Evidence of NW extension of the North Anatolian Fault Zone

Geo-Mar Lett (2002) 21: 183–199
DOI 10.1007/s00367-001-0088-0
Erkan Gökaşan Æ Cem Gazioğlu Æ Bedri Alpar
Zeki Yaşar Yücel Æ Şükrü Ersoy Æ Oğuz Gündoğdu
Cenk Yaltırak Æ Buser Tok
Evidence of NW extension of the North Anatolian Fault Zone
in the Marmara Sea: a new interpretation of the Marmara Sea (I_zmit)
earthquake on 17 August 1999
•
Received: 29 May 2001 / Accepted: 4 December 2001 / Published online: 13 February 2002
Springer-Verlag 2002
Abstract Active faults aligning in NW–SE direction
and forming flower structures of strike-slip faults were
observed in shallow seismic data from the shelf offshore
of Avcılar in the northern Marmara Sea. By following
the parallel drainage pattern and scarps, these faults
were traced as NW–SE-directed lineaments in the
morphology of the northern onshore sector of the
Marmara Sea (eastern Thrace Peninsula). Right-lateral
displacements in two watersheds of drainage and on the
coast of the Marmara Sea and Black Sea are associated
with these lineaments. This right-lateral displacement
along the course of these faults suggests a new, active
strike-slip fault zone located at the NW extension of the
northern boundary fault of the Çınarcık Basin in the
Marmara Sea. This new fault zone is interpreted as
the NW extension of the northern branch of the North
Anatolian Fault Zone (NAFZ), extending from the
Çınarcık Basin of the Marmara Sea to the Black Sea
coast of the Thrace Peninsula, and passing through
Büyükçekmece and Küçükçekmece lagoons. These data
suggest that the rupture of the 17 August 1999 earthE. Gökaşan (&) Æ C. Gazioğlu Æ B. Alpar Æ Z.Y. Yücel
Istanbul University, Institute of Marine Science
and Management, 34470, Vefa, Istanbul, Turkey
E-mail: egokasan@istanbul.edu.tr
Tel.: +90-212-5286022
Fax: +90-212-5268433
Ş. Ersoy
Istanbul University, Faculty of Engineering,
Geological Engineering Department,
34850, Avcılar, Istanbul, Turkey
O. Gündoğdu
Istanbul University, Faculty of Engineering,
Geophysical Engineering Department,
34850, Avcılar, Istanbul, Turkey
C. Yaltırak
Istanbul Technical University Mining Faculty,
Geology Department, 80626 Ayazağa, Istanbul, Turkey
B. Tok
Turkish Navy, Department of Navigation,
Hydrography and Oceanography,
81647 Çubuklu, Istanbul, Turkey
quake in the NAFZ may have extended through
Avcılar. Indeed, Avcılar and I_ zmit, both located on the
Marmara Sea coast along the rupture route, were
strongly struck by the earthquake whereas the settlements between Avcılar and I_ zmit were much less
affected. Therefore, this interpretation can explain the
extraordinary damage in Avcılar, based on the newly
discovered rupture of the NAFZ in the Marmara Sea.
However, this suggestion needs to be confirmed by
further seismological studies.
Introduction
The North Anatolian Fault Zone (NAFZ) is a dextral
strike-slip fault zone, extending from the Karlıova
region to I_ zmit Bay along Anatolia, and south of
Thrace as the Ganos Fault (e.g. Ketin 1948, 1968;
Şengör 1979, 1980; Şengör et al. 1985; Şengör 1987;
Barka and Kadinsky-Cade 1988; Şaroğlu and Yılmaz
1991; Barka 1992; Şaroğlu et al. 1992; Yılmaz et al.
1997; Yaltırak et al. 1998; Fig. 1a). Although it has
been the object of research for over 50 years, its
geometry in the Marmara Sea still controversially
since it was proposed by Pınar (1942) to be an
E-W-directed master fault (Fig. 1a).
Previous studies have generally focused on the development of four basins of the Marmara Sea, using
seismic, seismologic and bathymetric data. Some authors suggest that the NAFZ is a single shear zone
(Pınar 1942; Þengör 1979; Şengör et al. 1985), whereas
others propose that the Marmara Sea is cut by an E-W
trending graben (Ketin 1968; Crampin and Evans 1986;
Smith et al. 1995). The NAFZ has also been considered
as a combination of en-echelon strike-slip faults and
pull-apart basins (Barka and Kadinsky-Cade 1988;
Barka 1992; Wong et al. 1995; Görür et al. 1997; Armijo
et al. 1999).
Since the Marmara (I_ zmit) earthquake of 17
August 1999, the NAFZ has been considered with
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185
b
Fig. 1 a Geographic location of the study area on a map of the
eastern Mediterranean region and on a three-dimensional morphological image of Anatolia (extracted from Algün et al. 1999).
