3D crustal velocity and Vp/Vs structures beneath Southeast Anatolia and their geodynamic implications
We applied a seismic tomography method to arrival time data generated by local crustal earthquakes in Southeast Anatolia to study the shallow, three-dimensional, velocity and VP/VS structures beneath the area.
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irk-123456789-1585042019-09-05T01:25:42Z 3D crustal velocity and Vp/Vs structures beneath Southeast Anatolia and their geodynamic implications Salah, M.K. Şahin, Ş. We applied a seismic tomography method to arrival time data generated by local crustal earthquakes in Southeast Anatolia to study the shallow, three-dimensional, velocity and VP/VS structures beneath the area. Применен метод сейсмической томографии к данным по времени вступления продольных и поперечных волн от локальных коревых землетрясений в Юго-Восточной Анатолии для изучения мелких трехмерных скоростных структур и соотношение VP / VS под этим регионом. Застосовано метод сейсмічної томографії до даних щодо часу вступу поздовжніх і поперечних хвиль від локальних корових землетрусів у Південно-Східній Анатолії для вивчення дрібних тривимірних швидкісних структур і співвідношення VP/VS під цим регіоном. 2019 Article 3D crustal velocity and Vp/Vs structures beneath Southeast Anatolia and their geodynamic implications / M.K. Salah, Ş. Şahin // Геофизический журнал. — 2019. — Т. 41, № 2. — С. 122-140. — Бібліогр.: 82 назв. — англ. DOI: 10.24028/gzh.0203-3100.v41i2.2019.164460 0203-3100 http://dspace.nbuv.gov.ua/handle/123456789/158504 en Геофизический журнал Інститут геофізики ім. С.I. Субботіна НАН України |
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We applied a seismic tomography method to arrival time data generated by local crustal earthquakes in Southeast Anatolia to study the shallow, three-dimensional, velocity and VP/VS structures beneath the area. |
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Salah, M.K. Şahin, Ş. |
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Salah, M.K. Şahin, Ş. 3D crustal velocity and Vp/Vs structures beneath Southeast Anatolia and their geodynamic implications Геофизический журнал |
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Salah, M.K. Şahin, Ş. |
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Salah, M.K. |
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3D crustal velocity and Vp/Vs structures beneath Southeast Anatolia and their geodynamic implications |
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3D crustal velocity and Vp/Vs structures beneath Southeast Anatolia and their geodynamic implications |
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3D crustal velocity and Vp/Vs structures beneath Southeast Anatolia and their geodynamic implications |
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3D crustal velocity and Vp/Vs structures beneath Southeast Anatolia and their geodynamic implications |
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3D crustal velocity and Vp/Vs structures beneath Southeast Anatolia and their geodynamic implications |
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3d crustal velocity and vp/vs structures beneath southeast anatolia and their geodynamic implications |
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Інститут геофізики ім. С.I. Субботіна НАН України |
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3D crustal velocity and Vp/Vs structures beneath Southeast Anatolia and their geodynamic implications / M.K. Salah, Ş. Şahin // Геофизический журнал. — 2019. — Т. 41, № 2. — С. 122-140. — Бібліогр.: 82 назв. — англ. |
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Геофизический журнал |
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AT salahmk 3dcrustalvelocityandvpvsstructuresbeneathsoutheastanatoliaandtheirgeodynamicimplications AT sahins 3dcrustalvelocityandvpvsstructuresbeneathsoutheastanatoliaandtheirgeodynamicimplications |
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2025-07-14T11:05:13Z |
| last_indexed |
2025-07-14T11:05:13Z |
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1837620122167017472 |
| fulltext |
MOHAMED K. SALAH, ŞAKIR ŞAHIN
122 Геофизический журнал № 2, Т. 41, 2019
Introduction. Southern Turkey, a part of
the long Alpine-Himalayan Orogenic Belt
(AHOB), suffers from ongoing differential im-
pingement of Arabia and Africa into the weak
Anatolian collisional complex which results
from the subduction of the Neotethyan Ocean
along the Cyprus Arc [e.g., Reilinger and Mc-
Clusky, 2011]. This coupling has produced
one of the most complex crustal interactions
along the AHOB. Several major transform
faults with distinctive motions, including the
northward extension of the Dead Sea Fault
DOI: 10.24028/gzh.0203-3100.v41i2.2019.164460
3D crustal velocity and VP /VS structures beneath
Southeast Anatolia and their geodynamic implications
M. K. Salah1, Ş. Şahin2, 2019
1Department of Geology, American University of Beirut, Beirut, Lebanon
2Department of Geophysics, Suleyman Demirel University, Isparta, Turkey
Received 14 November 2018
Застосовано метод сейсмічної томографії до даних щодо часу вступу поздо-
вжніх і поперечних хвиль від локальних корових землетрусів у Південно-Східній
Анатолії для вивчення дрібних тривимірних швидкісних структур і співвідношен-
ня VP/VS під цим регіоном. Багато попередніх сейсмологічних досліджень регіону
виконано на регіональному або навіть глобальному рівні. У цілому з 2150 ретель-
но відібраних подій, що генерують 13690 і 12560 часів вступу P- і S-хвиль, було ви-
користано в томографічній інверсії. За результатами тестування на розв’язання
«шахової» задачі припускають, що отримані аномалії швидкості та співвідношення
VP/VS відображують особливості будови розрізу. Великі поперечні неодноріднос-
ті кори у вигляді аномалій швидкості нижче середньої виявлено під Південно-
Східною Анатолією. Слабкі аномалії швидкості відображаються поблизу сег-
ментів з активними порушеннями. Крім того, високі співвідношення VP/VS
закартовано поблизу основних поділів земної кори, особливо на глибинах 10 і 22 км,
що узгоджується з розміщенням офіолітових поясів. Зони високих співвідношень
VP/VS викликані можливим існуванням у корі та, можливо, у верхній частині мантії
флюїдів, що зазнають до надвисокого тиску. Поява цих флюїдів за інтенсивної текто-
нічної активності може стати пусковим механізмом для великих корових землетру-
сів уздовж західного сегмента Східноанатолійськой зони порушень. Це землетруси
можливі в зонах високих швидкостей, однак більшість великих корових землетрусів
розподіляються поблизу зон середніх аномалій швидкості/високих аномалій VP/VS.
