SURFACE WAVE PHASE VELOCITY DISTRIBUTION
OF TURKEY
Taciser ÇETİNOL*MEE06018

Supervisor: Kazunori Yoshizawa**

ABSTRACTThe crustal and upper mantle structure beneath Turkey is investgated by using phase velocity offundamental-mode Rayleigh waves working with a two-station method. We used vertical component
seismograms of 72 broad-band seismic stations in Turkey and surrounding areas from 172 teleseismic
events from April 2005 to June 2007. The average dispersion curve of the entire region of Turkey isestimated from path-average dispersion curves in a period range from 20 to 160 s. The average phasevelocity of Turkey is found to be about 3-9 % slower than the global average even if we consdier acrustal correction for Turkey. We generate path coverage maps as a function of frequency indicating
the path-specific phase velocity with colors, and perform two kinds of analysis using the paths withhigh accuracy to clarify the regional variations of phase velocity in Turkey. Both the phase velocity
coverage maps and two studies on localized phase velocity distributions indicate that the fastest phasevelocities are observed in the north-western part of Turkey (i.e., the Marmara Region) and the phasespeed becomes slow in the eastern part of Turkey. We may explain the observed fast phase velocities
in the northwestern part of Turkey by the cooling of the upper mantle, which can affect the immersionof oceanic lithosphere of the Black Sea beneath southward margin of the Marmara region and the slow
phase velocities in the eastern Turkey by the presence of conjugate strike slip fault system (EAFZ) and
Bitlis thrust zone as well as partially molten litospheric mantle which can be inferred from the wide
spread of young volcanism (< 2Ma). The previous geological (i.e. heat flow, volcanism and tectonics)
and seismological (i.e. Pn-wave velocity distribution, Sn-wave attenuation) studies strongly support
our results.
Keywords: Surface waves phase velocity, crustal and upper mantle structure of Turkey
INTRODUCTION
Turkey is located in the Alpine- Himalayan earthquake zone at the Eurasian Geological Plate whichcauses the numerous destructive earthquakes. The tectonic framework of Turkey is controlled by thenorthward motion of Arabian Plate against Eurasian Plate which contributes the westward motion ofTurkey towards the African Plate (McKenzie, 1972). Turkey accommodates a wide variety of tectonic
processes such as major strike slip faults (the North Anatolian Fault Zone –NAFZ & the East
Anatolian Fault Zone –EAFZ), continental collision (EAFZ), continental extensions (Western Turkey
Graben Complex, Marmara sea) (Ketin, 1969; McKenzie, 1972; Barka and Reilinger, 1997; McClusky
et. al., 2000). Turkey has a high level of seismicity due to these tectonic activities and has sufferedfrom noteworthy loses of life and property as a result of earthquakes in its long history. Many
researchers (e.g. Canıtez and Toksöz , 1980; Horasan et al., 2002; Gök et al., 2003; Barış et al., 2005;
Zor et al.,2006) investigated the different parts of the Turkey using various types methods, only
several structural studies were completed for the entire Turkey (e.g. Midevalli and Mitchell, 1989;
Villasenor et al, 2001; Al-Lazki et al, 2004; Maggi and Priestly 2005). In this study, we investigate*

Kocaeli University, Engineering Faculty, Department of Geophysical Engineering, Kocaeli, TURKEY
Hokkaido University, Graduate School of Science, Department of Natural History Sciences, Sapporo, JAPAN**1

the phase velocity distribution of the fundamental-mode Rayleigh waves to clarify the regionalvariations of the crust and uppermost mantle beneath Turkey.
DATA and METHOD
The seismograms are retrieved from broad-band seismic stations of the Boğaziçi University
Kandilli Observatory and Earthquake Research Institute (KOERI). The locations of the 72 broadband
seismic stations (66-KOERI, 2-IRIS, and 4-GEOFON) are given in Figure 1.
The stations of the KOERI consist of the Guralp product seismometers (CMG 3TD-3ESPD6T-40T). The stations of the IRIS and GEOFON include STS type (STS1 and STS2) seismometers. In
this study, vertical component seismograms of each station are used.