The course of the NAFZ is seen in the morphology. Dashed line
NAFZ route first suggested by Pınar (1942). A Avcılar, Ç-KES
Çatalca-Kocaeli erosional surface, GF Ganos Fault, İ
I_ ğneada, I_ zI_ zmit, KP Kocaeli Peninsula, MS Marmara Sea, sA
Study area, sCA Seismic cruise area, SM Stradnja Mountain, TP
Thrace Peninsula. b Three-dimensional image of the Marmara
region and the NAFZ in the Marmara Sea (Gazioğlu et al.,
unpublished data). The NAFZ separates two branches in the
Çınarcık Basin (ÇB), along the northern and southern slopes (Okay
et al. 2000; Aksu et al. 2000; I_ mren et al. 2001). On the western side,
the Ganos Fault (GF) extends to the east, cutting all basins and
ridges of the western Marmara Trough (MT) offshore of the
Büyükçekmece Lagoon (BL; Okay et al. 2000; I_ mren et al. 2001).
The inset map of the Marmara region shows the sources of the data
for the three-dimensional image. Darkly shaded area shows
topographic data, nonshaded area shows the multibeam bathymetric data, and lightly shaded area shows the earlier available singlebeam bathymetric data. BL Büyükçekmece Lagoon, İB I_ zmit
Bay, KÇ Küçükçekmece Lagoon, l Linear slope of Çınarcık Basin.
c Seismic profiles on three-dimensional image of the southern part
of the area. Bold lines show the selected sections in Figs. 5A–F.
ER Eastern ridge, GH Golden Horn
different models in the Marmara Sea. Some authors
interpreted the NAFZ as a few E-W-directed branches
(Parke et al. 1999; Siyako et al. 2000). However, detailed multichannel seismic and multibeam bathymetric
data indicate that an approximately E-W-directed
strike-slip fault following the Marmara Trough controls the active tectonics of the Marmara Sea (Okay
et al. 1999, 2000; Alpar and Yaltırak 2000; Aksu et al.
2000; I_ mren et al. 2001; Fig. 1b). Seismic, seismologic
and geologic studies carried out in I_ zmit Bay after the
earthquake clearly showed that the rupture of the
earthquake had right-lateral strike-slip motion (Barka
et al. 2000; Lettis et al. 2000; Örgülü and Aktar 2001;
Gökaşan et al. 2001). The aftershocks of the earthquake were traced on the northern and southern
slopes of the Çınarcık Basin, with right-lateral strikeslip movement (Örgülü and Aktar 2001). Okay et al.
(2000), Aksu et al. (2000), and I_ mren et al. (2001)
suggested that the rupture of the NAFZ separates two
branches in the Çınarcık Basin, aligning both northern
and southern slopes of the basin. According to Okay
et al. (2000), the northern branch of this fault extends
to the west, bending from NW to ENE off
Küçükçekmece Lagoon, whereas I_ mren et al. (2001)
proposed this branch dies and is replaced with reverse
faulting off Küçükçekmece Lagoon, the southern
branch possibly merging with the Ganos Fault in the
west.
To date, most of these investigations have assumed
that the NAFZ does not exist in the north of the
Marmara Trough. However, active faults have been
observed on the Istanbul (Trakya) and Kocaeli
peninsulas in the north of the Marmara Sea (Koral
1995; Koral and Şen 1995; Gökaşan et al. 1997; Koral
1998; Demirbağ et al. 1999; Oktay et al. 2002). Also,
seismological records show evidence of microseismic
activity on these peninsulas, particularly around the
Küçükçekmece and Büyükçekmece lagoons (KOERI
2000; Gürbüz et al. 2000; Fig. 2). A major surface
rupture 3 km long, and also a couple of shorter ones
which formed in 1894 along the Avcılar shoreline,
have been reported in historical records (Öztin 1994).
Perincek (1991) interpreted a north-western-trending
inactive strike-slip fault (Thrace Fault) from seismic
sections in central Thrace, and suggested that this
fault zone is a possible strand of the NAFZ in Thrace.
Perincek (1991) also suggested that an erosional
surface marking the end of deformation related to the
particular fault has been folded and faulted. This
author related this deformation on the erosional surface with reactivation of the Thrace fault system or
consequence of the recent formation of the Marmara
Sea basin (Perinçek 1991).
In addition, a significant linear morphology of the
NW-oriented eastern side of the northern slope of the
Çınarcık Basin was replaced by a convex shape off
Küçükçekmece Lagoon (Gazioğlu et al., unpublished
data; Fig. 1b, c). This linear slope aligns with the
NW-SE-directed right-lateral displacement on the eastern coast of the Küçükçekmece Lagoon (Fig. 1b, c).
This direction is also the same as that of the parallel
drainage system of the study area on the eastern Thrace
Peninsula (Figs. 1a–c, 2a).