Подібні зони швидкості й співвідношення VP/VS, нанесені на карту, відповідають
даним багатьох попередніх геофізичних досліджень під Південно-Східною Анато-
лією, зокрема, низьким швидкостям Pn- і Sn-хвиль, високою швидкості загасання,
Sn-хвиль високому тепловому потоку тощо. Результати, отримані під цим регіоном
Анатолійського плато, також подібні до даних щодо інших континентальних плато,
таких як Тибет, і вказують на гарячу, частково розплавлену верхню мантію.
Ключові слова: Південно-Східна Анатолія, структура кори, сейсмічна томографія,
структура сейсмічної швидкості, співвідношення VP/VS.
zone (DSF), meet in this region [Tatar et al.,
2004]. Southeast (SE) Anatolia, the target
area of this study, is situated at the junction
between the Arabian plate to the southeast,
the African plate to the southwest, and the
northerly-located Anatolian plate (Fig. 1, a).
It contains a number of tectono-magmatic/
stratigraphic units as, for example, the meta-
morphic massifs, the volcanic arc units, gran-
itoid rocks, and the ophiolites [Karaoğlan et
al., 2013]. In addition, intrusive and extrusive
volcanic igneous (and associated sedimenta-
3D CRUSTAL VELOCITY AND VP/VS STRUCTURES BENEATH SOUTHEAST ANATOLIA ...
Геофизический журнал № 2, Т. 41, 2019 123
Fig. 1. Tectonic elements at the northeast Mediterranean comprising the Anatolian, Eurasian, and Arabian plates
with their complex interactions (from [Schmincke et al., 2014]) (a). Tectonic elements partly after [Taymaz et al.,
2007]. NAF ― North Anatolian Fault; EAF ― East Anatolian Fault; DSF ― Dead Sea Fault Zone. The yellow ar-
rows point to the direction of the motion of the Arabian and Anatolian plates. Seismicity of the Eastern Mediter-
ranean, Cyprus and the Anatolian plate from the U. S. Geological Survey catalogs (http://earthquake.usgs.gov/
earthquakes/search/1976-2014) (b). Circles vary in size according to magnitude and in color according to focal
depth. The black rectangle shows the present study area.
MOHAMED K. SALAH, ŞAKIR ŞAHIN
124 Геофизический журнал № 2, Т. 41, 2019
ry) rocks are reported in SE Anatolia [Robert-
son et al., 2006, 2007]. Continental collision
along the Bitlis-Zagros suture zone began in
the early Miocene [Dewey et al., 1973; Dewey,
Şengör, 1979] and lead to the westward ex-
trusion of the Anatolian block between two
prominent strike slip fault zones [Rotstein,
Kafka, 1982], the left-lateral East Anatolian
fault zone (EAFZ), and the right-lateral North
Anatolian fault zone (NAFZ). The NAFZ is a
prominent tectonic feature in northern Anato-
lia with a confirmed history of seismicity [Öz-
türk, 2011]. The EAFZ is almost 550 km long,
approximately northeast-trending, sinistral
strike-slip fault zone comprising a group
of faults arranged parallel, subparallel, or
even obliquely to the general trend [Öztürk,
Bayrak, 2012]. These two fault zones form
parts of the boundary between the Anatolian
and the Eurasian plates, and between the Ara-
bian and African plates [Westaway, 1994]. The
eastern Turkey crust and part of the Lesser
Caucasus is hot and weak, and composed of
crustal blocks that are in relative motion to
one another [Al-Damegh et al., 2005]. The
EAFZ joins the NAFZ at the Karliova triple
junction north of the Arabian plate, and marks
the boundary between Arabia and Anatolia
[Nocquet, 2012].