Figure 1 The locations of the broadband seismic stations (the red
triangles: KOERI stations-KO, the blue squares: IRIS stations-IU,
the violet stars: GEOFON-GE).

Figure 2 The distribution of the events (redcircles). The red frame indicates Turkey andsurrounding area

We employed 172 events with moment magnitude greater than 6.0 and focal depth shallower
than 100 km which occurred in the world between April, 2005 and June, 2007 (Figure 2).
In this study, we estimate the dispersion of the Rayleigh waves employing two-station methodthat is one of the most classical but useful technique to determine regional variations of surface-wave
phase velocities, which should be of great help to delineate the crust and upper mantle of Turkey. We
employed the phase difference aproach (Dziewonski and Hales, 1972) to determine the phase velocityof the fundamental mode Rayleigh waves.
We can summarize the automatic procedures for data processing in the four steps as follows;
1. In the first step, the instrument responses are corrected so that the effect of the instrumental phaseshift of each station can be eliminated.
2. In the second step, a proper time-window for the fundamental-mode Rayleigh wave is chosen byusing theoretical arrival times based on a range of group velocities between 2.6 km/s and 5.1 km/s.
3. A list of station-pairs located on or near acommon great circle path are created.
The phase speed for each station-pair isthen calculated as a function of
frequency from unwrapped phases of two
stations based on a great circle
approximation.
4. In the final step, we check the results
visually and choose a proper frequency
range in which the calculated dispersion
curve is smooth enough with smaller
Figure 3 The distribution of the paths which are obtained using
variance.
two-station approximation. The used profiles are overlapped on
It should be noted that phase velocity
the retrieved profiles using lilac color.
measurements for two closely locatedstations exhibit significant errors in the longer period for which the scale of wavelength is close to the2propagation distance. On the contrary, for the excessive long distance paths between two stations, thedispersion curves may be affected significantly by 2π ambiguities in phase cycles. To obtainsatisfactory results based on a plane-wave approximation, the station pairs should not be too close nor
too far.
Although 687 great circle paths for
Table 1 The distribution of the paths against to years.
Year
Total
Employed
Total
Employedstation pairs can be derived from 172 events,
eventseventsdispersion dispersion
only 104 dispersion curves are employed tocurvescurvesconstruct the average dispersion curve of
2005
71
-63
-turkey in the next section. The numbers of the
2006
61
6
220
13
events and the paths are given in table 1. The
2007
40
13
404
91
April 2005 172
19
687
104
distribution of all the analyzed paths as well as
June 2007
employed paths are shown in Figure 3.
The stability and reliability of the two-station measurements depends on the distance between
the stations and the wavelength of surface waves to be considered. Moreover, the impacts of non-planewaves on two-station measurements may not be neglected. The non-plane waves may affect observed
seismograms as a summation of the plane waves propagating with a variety of amplitudes anddirections.
RESULTS AND DISCUSSION
Average dispersion curve of Turkey
To determine the average phase velocity of the entire
Turkey, we first calculate the moving averages of phasevelocity for a single dispersion curve in a frequency
domain. Then, the average phase velocities of the entire
region of Turkey as well as corresponding standard
deviations are estimated from all the averaged dispersion
curves. The average dispersion curve of Turkey is shown (a)in the Figure 4a between 20 and 160 seconds of periods.
The perturbation of the average velocity dispersion from a
global average model is also calculated (Figure 4b). The
reference global average model is created using PREMC (amodified version of PREM (Dziewonski & Anderson,1981) to better represent the continental structure) for the
mantle structure and 3SMAC model (Nataf & Ricard,
1996) for the crustal structure of Turkey.
(b)
The average phase velocity of Turkey isslower than the global average with perturbations Figure 4 Comparison of the average velocity dispersion curvesof Rayleigh waves
of about 3 - 4 % at period over 100 seconds, 4 - (a) Dashed blue line : Global model velocity (PREMC and
6 % at 30 - 100 seconds and about 9% at 20
3SMAC), Black circles and error bars: Average phasevelocity of Turkey (this study).
seconds (Figure 4b). Previous studies reveal thatthe average velocity of entire Turkey is (b) Perturbation of the average phase velocity of Turkey fromsignificantly slower than the global average. PREMC
Global and large scale phase velocity (Ekstrom et al, 1997, Villasenor et, al, 2001) and shear wave
velocity (Debayle et al, 2001, Maggi, A., and Priestley, K., 2005) studies support our result.
Phase speed distribution of Turkey
Now we investigate regional variations of the Rayleigh wave phase velocity distribution over Turkey.
We observe that the majority of the obtained dispersion curves tend to be more stable (less scattered)in the period range between 20 and 160 s.