The Marmara (I_ zmit) earthquake of 17 August
1999 also struck the town Avcılar close to the
Küçükçekmece Lagoon, located 105.5 km from
the main impact area in I_ zmit. Avcılar was badly
destroyed and many people lost their lives in the area
whereas the other settlements between Avcılar and the
centre of the earthquake (I_ zmit) were less affected by
the earthquake.
After the 1999 I_ zmit earthquake, high-resolution
single-channel shallow seismic data were gathered
offshore of Avcılar from aboard the R/V Arar, with
the aim of identifying the reasons for this large-scale
destruction. In order to achieve this aim, we investigated the possible existence of active faults in the
north of the Marmara Trough, as well as any
relationship with the rupture of the NAFZ on the
northern slope of the Çınarcık Basin, using shallowwater seismics in combination with regional bathymetric and topographic maps as well as satellite
imagery.
Materials and methods
Seismic surveys (220 km in all) were conducted in water
depths of 11–300 m in the Marmara Sea on the
northern continental shelf off the Büyükçekmece
and Küçükçekmece lagoons (Fig. 1c). We used a
1.25-kJ multi-electrode spark array and an 11-element,
10-m-long surface-towed hydrophone streamer. The
seismic source had 30 discharging electrodes (6 kV,
30 mF), offsetting about 5 cm. Sampling interval was
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Fig. 2 Seismic activity in the Marmara Sea and surrounding region
between 1900 and 2000 (KOERI 2000)
0.25 ms, recording window length was 250 ms
(two-way-travel time), and shot interval was two (about
4.1 m). These recording parameters provide information
on sedimentary deposits up to 150 m below the seabed.
Positioning was carried out by means of a differential
geographic positioning system (GPS) with an accuracy
of ±20 m.
Three-dimensional surface images were extracted
from Gazioğlu (2001), combining multibeam and earlier
available bathymetric and topographic data. For the
land surfaces, contour lines were digitised on 1:25,000
topographic maps (Fig. 1b) for every 20-m contour line,
using the AutoCAD mapping software for the area.
However, for closer similarities to real morphology, in
certain morphological units (such as hills, rivers, lakes
and coastlines) additional digitising processes were
performed. Digitised lines and points were converted
into shape files by means of the ArcView 3.1 software.
Erdas Imagine 8.3.1 image processing software was
used to generate a digital terrain model (DTM) in the
form of a raster structure. The resulting DTM of the
land area has a cell size of 40 m and a residual mean
square (RMS) of ±13.05 m.
Multibeam data were collected from the Marmara
Sea from aboard the TCG Çubuklu (Fig. 1b). The
single-beam data used for the area were from the multibeam data coverage. ArcView 3.2 was used to merge
these two data sets. Multibeam data were preferred in
overlapping data sets. The resulting DTM has a cell size
of 40 m and an RMS of ca. –18.87 m (Gazioğlu 2001) in
the total area, and an RMS of ca. –15.65 m in the
multibeam data coverage area.
The ERDAS software model was used to merge the
land and sea DTMs, without any interpolation between
bathymetric and topographic models. Generation of an
image with grey level corresponding to elevation allows
visualisation and integration with other data. Image
processing software mapping between elevation and grey
level is linear (minimum value of 0 to maximum value of
255).
DTMs were viewed by means of Image Drape, a
remote sensing and GIS technique which overlays
satellite images over the models. For this purpose, the
Landsat 5 TM images of May 1992 and September 1999
were used (cf. full coverage of study area). The images
were converted into the same projection of UTM
(Universal Transverse Mercator Projection, zone number 35) by using ground control points. Various band
combinations of the Landsat images were used.
187
Results
Morphology of eastern Thrace Peninsula
Background information
Major morphological features on eastern Thrace Peninsula include a mature erosional surface and the southwestern edge of the Strandja Mountains (Figs. 1a, 3a).
The erosional surface has been recognised since the 20th
century, and has been dated as Upper Miocene–Upper
Pliocene (Cvijic 1908; Chaput 1936). This surface has
been cut by rejuvenated streams (Fig. 3a). The largely
parallel drainage patterns of the major valleys developed
on Miocene rocks, and lattices on Paleozoic rocks (Erinç
2000). The Strandja Mountains also present a rejuvenated morphology with linear scarps (Fig. 3a). The
scarps and drainage system generally lie in a NW–SE
direction (Fig. 3a).
Present study
In the present study, numerous lineaments were observed for the parallel pattern drainage, linear slopes,
and right-lateral displacements on the watersheds and
shoreline of the Thrace Peninsula in the study area
(Fig. 3a, b). In this study we focused on 18 NW-SEoriented lineaments (L1–L18) as well as two N–S and
NE–SW lineament groups (L19–L20). Five of these
lineaments (L1–L5), passing through the whole area
continuously and cutting the others, are considered as
major lineaments (Fig. 3b). There are also a number of
shorter NE-SW-aligned lineaments between the NWSE-oriented lineaments which, in turn, are cut by the
NW–SE ones (Fig. 3a–f).