While rotating anticlockwise, the Anato-
lian plate is also affected by approximately
N-S and NNE-SSW shortening induced by
the collision between the African and Ana-
tolian plates along the Cyprean arc [Şengör,
1979; Rotstein, 1984; Şengör et al., 1985; Tatar
et al., 1995; Bozkurt, Koçyiğit, 1996; Barka,
Reilinger, 1997; Reilinger et al., 1997; Tan et
al., 2014]. The seismic activity is widely dis-
tributed in most parts of the northeast Medi-
terranean; but highly intense along the active
segments of the NAFZ, EAFZ, the Cyprian
Arc, and western Anatolia (Fig. 1, b), with
often many moderate and frequent major
events. This notable seismic activity is closely
related to the active tectonics in the region
[e.g., Tsapanos et al., 2014]. The majority of
earthquakes in the northeastern Mediter-
ranean are shallow with some intermediate-
depth events along the Cyprian Arc. Overall,
complex tectonic activity is responsible for in-
tense earthquake activity, deformation, differ-
ent types of faulting mechanisms, tsunamis,
volcanism and geological structures in the
Eastern Mediterranean [Yolsal-Çevikbilen,
Taymaz, 2012].
Using seismic tomography techniques,
scientists decode the information contained
in seismograms’ squiggles to develop images
of individual slices through the deep Earth.
These images are used to understand not
only the composition of Earth’s interior, but
also to help explain geologic mysteries like
concealed structures, interstitial fluids, earth-
quake nucleation zones, composition of rocks
at the crust-mantle transition, different types
of crust-mantle boundary, as well as many
other geological processes (see a review by
[Zhao, 2001; Janik et al., 2007, 2009; Janik,
2010]). The crustal and upper mantle struc-
tures beneath some parts in Anatolia have
been studied recently by seismic tomogra-
phy on different scales [e.g., Sandvol et al.,
2001; Nakamura et al., 2002; Al-Lazki et al.,
2003, 2004; Bariş et al., 2005; Lei, Zhao, 2007;
Salah et al., 2007, 2011, 2014a,b; Schmid et
al., 2008; Gans et al., 2009; Koulakov et al.,
2010; Mutlu, Karabulut, 2011; Bakırcı et al.,
2012; Warren et al., 2013]. However, there is
no detailed study dealing with both the com-
pressional- and shear-wave velocities (VP and
VS, and consequently their VP/VS ratio) of the
northeastern tip of the Mediterranean includ-
ing the northwestern corner of the Arabian
plate and the southeastern part of the Anato-
lian plate. Recently, the P-wave data set from
the local events used in this study, along with
teleseismic P-wave travel time residuals were
used jointly to study the lithospheric struc-
ture beneath SE Anatolia [Salah, 2017]. In the
present work, we use both the P- and S-wave
arrival times generated by local earthquakes
to study the three-dimensional (3D) crustal
velocity and VP/VS structures of this region.
Then, we relate the obtained VP, VS, and VP/
VS models with the available geological and
geophysical investigations at SE Anatolia.
Data and Methods. A total of 7132 events
which occurred during the period from 2007
to 2012 between latitudes 36,0―38,0° N and
longitudes 34―37.6° E was initially collected
3D CRUSTAL VELOCITY AND VP/VS STRUCTURES BENEATH SOUTHEAST ANATOLIA ...
Геофизический журнал № 2, Т. 41, 2019 125
for the present study as explained in [Salah,
2017]. These events were recorded by 41 seis-
mic stations operated by different seismic net-
works in Turkey, including the EDR-Republic
of Turkey Prime Ministry, Disaster and Emer-
gency Management Presidency, National
Seismological observation network, and the
DAF-TUBITAK (The Scientific and Technol-
ogy Research Council of Turkey). The routine
determination of hypocentre parameters is
conducted using the crustal model of [Her-
rin, 1968] and the HYPO71 source code [Lee,
Lahr, 1972]. However, the majority of events
have focal depths of 7―8 km and the errors in
their epicentre locations are relatively large.
These location uncertainties are reflected in
the large value of the travel-time residual ob-
tained after initial tomographic inversion of
the original data set. For this reason, we first
relocated the crustal events using an adopted
initial 1D P-wave velocity model of 5,8, 6,1,
and 6,5 km/s for depths above 5, 18, 42 km,
respectively. The Moho is set at a depth of
42 km with a P-wave velocity (VP) of 7.78 km/s
for the uppermost mantle (e.g., [Mooney et
al., 1998]). A VP/VS ratio of 1.73 is used to ob-
tain the initial S-wave velocity model (Fig. 2).
This velocity model is in general agreement
with many previous results in and/or close to
the study region (e.g., [Horasan et al., 2002;
Erduran et al., 2007; Bilim, 2011]). After re-
location, we removed all the poorly-located
events, thus, the number of local earthquakes
is greatly reduced to 2130 [Salah, 2017]. Each
event is recorded at a minimum of 6 seismic
stations and the errors in the focal depths of
all events are lower than 6 km. These 2130 lo-
cal events produced 13690 and 12560 P- and
S-wave arrival times, respectively, recorded
by 41 seismic stations. Before relocation,
some events were located in the uppermost
mantle with focal depths upto 90 km. Due to
the uncertainty associated with these events,
they were excluded from the final tomograph
inversion. The accuracy of arrival times is es-
timated to be lesser than 0,12 s for P-wave
data and less than 0,18 s for S-wave data. The
majority of the local earthquakes are located
in the central and eastern parts of the study
area with prominent concentration along the
active segments of the EAFZ. The seismic sta-
tions, on the other hand, are uniformly distrib-
uted in the land portion located in SE Anatolia
and northwest Arabia (refer to [Salah, 2017]
for more details). This distribution pattern af-
fects the spatial resolution and reliability of
the obtained velocity structures.