3For this period range, the fundamental mode
Rayleigh waves are mainly sensitive to the depth of thecrust and uppermost mantle of the earth (i.e.,
approximately 30 - 300 km depth) reflecting the shear
wave structure of the upper mantle beneath Turkey.
Two examples of surface wave paths at 30 s and70 s of periods are presented in Figures 5a and b
respectively, indicating the path-average phase velocity
using colors. A general feature of the phase velocity
distributions is that the phase velocity is apt to decrease
from the west to east. The highest phase velocities areobserved in the northern part of Turkey; i.e., theMarmara Region. The phase speed in east Turkey
becomes slow. The number of paths in the southern
margin of Turkey is not sufficient to make an accurate
estimation. However, this area seems to be slower than
the average velocity of Turkey.
Regional variations of Rayleigh-wave phase velocity

(a)

(b)
Figure 5 The path coverage of Rayleigh waves

phase speed distribution (a) at 30 s ( 58 paths), (b)Here we investigate the regional variations of phaseat 70 s period (49 paths).
velocity using the paths with high accuracy. To examine
regional variations of the phase speed, we divide Turkey
into some regions depending on the numbers of available paths. We have performed two kinds ofanalysis; i.e., study-1 and study-2 as follows.

Study-1: Phase dispersion in 6 corridors over Turkey
In the study-1, we selected the some stable velocity
profiles which contain highly reliable dispersion curves
and created 6 corridors of Rayleigh wave paths(Figures 6). The Corridors 1 to 4 have NW-SE trend
and are aligned from north to south. The Corridor-5 isextending NNW-SSE direction traversing western
Turkey. The Corridor-6 is extending NS-SW in analmost perpendicular direction to the others.
The
phase speeds of the Corridor-1 (northeast Turkey) andCorridor-4 (southwest Turkey) are the slowest among
the 6 corridors, whereas the phase speed of corridor-6(northwest part of Turkey) displays the highest phasespeed. The dispersion curve for the Corridor-5 shows
almost the same velocity as the average phase velocityof Turkey.
In Figure 6b, we can see a monotonicallyincreasing trend of phase speed dispersion from 20 s to
40 s for all the Corridors. However, the Corridor-2
Figure 6 study-1
change its characteristics of the dispersion curve and(a) Selected 6 corridors of Rayleigh-wave paths,
show low phase speed anomaly in the period range over(b) Average phase velocities of the 6 corridors.
Red line and stars: Corridor-1, Yellow line and40 s. We may expect that shear wave speeds of thetriangles: Corridor-2, Green line and diamonds:
upper mantle beneath this corridor may have a low
Corridor-3, Blue line and squares: Corridor-4, Brown
velocity zone at about 60-100 km depth. The dispersion
line and hexagonals: Corridor-5, Purple line andcurve for the Corridor-3 shows relatively steady phasecircles:Corridor-6, Black circles and error bars :Average phase velocity of Turkey ( this study).
speeds at the period over 40 s, and is closer to the4average dispersion curves of Turkey. The phase velocities of the Corridor-1 and Corridor-4 show amonotonic trend of increasing phase speed with period.
Among all the dispersion curves in Figure 6, the Corridor 6 shows the fastest phase speed at
all the period. Since the direction of this corridor is totally different from (or nearly perpendicular to)
