istan-
The crustal velocity structure in Table 5 and the propagating
bul metropolitan area. The possible future moderate events in the
rupture model are tested for a near-field event before the scenario
previous epicentre areas are assumed, and their source mechanisms
simulations. For this purpose, the latest 2021 Kartal earthquake
are derived from the solutions given in Table 1 and Fig. 5. The five
(ML 4.1) is simulated. The event’s CSM area is 1.5 × 1.5 km2
epicentres (Kartal, Pendik, Durusu Lake, Esenyurt and Sultangazi)
divided into 30 m × 30 m subfaults to obtain 0.01 s-sampling
with approximate coordinates and different faulting mechanisms at
synthetic seismograms. The stress drop of the ASP part is 1.2 MPa,
each location are given in Table 6. Because the event focal depths are
according to the recipe by Irikura & Miyake (2011). Two examples
between 6 and 15 km, the same depth (10 km) is used for all models
of the simulation results are given in Fig. 13 for one weak- and one
(A–E). The fault planes are chosen according to the interpretations
strong-motion station.
based on the right-lateral transtensional stress regime, summarized
The epicentral distance of the ISK week-motion station at KOERI
in Fig. 8. The first source models (#1) in Table 6 are the same as
on Kandilli Hill in the Bosporus is ∼21 km. The synthetics with a
the observed focal mechanisms. The successive nine models (#2–7)
length of 20.48 s are calculated using Bouchon’s DWNM algorithm
represent the uncertainties of the strike, dip and rake angles given
on the seismic bedrock since the site is covered with the early
in Table 1. Models #8 and #9 have similar faulting orientation and
Carboniferous limestone units (Fig. 13a). The S-wave amplitude on
dip angles but different rake angles (±45◦ ). The last models (#10)
the seismic bedrock is greater than P wave because the station is
have a low dip angle (45◦ ). These different source models allow
close to the NNW–SSE nodal plane. The waveform shapes, arrival
obtaining the effect of radiation patterns for S waves.
times and spectra of the observed and synthetic waveforms in the
The 10 scenarios (Mw 5) at each epicentre location are done for
three components are in good agreement. The second example is the
the 11 strong motion sites to sample different source models and
simulation of integrated acceleration records at the AFAD strong-
site classes in the İstanbul metropolitan area. The distance between
motion station, 3406, located 12 km northwest of the epicentre
the source and site ranges from ∼2 to ∼80 km. A total of 1100
(Fig. 13b). The waveforms are bandpass filtered (0.05–2.0 Hz) to
horizontal velocity seismograms (NS, EW) of the 550 simulations
show a low-frequency waveform fit. The synthetics are shifted 1.2 s
are generated. Selected waveforms calculated on the different site
to match the observed P and S wave because of the uncertainty
classes are shown in Fig. 14. The scenario events with a short epicen-
in the velocity model. The results indicate that the 1-D crustal
tral distance ( 1 Hz)
velocity model is utilizable in the scenario simulations for the hazard
on the rock sites (3405, 3418). The seismograms calculated at the
assessment of a future moderate event in the metropolitan area.
stiff soil sites (especially at 3415-Küçükçekmece and 3428-Avcılar)714 O. Tan et al.
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Figure 13. Comparison of observed (obs) and synthetic (syn) velocity waveforms of the 2021 Kartal earthquake at the stations ISK (a) and 3406 (b). The
synthetic waveforms are shifted 1.2 s because of the velocity uncertainty in the model. Time axes are plotted according to the event origin time (12:07). The
location of the stations is shown on the focal mechanism solution.