To invert the local arrival time data in SE
Anatolia for a 3D seismic velocity structure
(δVP and δVS), we used the tomographic meth-
od of [Zhao et al., 1992, 1994, 2012]. A 3D grid
net, with the proper spacing, is set up in the
model to express the 3D velocity structure. A
grid spacing of 0,33° in horizontal directions is
adopted for the present study (Fig. 3, a). Four
layers of grid nodes in the vertical direction,
on the other hand, are set up at 10, 22, 36, and
55 km depths (Fig. 3, b). We used the initial
P-wave velocity model described before (See
Fig. 2) for the tomograph inversion, whereas
the initial S-wave velocity model is calculated
using a VP/VS ratio of 1,7296, which is derived
from the Wadati diagram constructed using
the whole data sets [Salah, 2017]. The differ-
ent layers were assigned the given VP val-
ues which are constant within an individual
layer but change at the boundaries between
the layers. These, finally-adopted, initial ve-
locity models give the minimum root-mean-
square (RMS) travel time residuals when used
Fig. 2. The adopted initial P- and S-wave velocity models
used in the relocation of events (see text for details).
MOHAMED K. SALAH, ŞAKIR ŞAHIN
126 Геофизический журнал № 2, Т. 41, 2019
as starting models for the final tomographic
inversion. Before adopting these initial VP and
VS models, we tested slightly different models
by changing the velocities within the limits of
±5 % and noticed that the changes in the final
velocity structures are very low and does not
exceed 1 % from each other.
After computing the P- and S-wave velocity
models we calculated the VP/VS ratio at the
different crustal layers. This ratio (or the Pois-
son’s ratio) is more significant in character-
izing the petrophysical properties of crustal
rocks and provides better constraints on the
crustal composition and interstitial fluids than
the seismic wave velocities (e.g., [Christensen,
1996; Zhao et al., 2004; Bariş et al., 2005; Salah
et al., 2007, 2011; Janik, 2010]). Based on the
NEIC (the National Earthquake Information
Center, USGS) catalogs, we found a total of 12
moderate/large crustal events (Mb or Mw≥5,0)
that occurred in the study area since 1979. We
plot the epicenters of these events on the ob-
tained velocity and VP/VS anomalies to char-
acterize the seismogenic zones in the area.
Resolution and results. One of the well-
established and straightforward methods to
assess the reliability of the obtained velocity
models is the checkerboard (CKB) resolu-
tion test (e.g., [Zhao et al., 1992]). Synthetic
velocity anomalies of ±3 %, whose image is
straightforward and easy to remember, are
alternatively assigned to the 3D grid nodes
in order to generate a checkerboard pattern.
The corresponding synthetic arrival times are
then calculated for this input model. Stations
locations, event numbers and locations, and
accordingly ray paths, in the synthetic data
are the same as those in the real data. Random
errors of similar magnitude to those of the real
data are also added to the synthetic arrival
times and are then inverted with the same
tomographic method. The inverted image
of the checkerboard distinguishes between
areas of good and poor resolution. Figs. 4
and 5, display the resulting images of both
the δVP and δVS structures, respectively, at
three crustal layers. The CKB test points to a
good and uniform resolution of about 35 km
horizontally for the two data sets. Due to the
scarce distribution of both the seismic events
and the recording stations in the southern and
western parts of the study area, the resolu-
tion of the obtained velocity images at these
areas, especially those at a depth of 36 km, is
relatively poor (See Fig. 5).
Adopting the tomographic method de-
scribed in section two to the SE Anatolia data
set, we found that the sum of squared travel-
time residuals was reduced by more than 50 %
of its initial value. We conducted a number of
tomograph inversions using different values
for the damping and generated a trade-off
curve of the norm of the solution versus the
final RMS travel-time residual (Fig. 6). Con-
sidering the balance between the reduction
of travel time residuals and the smoothness of
the obtained velocity model, a damping value
of 8 was selected as the best damping for the
present tomograph inversion and regulariza-
Fig. 3. Configuration of the grid net adopted for the
present study in horizontal (a) and depth (b), directions.
The grid spacing is 0,33° and 6―20 km in horizontal
and depth directions, respectively. Straight lines in (a)
denote the location of five vertical cross sections shown
later. Black lines denote active faults. Inverted red tri-
angle denotes the Holocene Kilis volcano.
3D CRUSTAL VELOCITY AND VP/VS STRUCTURES BENEATH SOUTHEAST ANATOLIA ...
Геофизический журнал № 2, Т. 41, 2019 127
tion. The final RMS travel time residuals are
0,34 s and 0,46 s for the P-wave, and S-wave
data sets, respectively. Significant parts of
the study area have enough ray coverage at
the three depth layers (10, 22, and 36 km) in
which the number of P and S rays hitting each
grid node is adequate to retrieve the velocity
structures (Figs. 7 and 8). Although the two 22
and 36 km depth layers are assigned the same
initial velocity values (See Fig. 2), we expect
different final velocity structures because the
velocity at any point in the model is calculated
by linear interpolation of the eight surround-
Fig. 4. Results of the checkerboard resolution test for
P-wave velocity at three crustal depths (see text for
details). Black and white rhombs denote high and low
velocities, respectively. The perturbation scale (±3 %) is
shown at the bottom. The depth of each layer is shown
below every map.