Lineament L1 can be traced from the east of the
Strait of Istanbul to the west of the Terkos Lake.
The NW–SE aligned linear Marmara Sea shoreline on
the Kocaeli Peninsula and the Golden Horn are
considered as the eastern edge of L1 (Fig. 3b). L1
follows the northern scarp of the Derbent Hill, and the
source of the Alibey Stream. It passes through the Terkos
Lake and combines with lineament L2 on the Black Sea
coast of the Thrace Peninsula (Fig. 3b–f).
The NW–SE central axis of the Küçükçekmece
Lagoon and the 3.5-km-long right-lateral displacement
on the Marmara Sea coast of the lagoon are taken as
indicators of lineament L2 (Fig. 3a–d). L2 extends
northwards following the NW–SE valley of the
Koğukdere Stream. This lineament passes through a
right-lateral displacement in the watershed between the
Koğukdere and Sazlıkçayırdere streams (Fig. 3a, b, d),
and follows the western side of this watershed (eastern
side of Sazlıkçayırdere Stream) to the north (Fig. 3a–d).
Scarps along the north-eastern edge of the Strandja
Mountains constitute the northern extension of L2
(Fig. 3a, b, d, e). In the northern sector of these scarps,
L1 joins L2 on the Black Sea coast. L2 passes through a
right-lateral displacement on the Black Sea shoreline
(Kıyıköy Peninsula), and it extends along the southern
edge of I_ ğneada Bay to the NW corner of the study area
(Fig. 3a, b, d, e).
A slight right-lateral displacement on the Marmara
Sea shoreline delineates the southernmost extension of
lineament L3 (Fig. 3a, b). The NW–SE Haramidere
stream valley and the eastern side of Büyükçekmece
Basin indicate the course of lineament L3 (Fig. 3a–e).
This lineament passes through a right-lateral displacement in another watershed between the Haramidere and
Sazlıkçayırdere streams (Fig. 3a, b, d). The northern
extension of L3 is revealed by south-west-facing scarps
of the Strandja Mountains.
Lineaments L4 and L5 are indicated by the northand south-facing scarps of the Çatalca Rise (Fig. 3a–
d). The south-western side of the Büyükçekmece Basin
and the Tepecik stream valley follow the alignment of
L4.
Lineaments L6 and L7 parallel lineament L1 in the
north of the study area (Fig. 3a, b, d–f). L6, and are
situated north of L1, starting from the southern entrance
of the Strait of Istanbul and following the Alibey stream
valley and the northern shoreline of Terkos Lake to
eventually pass through the right-lateral displacement
on the Black Sea coast of Thrace Peninsula. L7 extends
from the Kocaeli Peninsula in the east from where it
crosses the Bosphorus, passes through the Kağıthane
stream valley to cut the Black Sea coast north-east of
Terkos Lake (Fig. 3b, f).
Lineament L8 follows the north-eastern slope of the
Strandja Mountains until it merges with L2 (Fig. 3a, b,
d, e). Lineament L9 is aligned with the Sazlıdere stream
valley (Fig. 3a–f). Lineament L10 is identified as a
right-lateral displacement along the northern shoreline
of I_ ğneada Bay (Fig. 3a, b, d, e). Lineaments L18
represent the shorter NW-SE-oriented lineaments in
Fig. 3a–f.
The linear NW-SE-aligned northern slope of the
Çınarcık Basin constitutes the offshore member of the
lineaments (labelled as L in Fig. 1b, c). This lineament, located between the Marmara entrance of I_ zmit
Bay and the south-eastern shore of Küçükçekmece
Lagoon, forms a NW-SE-oriented linear offshore
slope. To the south and south-west of Küçükçekmece
Lagoon, this linear slope curves in an E–W direction
(Fig. 1b, c).
Seismic profiles on the northern shelf
of the Marmara Sea
Seismic data from the shelf area off the Küçükçekmece
and Büyükçekmece lagoons (Fig. 1c) were interpreted
stratigraphically and structurally by tracing the submarine continuation of the lineaments onshore.
188
189
b
Fig. 3A–F Lineaments and some morphological features of the
study area. A Simplified drainage and scarps on the morphology.
B Lineaments simulating parallel pattern streams, scarps, and
displacements on the shoreline. C A three-dimensional image
and lineaments of the Büyükçekmece and Küçükçekmece lagoons
and the Marmara Sea coast from the north. T Terkos Lake. D A
three-dimensional image and lineaments of the Terkos Lake and
Black Sea coast from the south-east. The three-dimensional image
in the inset shows right-lateral displacements in the watersheds
between the Sazlıkçayırdere, Haramidere, and Koğukdere streams.
