PAGEOPH, Vol. 135, No. 3 ( 1 9 9 1 )
0033-4553/91/030401-2051.50 +0.20/0
9 1991 Birkh/iuser Verlag, Basel
3-D Velocity Structure Beneath the Crust and Upper Mantle of
Aegean Sea Region
G. DRAKATOS l a n d J. DRAKOPOULOS l
Abstract--The region of the Aegean Sea and the surrounding areas in the Eastern Mediterranean
lies on the boundary zone between the Eurasian and the African plates. It is a zone of widespread
extensive deformation and, therefore, reveals a high level of seismicity.
Three-dimensional velocity structure, beneath the crust and upper mantle of the region between
33.0~176
and 18.0~176
is determined.
The data used are arrival times of P-waves from 166 earthquakes, recorded at 62 seismological
stations. In total, 3973 residual data are inverted.
The resultant structure reveals a remarkable contrast of velocity. In the top crustal layer, low
velocities are dominant in Western Turkey and on the Greek mainland, while a high velocity zone is
dominant in the Ionian Sea and in the southern Aegean Sea.
In the upper mantle, high velocity zones dominate along the Hellenic arc, corresponding to the
subducting African plate and in the northern part of the region, corresponding to the subducting African
plate and in the northern part of the region, corresponding to the margin of Eurasian plate.
A low velocity zone is dominant in the Aegean Sea region, where large-scale extension and volcanic
activity are predominant, associated with the subduction of the African plate.
Key words: Seismic tomography, Aegean, velocity structure, Greece, Mediterranean.
Introduction
T h e region o f the A e g e a n Sea a n d the s u r r o u n d i n g areas in the E a s t e r n
M e d i t e r r a n e a n lies o n the b o u n d a r y zone between the E u r a s i a n a n d the A f r i c a n
plates a n d consists o f the A e g e a n a n d s o m e small plates (MCKENZIE, 1970, 1972;
PAYO, 1976; DEWEY a n d SENGOR, 1979).
T h e A f r i c a n plate is s u b d u c t i n g u n d e r the A e g e a n p l a t e with a d i p o f 3 0 0 - 4 0 ~
(PAPAZACHOS a n d COMNINAKIS, 1971; PAPAZACHOS, 1973). V o l c a n i c activity
a s s o c i a t e d with the s u b d u c t i o n is f o u n d (NINKOVITCH a n d HAYS, 1972; FYTIKAS et
al., 1976) a l o n g the A e g e a n volcanic arc ( F i g u r e 1).
A large positive a n o m a l y o f gravity is o b s e r v e d in the A e g e a n Sea, while
I National Observatory of Athens, Seismological Institute, P.O. Box 20048, GR 118-10, Athens,
Greece.
402
G. Drakatos and J. Drakopoulos
PAGEOPH,
NORTHANt-TOLIAN TRANSFORM
NOR'fH STRAND ~--~------~'-'~t'--.,--.~,J T H S TR.AI'tO
.
0
s,~o~
\
40c~N
GULF OF
38ch
.
d"
. K,.
sc~mo.C ~oOF.S
B6o~
20oE
22%
24~
2~
28o~
Figure 1
Simplified summary of Greek tectonics, following HASHIDA et al. (1988). The heavy and broken lines
indicate faults and poorly defined faults, respectively. The stippled areas show Neogene-Quaternary
grabens. Solid triangles are volcanoes.
negative anomalies are observed in Turkey and on the Greek mainland (MAKRIS,
1976, 1978a).
Geomagnetic anomalies have been observed along the volcanic arc and in the
North Aegean Trench (VOGT and HIGGS, 1969; MAKRIS, 1973).
Heat flow data indicate a high heat flow with a mean value of 2.1 HFU in the
northern and central Aegean Sea (JONGSMA, 1974).
Crustal and upper mantle structure in the Eastern Mediterranean region was
studied by many investigators. The crustal thickness beneath the Aegean Sea is
estimated about 35 km, beneath Central Greece between 36 km and 42 km and
under Macedonian and southern Yugoslavia between 31 km and 47 km (CALCAGNILE et al., 1982). The same investigators also proposed that the thickness of the
lithosphere beneath the central and eastern Mediterranean lies between 51 km and
Vol. 135, 1991
3-D VelocityStructure
403
130 km. MAKRIS (1978b) found that crustal thickness under Peloponnesus is about
46 km and about 26 km in the central Aegean Sea.
SPAKMAN et al. (1988) have used a combination of regional and teleseismic data
in order to determine the major features of the upper mantle in the Hellenic
subduction zone using a tomographic technique.