Fig. 5. Results of the checkerboard resolution test for
S-wave velocity at three crustal depths. Other details
are similar to those of Fig. 4.
Fig. 6. Trade-off curve of the variance velocity perturba-
tion (%) and the root-mean-square travel time residual
at different values of the damping parameter (numbers
on the curve). A pen pointing to the best damping is
shown at a value of 8.
MOHAMED K. SALAH, ŞAKIR ŞAHIN
128 Геофизический журнал № 2, Т. 41, 2019
Fig. 7. Number of rays passing through each grid node
(hit count) for P-wave data at three depth slices. Grid
nodes with rays <8, and consequently poor coverage are
not included in the tomographic inversion.
Fig. 8. Number of rays passing through each grid node
for S-wave data at three depth slices. Other details are
similar to those of Fig. 7.
ing grid nodes in the horizontal and vertical
directions [Zhao et al., 1992]. Grid nodes of
less than 10 rays are not included in the final
tomograph inversion. It is clear that the cen-
tral, eastern and northeastern parts have large
hit counts and many nodes are hit by more
than 4000 rays at the first two crustal layers.
Inversion results of δVP , δVS , and VP/VS
distributions in map views at three crustal
layers are shown, respectively, in Figs. 9, 10,
and 11. In addition, Figs. 12―16 show the
δVP, δVS, and VP/VS images along five vertical
cross-sections in SE Anatolia (refer to Fig. 3
for the location of cross-sections). The veloc-
ity images show the velocity perturbations in
percentage from the initial velocity model at
each depth slice or cross section. Although
some researchers present absolute velocities
which increase generally with depth (e.g.,
[Hearn, Ni, 1994; Al-Lazki et al., 2004]), dis-
playing velocity perturbations (%) are much
more preferred as they show clearly and eas-
ily areas of high or low velocity anomalies
(e.g., [Zhao et al., 1992, 1994]).
Significant lateral and vertical variations
of up to ±8 % of velocity (δVP and δVS) and
VP/VS ratio are revealed in the study area.
Higher VP zones are visible at the upper crust
beneath the western side, in the central part
3D CRUSTAL VELOCITY AND VP/VS STRUCTURES BENEATH SOUTHEAST ANATOLIA ...
Геофизический журнал № 2, Т. 41, 2019 129
at middle crust, and in the central northern
regions at the lower crust (See Fig. 9, anoma-
lies A, C, D and E). These high-velocity areas
might represent asperities of highly consoli-
dated rocks. Lower-than-average VP anoma-
lies, on the other hand, are revealed at many
parts at middle crustal depths and in broad
zones at both the upper and lower crust (See
Fig. 9. P-wave velocity structures (in %) at three depth
slices beneath southeast Anatolia. Red and blue colors
denote low and high velocities, respectively. Numbers
between brackets show the depth range of the micro-
seismic activity plotted as crosses. Moderate and large
earthquakes (M≥5,0) in the same depth range of the
background seismicity are plotted as red stars. Thin
solid lines denote active faults in southeast Anatolia. In-
verted red triangles denote the Holocene Kilis volcano.
The perturbation scale (±8 % for the shallowest layer
and ±7 % for the remaining) is shown to the lower right.
Fig. 9, anomalies B, F, and G). A clear, low
shear wave velocity anomaly is clearly vis-
ible at a depth of 10 km in the central part
of the study area (See Fig. 10, a, anomaly A).
Another prominent low shear wave velocity
anomaly can also be seen at a depth of 36 km
(See Fig. 10, c, anomaly E). A number of low VS
anomalies to the south and west are also vis-
ible at this depth. The magnitude of the low-
velocity anomalies is larger near the EAFZ
and the Holocene volcano at the southeast
(See Figs. 9, b, c and 10, b, c). High VS zones
to the west of the study area at both the upper
and lower crust are also imaged (See Fig. 10,
anomalies B, C, D, and F). In addition, most
Fig. 10. S-wave velocity structures at the three depth
slices. Other details are similar to those of Fig. 9.
MOHAMED K. SALAH, ŞAKIR ŞAHIN
130 Геофизический журнал № 2, Т. 41, 2019
of the microseismic activity is concentrated
along the EAFZ and to some heterogeneous
zones of low- to average-velocity anomalies.
The background seismicity is scarce along the
strong asperities implying that such areas are
capable of resisting accumulated stress.
The VP/VS ratio varies between around
±7,5 % and hence, shows a high structural
heterogeneity at the different crustal layers
(See Fig. 11, anomalies A and B). It is gener-
ally higher than 1,73 at all depth slices, imply-
ing a general low S-wave velocity structure
compared to the P-wave velocity. Prominent
high VP/VS anomalies are evident beneath
Fig. 12. Vertical cross sections of δVP, δVS and VP/VS
structures along line AA’ (see Fig. 3 for the location of
the cross sections). The red color denotes low velocity
and high VP/VS ratio; whereas high velocities and low VP/
VS ratios are shown in blue. Crosses show the location
the microseismic activity in a 40 km-wide-zone around
the profile. The perturbation scales (±7 % for velocity
and 1,60―1,95 for VP/VS ratio) are shown to the right.