E A three-dimensional image and lineaments of the Terkos Lake
and Black Sea coast from the east. F A three-dimensional image
and lineaments of the Strait of Istanbul and Black Sea coast from
the south-east. SoI_ Strait of Istanbul (Bosphorus)
Seismic stratigraphy
Three sediment sequences and a truncation are distinguished using the reflection configuration of the sediment deposits. These sequences are named lower unit,
upper unit, and deltaic sequence (Figs. 4, 5a–f).
Although the reflection configuration of the lower
unit indicates a characteristic deformation, its initial
internal parallel reflectors are observed on the seismic
profiles. An angular unconformity marks the top of the
lower unit, indicating an erosional period after deposition. This surface has also been reported as developed
during the Würm glacial age from neighbouring areas
(Demirbağ et al. 1999; Oktay et al. 2002). The parallel
reflection configuration of the lower unit indicates initial
deposition under low-energy conditions. However, along
some narrow zones the lower unit is much more deformed than the parts on the seismic profiles called the
high-deformation zone in the lower unit (Figs. 4 and
5a–f). In addition, some gas charges veiling the strata of
the lower unit on seismic profiles occasionally occur in
these zones (Fig. 5 c, d).
Onshore deposits comprise Upper Miocene and older
sediments covering the coast and hinterland of the study
area (Fig. 6; Arıç 1955). The Upper Miocene sequence
consists of unconsolidated gravel and sand, the base
having a variable thickness, and passes through claymarl-mudstones and muddy limestone towards the top
(Arıç 1955). The lower unit is interpreted as a submarine
extension of the Upper Miocene and older sediments.
A deltaic sequence with an oblique sigmoidal reflection configuration is observed between the upper and
lower units on the truncation surface of the Marmara
entrance of the Strait of Istanbul (Fig. 5a). This sequence was interpreted by Oktay et al. (2002) as delta
deposits which originated during the Würm glacial period. An isopach map of this sequence is shown in
Fig. 7c. The evidence indicates that the delta sequence
was developed in the Bosphorus channel during the
Würm glacial age before deposition of the upper unit,
whereas the northern shelf of the Marmara Sea was
eroded (Oktay et al. 2002).
The upper unit is exposed on the truncation surface
on the northern shelf of the Marmara Sea (Figs. 4, 5a–f).
It is characterised by an internal parallel reflection
configuration, implying low-energy depositional conditions for this unit. A truncation surface is not observed
in the internal reflector. The top of this sequence forms
the modern bed, suggesting that this upper unit is still
actively accreting today. Deposition should thus have
commenced in the course of the Flandrian transgression.
The seismic profiles reveal that the upper unit is deformed together with the truncation surface and seafloor
in the highly deformed, lower sequence zones, the
thickness of the upper unit changing in these zones.
Structural interpretation
On the seismic profiles, the layers of the lower unit,
which is the submarine extension of the Upper Miocene
and older deposits, are observed as being gently deformed and truncated by the erosional surface during
the Würm glacial age (Figs. 4, 5a–f). This gentle deformation of the lower unit is not active today. However,
the lower unit is highly deformed along some narrow
zones, together with the truncation surface, upper unit
and seafloor (Figs. 4, 5a–f). These highly deformed
zones are frequently associated with gas charging.
Therefore, any faults in the deformed zone of the lower
unit cutting or affecting the truncation surface, the upper
sediment layers, and the seafloor may be interpreted as
being active faults. These active faults are identified
as fault groups on seismic profiles, and are interpreted as
active fault zones. Some of them extend in a NW–SE
direction, five faults being interpreted as major active
fault zones named faults F1, F2, F3, F4, and F5.
Faults are observed on the seismic profiles from the
eastern edge of the study area, close to the Marmara Sea
entrance of the Strait of Istanbul (Bosphorus; Figs. 5a,
b, 7a). These faults cut layers of the upper unit, truncation surface and seafloor, and together indicate the
formation of an active fault zone (Fig. 5a, b). On the
other hand, small steps in the seafloor and uplifting
along this zone are indicators of a strike-slip fault.
Right-lateral displacement of the shoreline along the
route of these faults supports this interpretation
(Fig. 7a, b). This displacement also indicates the course
of lineament L17, and thus lineament L17 should be an
onshore extension of these faults (Figs. 7a–d, 8a). An
isopach map of the delta sequence, and a palaeotopographic map of the lower sequence (Oktay et al. 2002) at
the Marmara Sea entrance of the Bosphorus indicate
right-lateral displacements along these faults (Fig. 7c,
d). At the same time, in the hinterland extension of this
fault zone, the Ayamama stream valley (L1; Fig. 3a, b)
is observed to be aligned in the same direction as the
active faults (Meriç et al. 1991). This suggests that the
faults form a NE-SW-directed active strike-slip fault
zone (fault F1).
Other NE-SW-aligned faults (fault 2), which affect
all units in the study area, occupy the area extending
from south of the Küçükçekmece Lagoon through to
the south-eastern edge of the study area (Figs. 5b, c,
190
Fig. 4A, B Detailed parts of
the seismic sections. Active
faults are indicated by highly
deformed lower sequence zones,
thickness changing and deformation at the upper sequence,
as well as deformation on the
truncation surface and seafloor.