This paper is an attempt to thoroughly investigate the velocity anomalies in the
upper part of the lithosphere in the Aegean Sea region, using local events. The
resolution and the reliability of the tomographic results strongly depend on the
degree of intersection of crossing rays. Teleseismic rays illuminate the deep mantle
structure but the resolution in tomography decreases with depth when teleseismic
data are used.
HASHIDA et al. (1988) investigated the structure of the crust and upper mantle
in the Aegean Sea region using a tomographic technique, but their data set includes
intensities data and not travel times residuals.
In this study local events have been used in combination with a dense station
network to achieve the maximum degree of crossing rays.
Method and Data
In the present study, the method proposed by AKI and LE~ (1976) has been
used.
Writing observed minus calculated time (O - C) as a vector d, we can write
d=Am
+e
(1)
where, A is a coefficient matrix consisting of partial derivatives of travel times and
e is the error vector.
The solution is given, using a damped least-squares method by
m = (.,~A + O ) - l A d
(2)
where, 0 is a diagonal matrix with positive elements qb (LEVENBERG, 1944),
Therefore, the resolution matrix is given by
R = (AA + 0) -1.~A.
(3)
As shown by WIGGINS (1972), the magnitude of diagonal element is a measure
of the resolution.
The covariance matrix D is given by
D = ~R(2A
+ O) - i
where, crj2 is the variance of errors in the data.
(4)
404
G. Drakatos and J. Drakopoulos
PAGEOPH,
Earth's crust and upper mantle, between 33.0~176
and 18.0~176
are divided into four layers, each of 40 km thickness. Each layer is divided into
10 x 10 rectangular blocks in E - W and N - S directions, almost parallel to the
tension axis (N-S) of the area. The block size is 110 km • 114 km in E - W and
N - S directions, respectively (Figure 2).
Sixty-two stations, located in the above-mentioned region, are used. Table 1
shows the parameters of their locations.
166 earthquakes are selected from ISC bulletins. The focal depths lie between
0 km and 160 km (Figure 2). We added records from local stations which do not
routinely report to the ISC in order to achieve the maximum degree of crossing rays
in the upper part of the lithosphere.
r
23.0
tB.o
ij3. o
":~a.0
9
R3.G
C
~_k c~-~
~8.0
-:~"a. 0
9
'~:].O
!
d
23.0
tS,0
~. f.glCElflT.l~
ZI.0
33.0
I ~ R~tars
qt,'At.t | l I ~ G
Figure 2
Map of Greece and surrounding regions is shown. Polygons, squares and triangles represent epicenters
of used earthquakes with focal depths less than 40 km, between 40 km and 80 km and greater than
80 kin, respectively. The two block configurations, are also shown.
Vol. 135, 1991
3-D Velocity Structure
405
Table 1
Seismological Stations
J
N
Code
v'N
1
2
3
4
5
6
7
8
APE
&RG
ATH
ITM
JAN
KZN
NPS
PLG
PRK
PTL
RLS
VAM
VLS
GaG
KNT
LIT
OUR
PAIG
SOH
SRS
THE
VAG
37.07
36.22
37.97
37.18
39.66
40.31
35.26
40.37
39.25
38.05
38.06
35.41
38.18
40.96
41.16
40.10
40.33
39.93
40.82
41.12
40.83
38.32
39.24
39.45
38.71
39.31
38.78
38.88
39.11
38.20
39.06
1:
II
12
13
14
15
11
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
VFI
VGL
VMA
VNE
VPA
VSI
VSK
VTH
ALT
~'E
31.53
28.13
23.72
21.93
20.85
21.77
25.61
23.45
26.27
'23.86
21.47
24.20
20.59
22.40
22.90
22.49
23.98
23.68
23.35
23,59
22.96
22.90
22.59
22.88
23.59
23.23
22.34
23.21
23.69
23.36
30.11
Alt(m)
S20
170
95
400
540
900
370
580
100
500
100
225
375
560
380
480
60
140
670
400
70
760
360
424
468
692
1084
448
374
756
1060
N
32
33
34
35
36
!~ 37
I
38
';,~ 3 9
40
41
42
43
44
45
46
47
i
48
1
49
i
50
' 51
52
53
54
55
56
57
58
59
60
61
; 62
Code
CIN
CTT
DST
EDC
ELL
EZN
GET
HRT
ISK
IST
IZM
MFT
YER --;
DIN
DMK
KDZ
VTS
BCI
KKS
OHR
PUK
SKO
TTG
VAY
KBN
PHP
SDA
SRN
TIR
VLO
LCI
yen
~'E
37.60
41.15
39.61
40.35
36.75
39.83
40.11
40.82
41.07
41.04
38.40
40,79
37.13
42.05
41.82
28.09
28.43
28.63
27.86
29.91
26,33
27.57
29.67
29.06
28.98
27.26
27.28
28.28
25,58
27.76
25.35
23.20
20.07
20.41
20.80
19.89
21.44
19.26
22,57
20.81
20.44
19.50
20.00
19.87
19.50
18.11
41.64
42,60
42.37
42.08
41.11
42.04
41.97
42.43
41,32
40.62
41.69
42.02
39,88
41,35
40.47
40.33
Alt(m)
1
324
685
269
1230
49
590
645
132
65
631
924
729
1
315
329
739
346
40
168
197
Inversion Procedure and Results
In total, 3973 residual data are inverted. The number of unknown parameters is
853. The value of damping factors for the X, Y, Z coordinates of the hypocenters,
for the origin time and for the velocity is set equal to 10.