Fig. 11. Distribution of VP/VS structures at three depth
slices. Red and blue colors denote high and low VP/VS
ratio, respectively. The variation scale (1,60―1,95) is
shown to the lower right. Other details are similar to
those of Fig. 9.
the active volcano in the south (See Fig. 11,
anomalies C and D), and along the active seg-
ments of the EAFZ. A number of high VP/VS
zones are also distributed in the lower crust
(See Fig. 11, c). These areas may represent
the conduits for the vertically ascending hot
fluids from the uppermost mantle beneath
the study area. All the moderate/large earth-
quakes are located in areas characterized by
low/average velocity and average/high VP/
VS (See Figs. 9―11). Microseismic activity,
on the other hand, is intense in the central
and eastern parts of the study area, which are
characterized by highly heterogeneous veloc-
ity and VP/VS ratio structures. Both low veloc-
3D CRUSTAL VELOCITY AND VP/VS STRUCTURES BENEATH SOUTHEAST ANATOLIA ...
Геофизический журнал № 2, Т. 41, 2019 131
ity and high VP/VS anomalies are clearly vis-
ible along the vertical cross sections AA’―EE’
(See Figs. 12―16). Prominent, low VS/high VP/
VS anomalies are visible along cross section
BB’ (which correspond to anomalies A and B;
Fig. 11). The seismic activity is concentrated
in low-velocity/high VP/VS zones (See Figs. 13,
15). Another clear low VS zone is evident at
the middle and upper crust along cross sec-
tion DD’ (See Fig. 15, anomaly A). This zone
is characterized by high VP/VS ratios (See
Fig. 15, anomaly B). Lower seismic wave ve-
locities are also visible along cross section EE’
with a high VP/VS anomaly at lower/middle
crustal depths (See Fig. 16). Background seis-
micity and large crustal earthquakes occur in
heterogeneous zones characterized by low-
velocity/high VP/VS anomalies (See Figs. 13,
Fig. 13. Vertical cross sections of δVP, δVS and VP/VS
structures along line BB’. Large stars show the loca-
tion of moderate-large earthquakes (M≥5,0) in a 40 km
wide-zone around the profile. Other details are similar
to those of Fig. 11.
15, 16). Although the P-wave velocity is close
to its average value at lower crustal depths be-
neath the Holocene volcano in the southeast;
low VS and high VP/VS anomalies are clearly
visible beneath the volcano, although shifted
slightly to the west (See Figs. 14, 16).
Discussion. Previous seismological obser-
vations. The results of many previous geo-
physical investigations at SE Anatolia indicate
that the region is characterized by heteroge-
neous structures and complex seismotectonic
setting. This is evident from the obtained to-
mographic images dominated by average- to
low-velocity and average to high VP/VS anom-
alies which arise mainly from the extensive
faulting, widespread magmatism, and the
existence of sedimentary basins in the study
area (e.g., [Jaffey et al., 2004]). Hearn and Ni
[1994] previously detected low Pn velocity of
about 7,8 km/s beneath the Anatolian plate,
whereas Rodgers et al. [1997] documented
Fig. 14. Vertical cross sections of δVP, δVS and VP/VS
structures along line CC’. Other details are similar to
those of Fig. 11.
MOHAMED K. SALAH, ŞAKIR ŞAHIN
132 Геофизический журнал № 2, Т. 41, 2019
Fig. 15. Vertical cross sections of δVP, δVS and VP/VS
structures along line DD’. Other details are similar to
those of Figs. 11 and 12.
Fig. 16. Vertical cross sections of δVP, δVS and VP/VS
structures along line EE’. Other details are similar to
those of Figs. 11 and 12.
an inefficient Sn propagation, low Pn veloc-
ity, and volcanism indicating possible partial
melt in the upper mantle beneath Turkey.
Large-scale (~500 km) zones of low (<8 km/s)
Pn velocity anomalies were mapped beneath
the Anatolian plate and the Anatolian plateau
by Al-Lazki et al. [2004] and Al-Damegh et al.