A Section of Fig. 5B. B Section
of Fig. 5E
7a, b). These active faults cut the seafloor without any
dip-slip component. The Küçükçekmece Lagoon and
lineament L2 are located on the hinterland extension of
these faults, along which some microseismic activity
has been recorded (KOERI 2000; Figs. 2, 3b, 7a, b,
8a). Also, the shoreline is laterally displaced 3.5 km
right through the south-eastern corner of the
Küçükçekmece Lagoon (Fig. 3a). The contours of delta
deposits and the palaeotopography of the lower
sequence in the Bosphorus are right-laterally displaced
along fault F2 (Fig. 7c, d).
A third fault group has been traced to extend in a
NW–SE direction (fault F3; Figs. 5b, c, 7a, b). A slight
dip-slip movement is observed on the irregular seafloor
(Fig. 5c). The shoreline is right-laterally displaced as
well as the Haramidere stream valley (Fig. 3a), which
191
Fig. 5a–f Interpreted and uninterpreted seismic sections. A detailed explanation of the active faults which cut the seafloor is given
in the text
indicates lineament L3 developed along the course of
fault F3 (Figs. 3b, 7a, b, 8a). This indicates that fault F3
is a major, active strike-slip fault zone linked to fault F2
offshore of Avcılar (Fig. 7a, b).
The faults are interpreted to form a NW-SE-directed
fault zone both inside and offshore of the Büyükçekmece
Lagoon (fault F4; Figs. 5d, f, 7a, b). Although a narrow
depression is observed on the truncation surface, the
faults cut the seafloor without any dip-slip component
being visible on the seismic profiles from within and
south of the Büyükçekmece Lagoon (Fig. 5f). Microseismic activity observed in the Büyükçekmece Lagoon
(KOERI 2000; Fig. 2) implies that these areas are
seismically active. A NW-SE-directed fault (Çatalca
Fault), which can be traced from the inland morphology
as lineament L4 (Fig. 3a, b), has been reported from the
north-west of Büyükçekmece Lagoon (Koral 1995, 1998;
Figs. 3a, b, 7a, b, 8b).
The last group of faults, which forms the major fault
zone, is observed along the south-western edge of the
study area (fault F5; Figs. 5d–f, 7a, b). These faults cut
not only the truncation surface but also the seafloor. The
thickness of the upper unit is affected by this fault group
along its course (Fig. 5c, e, f). The linear shoreline along
the west coast of Büyükçekmece Lagoon and lineament L5 may be interpreted as the hinterland extension
of these faults (Figs. 3b, 7a, b, 8b) being associated with
the active NW-SE-directed strike-slip fault zone.
The other active faults affecting all of the stratigraphic units seem to have a strike-slip character due to
192
Fig. 5 (Contd.)
193
Fig. 5 (Contd.)
their high angularity with respect to the seafloor, their
dip-slip components slightly affecting the strata (Figs. 5a–f, 7a). These fault groups are numbered as F6, F8,
F12, F13, F14, F15, and F16.
Faults F12, F13 and F15 are bounded by the major
faults F4 and F5 (Figs. 5d, e, 7a, b). Fault F6, aligned
NW–SE, is located on the western side of fault F5 and
connects to this fault in the SE, i.e. offshore of
Büyükçekmece Lagoon. Faults F9 and F10 were
originally observed by Oktay et al. (2002) and are
194
Fig. 6 Simplified lithological
map of the terrestrial part of
the study area
recognised in our seismic profile as dip-slip faults
(Fig. 5a). The isopach maps of the Marmara entrance
of the Bosphorus indicate that fault F10 consists of at
least seven normal subfaults which bound the Bosphorus channel (Fig. 7c, d). Fault F11 is interpreted as
a strike-slip fault within a dip-slip component between
faults F1 and F2 (Figs. 5b, 7a–d). Faults F7 and F17
are interpreted as dip-slip faults. With their synthetic
faults they form the westernmost part of the northern
slope of the Çınarcık Basin (Figs. 1b, 5c–e, 7a, b). The
present shape of the slope is not only controlled and
developed by the dip-slip faults (such as faults F7
and F17) but also by submarine landslides which are
probably triggered by normal faulting (Fig. 5c, e).
Fault F18 is interpreted as a strike-slip fault with a dipslip component along the north-eastern edge of the
study area (Fig. 5a). Right-lateral displacements on the
shoreline and isopach maps of the Bosphorus entrance
are considered as evidence for the right-lateral activity
of this fault (Fig. 7a–d).