the other 5 corridors, this significant fast anomaly may imply the existence of azimuthal anisotropy in
the north-western Turkey, although we cannot distinguish the effects of anisotropy and heterogeneityfrom the current analysis with small numbers of paths. Such complicated effects should be
investigated in more detail in the future.
Study-2: Phase dispersion in 3 parallel corridors
In the study-2, all the corridors have the NW-SE trend(Figures 7). Corridor-A is the same as Corriodor-1,
and Corridor B and C are the new versions of theCorridor 3 and 4 of the study-1.
The fastest phase velocities are observed in the
Corridor B, whereas the slowest in the Corridor A.
These results show that the phase speed of thenorthwest margin and the center of Turkey is higher
than the northeastern margin and southwest region ofTurkey. The phase speed of Corridor-B located in thecentral part become steady in the period range longer
than 40 s, indicating a small variation in shear wave
speed at the depth of about 60-100 km.
We may explain the observed fast velocities in
the northwestern part of Turkey (Marmara region) by
the cooling of the upper mantle, which can affect theimmersion of the Black Sea oceanic lithosphere Figure 7 study-2(a) Selected 3 corridors of Rayleigh-wave paths.
beneath southward margin of the Marmara region(b) Average phase velocities of the Corridor-A,(Gülen and Kuleli, 1995). In this region, higher Pn
Corridor-B and Corridor-C.
velocities has also been found by Horasan et al (2002).
Red line and stars: Corridor-A, Blue line andsquares: Corridor-B, Purple line and diamonds:
The lowest phase velocities in the eastern
Corridor-C, Black circles and error bars : Average
Turkey can be interpreted by the presence of conjugate
phase velocity of Turkey ( this study)
strike slip fault system (EAFZ) and Bitlis thrust zone
as well as wide spread of young volcanism (< 2Ma).
The fissure-fed mantle-derived alkaline volcanism was observed until recent times (0.4 Ma) andlitospheric mantle can be observed in the fault area as the presence of alkali basalt (Yürür andChorowicz, 1998). The evidence from previous studies such as the low Pn velocities (Al-lazki et al.,2004), high Sn attenuation (Rodgers et al, 1997; Gök et al., 2003), and geological features such as hot
rising asthenosphere (Yürür and Chorowicz, 1998; Şengör et al., 2003), thin litospheric mantle (Gök etal., 2007) and high heat flow (40-140mW/m2-Tezcan, 1995) can be strong support for the observed
slow velocities in this study.
CONCLUSIONSIn this study, we have estimated the phase velocity distribution of Turkey using seismic records of the
72 broad-band stations located in Turkey and surrounding areas from 172 events with moment
magnitude greater than 6.0 and focal depth shallower than 100 km which occurred in the world
between April 2005 and June 2007. We determined 104 phase velocity dispersion curves by applying
the two-station method for the vertical component of the fundamental mode Rayleigh waves.
The average dispersion curve of Turkey is created by using observed dispersion curves
covering the entire Turkey, and is compared with the global average with a correction for the crustal
structure of Turkey. The estimated average dispersion curve of Turkey indicates about 3-9% slower
phase velocity than the global average.