[2004]. These are thought to be hot and unsta-
ble mantle lid zones, and may partially result
from the subduction of the Tethyan oceanic
lithosphere beneath Eurasia. A region of very
low Pn velocities (<7,8 kms−1) is imaged east of
the central Anatolian fault zone, with a tran-
sition to faster velocities (>8,1 kms−1) west of
the fault [Gans et al., 2009]. These anomalies
are also consistent with the heterogeneous Pn
structures across Turkey [Mutlu, Karabulut,
2011] with widely distributed low Pn velocities
beneath SE Anatolia (east of 35°E). The low-
est Pn velocities coincide with the volcanics
in eastern Anatolia and the central Anatolian
volcanic zone. Erduran et al. [2007] detect-
ed shallow, very low shear wave velocity of
2,2 km/s that increases to 3,6 km/s at a depth
of 10 km beneath Anatolia. They reached to
the conclusion that the estimated velocities
in almost all depth ranges are significantly
lower than the PREM values. In addition, the
uppermost mantle beneath the Anatolian
plate is represented by a relatively low shear
wave velocity of 4,27 km/s. Anomalously low
shear wave velocities are found underneath
the Anatolian block and the Anatolian plateau
in eastern Turkey [Gök et al., 2007; Warren
et al., 2013] throughout the whole crust. In
addition, recent results of Delph et al. [2015]
show that the overall shear wave velocities of
the Anatolian crust are low. These low veloci-
ties point to upwelling of hot materials from
an underlying hot asthenosphere (e.g., [Jiang
3D CRUSTAL VELOCITY AND VP/VS STRUCTURES BENEATH SOUTHEAST ANATOLIA ...
Геофизический журнал № 2, Т. 41, 2019 133
et al., 2015]). The recent study of Bakırcı et
al. [2012] points to a deep source for these
low shear-wave velocities. Low Sn velocity of
4,3―4,5 km/s [Gök et al., 2003; Tezel et al.,
2007], and upper mantle low P-wave velocities
[Lei, Zhao, 2007] detected beneath eastern
Anatolia support this hypothesis. These low
velocity anomalies are probably related to the
absence of a lithospheric mantle lid and its
replacement with asthenospheric materials.
Inversion results of Tezel et al. [2007] indicate
that the Anatolian region has a shallow low
shear-wave velocity zone at 7―20 km depths,
which are consistent with the low shear wave
velocity anomalies detected at a depth of
10 km (See Figs. 9, b, 12, 14―16).
Fig. 17. Distribution of the Neotethyan ophiolites superimposed on the major tectonic features in the eastern
Mediterranean region (from [Dilek, Flower, 2003]). AC ― Antalya complex; IPO ― Intra pontide ophiolites; BHN
― Beyşehir-Hoyran nappes; IAESZ ― Izmir-Ankara-Erzincan suture zone; MO ― Mersin ophiolite; AO ― Aladağ
ophiolite; DO ― Divriği ophiolite.
MOHAMED K. SALAH, ŞAKIR ŞAHIN
134 Геофизический журнал № 2, Т. 41, 2019
High attenuation (low Q) zones beneath
different parts of Anatolia have been detected
by many researchers [Zor et al., 2007; Pasya-
nos et al., 2009]. Lower values of Lg Q0 were
also detected recently by Kaviani et al. [2015]
over the Turkish-Anatolian plateau (<150)
than those observed over the Iranian plateau
(150―400).
The ophiolites and their associated low-
velocity/high VP/VS ratio. The ophiolites of
SE Anatolia occur in two main belts, namely
the Peri-Arabic belt and the SE Anatolian
ophiolites (Fig. 17). The former were extruded
directly onto the Arabian platform; whereas
the latter were tectonically emplaced beneath
the Tauride platform and cut by calc-alkaline
volcanic granitoid rocks [Karaoğlan et al.,
2013]. The ophiolites in the two belts exhibit
a complete ophiolite pseudostratigraphy and
are geochemically classified as suprasubduc-
tion zone (SSZ) type (for more details, refer
to [Parlak et al., 2004; Bağci et al., 2005, 2006,
2008]). Studies of [Parlak et al., 2009] on the
petrology and geochemistry of Neotethyan
ophiolites point also to extensive distribution
of Late Cretaceous ophiolitic belts running
NE-SW; almost parallel to the orientation of
the EAFZ (in our study area, these are the
MO-AO-DO and the EAFZ trends). It is well
known that the presence of serpentine and
other hydrous minerals have significant ef-
fects on the physical and mechanical prop-
erties of crustal and upper mantle rocks,
including a decrease in seismic velocity, an
increase in Poisson’s ratio, generation of seis-
mic reflectivity, an increase in magnetization,
a reduction in density, an increase in electri-
cal conductivity, and reduction in mechanical
strength (e.g., [Christensen, 1978; Horen et
al., 1996]). Extensive research of Christensen
[1966, 1978] showed that original VP/VS val-
ues of 1,76―1,81 may increase to 1,87 at 15 %
serpentinization and even to 2,27 at 100 %
serpentinization. High VP/VS values between
1,75―1,83 indicate a composition of interme-
diate to mafic rocks [Paswan et al., 2016]. The
widely distributed high VP/VS zones in our re-
sults at the upper/middle crustal depths (See
Fig. 11, anomalies A, and B) are consistent
with the MO-AO-DO trend in SE Anatolia
[Dilek, Flower, 2003; Karaoğlan et al., 2013]
(See Fig. 17). In addition, high VP/VS zones
at middle/lower crustal depths (See Fig. 11,
anomalies C and D) are mapped near the
Holocene volcano. Low seismic wave veloci-
ties are also seen parallel to the two ophiolite
trends discussed above (for example anoma-
lies B and F in Fig. 9, and anomalies A and E
in Fig. 10).