Discussion and conclusions
Lineaments controlling the present morphology of the
terrestrial portion of the study area should be younger
than the Upper Miocene–Upper Pliocene mature erosional surface. The NW-SE-directed lineaments are
more continuous and cut or bound the NE-SW-directed
lineaments, indicating that the major lineament direction
is aligned NW–SE in the study area. Some of the NWSE-aligned faults, along the route of lineaments L2,
L10, and L13 in I_ ğneada Bay (Turoğlu 1995) and offshore of Küçükçekmece Lagoon (Yaltırak and Alpar
2000), L4 in Çatalca (Koral 1995, 1998), and L17 in the
Ayamama Stream (Meriç et al. 1991), have been reported using seismic, geological and geomorphological
studies. NW-SE-directed right-lateral displacements occur along the southern and northern coastlines of the
Thrace Peninsula, e.g. the entrances of Küçükçekmece
Lagoon, Ayamama and Haramidere streams and along
the Black Sea shoreline around Kıyıköy and I_ ğneada
along lineaments L2, L3, L5, L6, L10 and L17. The
watersheds between the Koğukdere, Haramidere, and
Sazlıkçayırdere rivers are also right-laterally displaced,
coinciding to the NW-SE-directed lineaments L2 and
L3. All this evidence on morphology is thought to be
expressed in the NW–SE-oriented right-lateral strike-slip
faults on the eastern side of Thrace Peninsula.
Faults on the seismic profiles are interpreted as active
right-lateral strike-slip faults, due to their flower-like
expression, right-lateral displacements in delta sediments, the basement palaeosurface in the Strait of
Istanbul (Oktay et al. 2002), and the coast of the Thrace
Peninsula. These faults can be traced from one seismic
profile to another in a NW–SE direction.
Most of the major lineaments are observed along the
route of the onshore extensions of the faults. They can
thus be connected to each other and are interpreted as
active faults (Figs. 8, 9). Some NW-SE-directed, rightlateral active strike-slip faults observed on the northern
shelf of the Marmara Sea are therefore extended to the
onshore area between Avcılar and I_ ğneada on the eastern Thrace Peninsula (Figs. 9, 10).
A block rotation model has been suggested to explain
the tectonic evolution of the Strait of Istanbul (Oktay
et al. 2002). This model may also be applicable to the
study area. According to this model, the NW–SE main
fault zones may be considered to represent the zonebounding faults identified by Peacock et al. (1998), and
the NE-SW-directed shorter ones might be the traces of
block-bounding faults observed in the morphology and
on seismic sections.
Okay et al. (2000), Aksu et al. (2000), and I_ mren et al.
(2001) identified that the northern segment of the
NAFZ separates two branches in Çınarcık Basin,
following the northern and southern slopes of the basin.
These authors also suggested that the Ganos Fault in
the western onshore region of the Marmara Sea extends
to the east in an ENE–WSW direction to the offshore
195
Fig. 7A–D Active fault map of coastal and offshore portion of the
study area. A Three-dimensional view of faults with some
interpreted sections. B Two-dimensional view of the active faults
on the northern shelf of the Marmara Sea. C Isopach map of the
deltaic sequence in the Marmara Sea entrance of the Bosphorus,
and active faults which are correlated with the right-lateral
displacements on contours and variations in thickness values of
this sequence (Oktay et al. 2002). Faults 1, 2, 11, and 18 align with
these variations. D Palaeotopographic map of Upper Miocene and
older sediments in the Marmara Sea entrance of the Bosphorus,
and active faults which are indicated by right-lateral displacements
on contours and variations in depth values of the contours (Oktay
et al. 2002). Faults 1, 2, 11, and 18 align with these variations
region of Küçükçekmece and Büyükçekmece lagoons
(Fig. 10). Recent seismological studies focused on the
Marmara Sea support these suggestions. A dominance
of strike-slip motions is observed along both northern
and southern slopes of the Çınarcık Basin, resulting
from the focus of aftershocks of the 17 August 1999
I_ zmit earthquake (Örgülü and Aktar 2001), and ENE–
WSW-oriented microseismic activity aligning the
suggested offshore extension of the Ganos Fault was
observed by Gürbüz et al. (2000). Okay et al. (2000)
suggested that the northern branch bends off
Küçükçekmece Lagoon and merges with the Ganos
Fault. Thus, this merged branch consists of the major
segment of the NAFZ in the Marmara Sea, whereas the
southern branch dies at the western edge of the Çınarcık
Basin. However, I_ mren et al. (2001) proposed that the
northern branch dies off Küçükçekmece Lagoon,
whereas the southern one merges with the Ganos Fault
and forms the major segment of the NAFZ in the
Marmara Sea.