with larger epicentral distances (>20 km) show low-frequency con- active areas including Turkey (Fig. 15). The GMPEs are calculated
tents (f ≤ 1 Hz). The peak ground velocities (PGV) of the synthetics for an M5 event with oblique faulting considering our source mod-
reach up to 13 cm s–1 . While the waveform durations increase at the els. The site response of the engineering bedrock (Vs = 760 m s–1 )
distant sites (>50 km), the amplitudes decrease due to attenuation. is assumed for a generic relation because different Vs30 values for
Therefore, a moderate event with a moment magnitude of ∼5 may soil classes do not show a remarkable difference in interpretation
strongly affect the structures on the same side of İstanbul as the (Fig. A3). The PGAs from the simulations are in good agreement
epicentre. with the B2014 model for the distance between 5 and 30 km. The
The velocitograms are derived in the frequency domain to obtain values are less than predicted at the rock sites with shorter distances
accelerograms, peak ground accelerations (PGAs) and spectral ac- (Rjb < 5 km). However, the B2014 curve is higher than the simu-
celerations for the scenarios. The maximum simulated PGA value lated PGAs for all site classes at larger distances (>30 km). The
from the synthetic models is about 0.3 g and corresponds to the felt AC2010 model predicts lower PGAs than the B2014 and shows a
intensity of MMI IX (great damage) according to the relationship by better fit for the same distances. Some extremely high values are
Bilal & Askan (2014). The simulated PGAs are compared with the also obtained from the Model-D simulations for the stiff soil sites
ground motion prediction equations (GMPE) to generate an overall 3412 and 3415 (VS30 < 300 m s–1 ) located near Büyükçekmece
image between the prediction curves and a set of different possible and Küçükçekmece lakes, respectively. These high values relate to
event sources. We implement two prediction models developed by the site amplification of stiff soil class (ZD) for these source-site
(1) Akkar & Çağnan (2010, AC2010) derived from the local Turkish pairs. Similar high PGA values above the empirical GMPE are ob-
strong ground motion database and (2) Boore et al. (2014, B2014) served at the stiff soil sites close to the shoreline in the Bornova
using the global strong-motion observations in the seismically Basin, İzmir, during the 30 October 2020 Samos earthquakeNeglected İstanbul earthquakes 715
Table 6. Fault plane parameters (strike/dip/rake in degrees) of the hypothetical source models (#1–10) used in numerical simulations.
Source locations (A–E) are shown in Fig. 5. Epicentre coordinates are also given.
Model S/D/R (◦ ) Model S/D/R (◦ ) Model S/D/R (◦ )
A: Kartal (40.94◦ N 29.25◦ E) B: Pendik (40.85◦ N 29.30◦ E) C: Durusu (41.35◦ N 28.63◦ E)
A1 238/83/135 B1 272/62/−139 C1 160/70/−10
A2 233/83/135 B2 267/62/−139 C2 155/70/−10
A3 243/83/135 B3 277/62/−139 C3 165/70/−10
A4 238/78/135 B4 272/57/−139 C4 160/60/−10
A5 238/88/135 B5 272/67/−139 C5 160/80/−10
A6 238/83/125 B6 272/62/−119 C6 160/70/−20
A7 238/83/145 B7 272/62/−159 C7 160/70/0
A8 240/80/180 B8 270/60/−90 C8 160/70/−45
A9 240/80/−135 B9 270/60/−45 C9 160/70/45
A10 240/45/90 B10 270/45/−90 C10 160/45/0
D: Esenyurt (41.13◦ N 28.68◦ E) E: Sultangazi (41.05◦ N 28.83◦ E)
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D1 335/85/15 E1 135/70/−60
D2 330/85/15 E2 130/70/−60
D3 340/85/15 E3 140/70/−60
D4 335/65/15 E4 135/60/−60
D5 335/80/−15 E5 135/80/−60
D6 335/85/0 E6 135/70/−45
D7 335/85/30 E7 135/70/−75
D8 335/85/−45 E8 135/70/−90
D9 335/85/45 E9 135/70/0
D10 335/45/0 E10 135/45/−45
Figure 14. Examples of simulated velocity waveforms and their spectra. Station name with site class and source model is given at the bottom right of each
seismogram.
(Akinci et al. 2021). The PGA differences among the different example is 3418, the nearest station for source B, and its PGA values
focal mechanisms at a source indicate the radiation pattern effects change from 0.07 to 0.18 g for different faulting types. Depending
on the waveforms. Two clear examples of the rock sites are seen in on the radiation pattern, the PGA value may vary three to four times.
Fig. 15 for the scenario models A and B. The simulated PGAs at It can be concluded that the faulting type is important for short epi-
station 3417 for the Model-A range from 0.02 to 0.07 g. The second central distances in hazard estimation for urban areas. On the otherYou can also read