Other geophysical investigations. The
Curie point depths estimated by Bektaş et
al. [2007] over the region indicate generally
high temperatures within the crust and a pos-
sibly demagnetized lower crust. The presence
of hot springs (>45 °C) and the young-aged
volcanic rocks in the region support these
anomalously high temperatures. These ob-
servations suggest that the uppermost mantle
and perhaps parts of the overlying lower crust
are partially molten and the asthenosphere is
close to the base of the crust; both of which
are consistent with the existence of the volca-
nism in the region. Hartlap hot spring located
at 37,53° N and 36,67° E, with a water tem-
perature of up to 37 °C, in the northeast part
of the study area only few kilometers to the
north of the EAFZ, is related to the faulting
system in Inner East Anatolia [Bektaş, 2013].
It is, however, believed that the mantle heat
flow in the region may not have made its way
to the surface. Instead, it is too deep to heat
the hot springs generally found in Inner East
Anatolia [Bektaş, 2013].
Conclusions. In addition to more detailed
VP structures compared to the results of Salah
(2017), the present study provides fine shear
wave velocity and VP/VS structures beneath
SE Anatolia which are determined by invert-
ing a number of P- and S-wave arrival times
generated by local earthquakes recorded at
41 seismic stations. Results of the checker-
board resolution test and hit counts imply that
the mapped structures are real down to the
depth of the Moho discontinuity. The follow-
ing points outline the main conclusions of the
present study.
1. The crustal velocity structure is highly
inhomogeneous and contains many lower-
than-average velocity anomalies. The low-
velocity zones are more widely distributed
3D CRUSTAL VELOCITY AND VP/VS STRUCTURES BENEATH SOUTHEAST ANATOLIA ...
Геофизический журнал № 2, Т. 41, 2019 135
beneath the active faults and areas of thick
sedimentary cover.
2. Distinct high VP/VS structures are clearly
seen at all depth slices, which might be related
to the presence of partial melt in the lower crust
and the uppermost mantle. The high VP/VS
areas are consistent with the existence of oph-
iolite belts.
3. Although some events occur in high
seismic velocity zones; most of the large
crustal earthquakes are found in average/
low velocity and average/high VP/VS ano-
malies. This indicates that the large crustal
events occur away from weak areas that may
deform aseismically, but not in those char-
acterized by higher-than-average veloc-
ity, which are capable of resisting relatively
large stresses.
SE Anatolia is an area that is characterized
by the presence of some sedimentary basins,
extensive faulting, ophiolite belts, and Ceno-
zoic volcanics. The presently mapped low-ve-
locity/high VP/VS zones are in agreement with
such circumstances. Many previous geophys-
ical observations such as low Pn and Sn veloci-
ties, high Sn attenuation, high heat flow, low
Lg Q0 values, and high geothermal potential
support the imaged low-velocity/high VP/VS
ratio zones. The results of these geophysi-
cal observations beneath southeast Anatolia
are analogous to other continental plateaus
such as the Tibet and are interpreted to be
an indication of a serpentinized hot mantle
that could be partially molten. The presence
of this partial melt in the uppermost mantle
feeds the widespread Cenozoic volcanism in
the Anatolian plateau.
Acknowledgements. The authors thank
Hakan Demirsıkan for his help in data prepa-
ration. Large earthquake information is ob-
tained from the earthquake catalogs report-
ed by the National Earthquake Information
Center (NEIC), (http://neic.usgs.gov/neis/
epic/). Most figures in this paper are made us-
ing GMT (Generic Mapping Tools) software
which is written by Wessel and Smith [1998].
3D crustal velocity and VP/VS structures beneath
Southeast Anatolia and their geodynamic implications
M. K. Salah, Ş. Şahin, 2019
We applied a seismic tomography method to arrival time data generated by local crustal
earthquakes in Southeast Anatolia to study the shallow, three-dimensional, velocity and
VP/VS structures beneath the area. Many of the previous seismological studies of the region
are of a regional, or even global, scale. A total of 2150 carefully-selected events generating
13690 and 12560 P- and S-wave arrival times are finally used in the tomographic inversion.
Results of the checkerboard resolution test imply that the obtained velocity and VP/VS
anomalies are reliable features. In addition, hit count maps indicate that all parts are hit by
an adequate number of rays to retrieve the crustal velocity structure. Strong lateral crustal
heterogeneities are revealed beneath southeast Anatolia with many lower-than-average
velocity anomalies. The low velocity anomalies are imaged especially near the active fault
segments. In addition, high VP/VS ratios are mapped at most crustal layers especially at
depths of 10 and 22 km which are consistent with the distribution of ophiolite belts. The
high VP/VS zones are induced by the possible existence of over-pressurized fluids in the
crust and perhaps the uppermost mantle. The existence of these fluids along with the in-
tense tectonic activity could trigger large crustal earthquakes along the western segment
of the East Anatolian fault zone. Although may occur in high velocity zones, the majority
of the large crustal earthquakes are distributed near zones of average velocity/high VP/VS
anomalies. Such mapped velocity and VP/VS zones are in agreement with many previous
geophysical investigations beneath southeast Anatolia such as low Pn and Sn velocities,
high Sn attenuation, high heat flow, and low Lg Q0 values. Results beneath this region of the
MOHAMED K. SALAH, ŞAKIR ŞAHIN
136 Геофизический журнал № 2, Т. 41, 2019
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