196
Fig. 8A, B Three-dimensional combined images with seismic
profiles and morphologic images. Lineaments L2, L3, L4, L5,
and L17 align with the faults F1, F2, F3, F4, and F5 as their
onshore extension along the eastern Thrace Peninsula. A Combined
image for Fig. 5b and morphologic image around Küçükçekmece
Lagoon. B Combined image for Fig. 6b and morphologic image
around Büyükçekmece Lagoon
We contend that, instead of dying or bending offshore of Küçükçekmece Lagoon, the northern branch
extends in a NW direction through the steps of the
northern slope whereas the linear slope becomes curved,
and forms the faults (F1–F18) on the northern shelf and
the lineaments (L1–L20) on the Thrace Peninsula
(Figs. 9, 10). In this case, the southern branch may
merge the Ganos Fault and form the major segment of
197
Fig. 9 Three-dimensional
image showing the combination of faults observed on the
shelf with lineaments on
terrestrial morphology and the
northern branch of the NAFZ
in the Çınarcık Basin of the
Marmara Sea controlling the
linear northern slope of the
Çınarcık Basin. D Dip-slip
movement caused by
submarine landslides and
normal faults
Fig. 10 The NW extension of the North Anatolian Fault Zone
between Avcılar and I_ ğneada. This fault is modelled as the NW
(eastern Thrace) extension of the northern branch of the NAFZ in
the Çınarcık Basin, considering the active faults on the northern
shelf of the Marmara Sea, lineaments on the eastern Thrace
Peninsula, and right-lateral displaced shoreline of the eastern
Thrace Peninsula (base map extracted from Taymaz et al. 2001).
Epicentres of microseismic activity on eastern Thrace Peninsula
support the existence of the faults. The southern branch of the
NAFZ in the Çınarcık Basin is presumed to merge with the Ganos
Fault (dashed line), and they consist of the major branch of the
NAFZ in the Marmara Sea and extend westwards into the northern
Aegean Sea. Thin dashed lines show the other possible active faults
due to morphology and microseismic activity
the NAFZ in the Marmara Sea, as suggested by I_ mren
et al. (2001).
In the light of this new interpretation, we propose a
different explanation of the Marmara (I_ zmit) earthquake
on 17 August 1999. Aftershock locations of the earthquake show that the rupture may have extended at least
onto the northern slope of the Çınarcık Basin (Örgülü
and Aktar 2001). Avcılar, which is located 105.5 km
away from the earthquake centre (I_ zmit), was also seriously affected by the event, whereas other settlements
between I_ zmit and Avcılar were much less affected.
198
According to historical records (Öztin 1994), some surface ruptures occurred during the Marmara earthquake
of 10 July 1894, which is thought to have originated
around I_ zmit. This indicates that Avcılar has also been
affected by earthquakes in the past. Avcılar is located
along the NW continuation of the northern boundary
fault of the Çınarcık Basin, which contained the aftershocks of the earthquake on 17 August 1999 (Örgülü
and Aktar 2001). This phenomenon can be explained by
extending the rupture of this earthquake through to
Avcılar. The other settlements were not damaged during
the earthquake, because the rupture between I_ zmit and
Avcılar is located in the Marmara Sea. We therefore
suggest that the rupture of the 17th August 1999
earthquake was not only activated between Düzce and
Hersek (Barka et al. 2000; Lettis et al. 2000) but also
along the fault section between Hersek and Avcılar. On
the basis of this interpretation, we should call the 17
August 1999 earthquake the Düzce-Avcılar earthquake.
The serious damage limited to the Avcılar region indicates that this supposed rupture on 17 August 1999
ended in Avcılar. Although microseismic activity aligns
with some ruptures of this newly suggested fault in the
eastern Thrace Peninsula (Figs. 2, 10; KOERI 2000;
Gürbüz et al. 2000 ; Taymaz et al. 2001), and the first
results of the Ocean Bottom Seismogram (OBS) data
express microseismic activity on the Thrace and Kocaeli
peninsulas after the earthquake on 17 August 1999
(Taymaz et al. 2001), the suggestion of this study needs
to be confirmed by further seismological studies.
If these arguments and interpretations are correct,
then the fault zone between Avcılar and I_ ğneada should
be considered as the Thrace extension of the northern
boundary fault of the NAFZ in the Çınarcık Basin, and
that the Düzce-Avcılar portion of this fault released its
stress during the 17 August 1999 earthquake. The area
between Avcılar and I_ ğneada should therefore be considered as a new hazard zone for future earthquakes in
the Marmara region, including the western part of the
Marmara Sea.
Acknowledgement We wish to express our sincere thanks to the
officers and crew of the research vessel Arar of the University of
Istanbul, and the TCG-Çubuklu of the Turkish Navy, Department
of Navigation, Hydrography and Oceanography for their guidance
and assistance in seismic and bathymetric data acquisition. We are
grateful to F.Y. Oktay and Y. Yılmaz for their valuable suggestions. We also thank O. Algan, Ö. Huvaz, T. Ustaömer, S. Gözenç,
C. Güneysu, E. Doğan, T. Taymaz, and O. Tan for their valuable
suggestions and for providing the topographic and seismologic
maps. This work was supported by the Research Fund of the
University of Istanbul. Project number YP-91/12122001.
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