5

We generated the phase-velocity coverage maps depending on the frequency. A generalfeature of the phase velocity distributions varies significantly from the west to east of Turkey. The
fastest phase speed is observed in the northern part of Turkey; i.e., the Marmara Region. The phasespeed becomes slow in the eastern part of Turkey.
To estimate regional variations of the phase speed, we have done two kinds of analysis using
several corridors of the Rayleigh wave paths. The phase speeds of the northeastern and southwestern
Turkey are determined to be slower than the northwestern Turkey in both studies.
The fast velocities in the northwestern part of Turkey (Marmara region) may be interpreted by
the cooling of the upper mantle due to dipping oceanic lithosphere of Black Sea under the southward
Marmara region (Gulen and Kuleli, 1995). The slow phase velocities in the eastern Turkey can be
explained by geological features of this area such as wide spread of young volcanism (< 2Ma), hot andrising asthenosphere or thin litospheric mantle and the presence of high heat flow (40-140mW/m2)(Tezcan 1995; Yürür and Chorowicz, 1998; Şengör et al., 2003; Gök et al., 2007). A variety ofevidences from previous studies can be strong supports for a significant regional variations of theRayleigh wave phase velocity obtained in this study.
ACKNOWLEDGMENT
I would like to express my sincere gratitude to the adviser T. Hara for their support during the courseof this study. We would like to thank the Boğaziçi Unv. Kandilli Observatory and Earthq. Res. Inst.
for making data available on their web site. In the study many figures were made using GMT (Wessel
and Smith, 1991).
REFERENCES
Al-Lazki, et al., 2004, Geophys. J. Int., 158, 1024-1040.
Barış, Ş., et al., 2005, Earth Planets Space, 57, 11, 1019-1038.
Barka, A., and R. Reilinger, 1997, Annali di Geofisica, XL (3), 587-610.
Canıtez, N., and M. N. Toksöz, 1980, , Eos Trans. Am. Geophys. Un., 61, 290.
Debayle, E. et al., 2001, , Earth and Planet. Sci. Lett., 193, 423-436.
Dziewonski, A., M. and A, L., Hales, 1972, Seismology, Surface W. and Earth Oscil., 39-85. Vol 11
Dziewoneski, A. M., and D. L. Anderson, 1981, Phys. Earth Planet Interr., 25, 257-356
Ekstrom, G. et al., 1997, J. Geophys. Res., 102 (B4), 8137-8157.
Gök, R. et al., 2003, Geophy, Res. Lett., 30, 24, 8042.
Gök, R et al., 2007, Geophys. J. Int., 169, 1029-1088.
Gülen, L., and H. S. Kuleli, 1995 , EOS, Trans, AGU, 76, 622-623.
Horasan, G. et al., 2002, Bull. Seis. Soc. Am. 92, 322-329.
Ketin, İ., 1969, Bull. Min. Res. Explor. Inst. Turkey, 72, 1-28.
Maggi, A., and K. Priestley, 2005, Geophys. J. Int. 160, 1068-1080.
McClusky, S et al., 2000, J. Geophys. Res. 105 5695-5719.
McKenzie, D., 1972, Geophs. J.R. astr. Soc., 30, 109-185.
Mindevalli, O. Y., and J B. Mitchell, J. B., 1989, Geophys. J., Int., 98, 93-106.
Nataf, H. C. and T. Ricard, 1996, Phys. Earth planet inter., 95, 101-122.
Rodgers, J. et al., 1997, Bull. Seis. Soc. Am., 87, 2, 396-413.
Şengör, A. M. et al., 2003, , Geophys. Res. Lett., 30, Issue 24, 8-1.
Tezcan, A. K., 1995, A., A. Balkema, Rotherdam, 23-42.
Villasenor, A. et al., 2001, , Phys. of the Earth and Planet. Int., 123, 169-184.
Wessel, P., and W. . F. Smith, 1995, , Eos, Trans. AGU 76,329.
Yürür, T. M, and J. Chorowiez, 1998, Journal of Volcanology and Geoth. Res., 85, 1-15.
Zor, E. et al., 2006, Geophys. J. Int., 167, Issue 1 213-222.

6