As initial values of velocities, 7.20km/sec, 7.75km/sec, 8.10km/sec and
8.20 km/sec are assigned to the four layers. Then, ray paths from each event to
station and travel time through each block are calculated. In Figure 3 ray paths are
shown from hypocenters to stations, which penetrate each block. In the first three
layers ray coverage is good. In the bottom layer ray coverage is rather poor, due to
the small number of deep events.
The data are inverted three times. Their standard deviation before the inversion
was 2.60 sec and took the values of 2.16 sec, 1.99 sec and 1.90 sec after the first, the
second and the third inversion, respectively. Then, solution, resolution and covariance matrices are computed.
In order to obtain slight variations of the velocity in the horizontal direction, the
whole block configuration is shifted by half block size to NW direction (Figure 2).
406
G. Drakatos and J. Drakopoulos
PAGEOPH,
D ~
m
z
sl
Vol. I35, 1991
3-D Velocity Structure
407
..=
m
.a
o
~o
II
O
~
r
z
..=
,r
"~
408
G. Drakatos and J. Drakopoulos
PAGEOPH,
Again, ray paths from each event to station and travel time through each block are
calculated. The number of unknown parameters is 857.
The data are inverted three times. The standard deviation before the inversion
was 2.60sec and 2.17sec, 2.01 sec and 1.92sec after three inversions. Again
solution, resolution and covariance matrices are computed.
In Figure 4, the solution of the inversion is shown for each block of the four
tLaYER I
(a)
-L+'.S
I':1..~ -L.l.!]
- 3 . 4 1"28.2
1"15.7
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+3.4
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1-1.3 -+1.9 + 9 , 4 /
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1-11.1 -0+,I 1-13.1 1-10.4
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','L.I
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1-3+0 - 1 . I I
*7.2
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+1.7
/
/
LAT~ 2 (b)
-9.0
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~'S.9 1"14.3 - 9 . 0
-1.9
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I.l.S
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1-0.3 - 3 . 2
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1-2,1 - 2 , 0
-1.9
1-1.4 - 0 . 0
-(1.9
-0.4
Figure 4(a)
/
/
-3.5/
Vol. 135, 1991
3-D Velocity Structure
/
LAYER 3,
411.2 -17.8 + 0 . 7
+5.1 ,2.8 - t . 4
~6{..6
-33.6
+9.6
~9.7
+2.7
(a)
+t,9
-1.1
+4.5
-0.0
-1,0
e2.6
+3.5
-3.3
-16.3 44.6 43.5 +2.6 ,8.1 +'/.5 -1.0
-0.9 --0.5 ~6.0 ~1.7 ~10.4 -1.3 40.!
48207 -14.8 +X.3 +2..* +6.4
3
LAYE~
/
409
i'6.2
~'8,8
+3,1
+4.2
+2.0
~-0.6
LAYER q
.(a)
+9.7
-z.7
+5.4
+z
(b)
-3.3
+5.6
-15.1
/
//
4
+0-.91 ~-4.6 -0.6 ~4,9 r
44.5 +0.7 42.5
+2,7
/
/
/
-0.4
-1.6
-3.0 +8.3 +2.8 45.4 1-2.5 -0.8
-2,7 +1.8 I - 5 , 0 4-7.0 43.1 ~-3.9
+1.1 +4.9 47.0 +2,8 +7.9 +2.3
-1.0 -7~1.2 -41.4 t.2.6 -1.8
-58.9
LAYER ~
~
/
+0.2
-61,0
-6.z
/ /
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(b)
415.5 -0.8 - 8 . 9
-67.6 -15.7 ~0.7 -6.0 -2.6
9
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J
/
+2,0
+0.3
~2.6
+0.{.
Vl.8
+5.t
-8.7
+3.6
,3.9
+7.5
Figure 4(b)
Figure 4
The solution of the inversion is shown, as slowness perturbation, in each block of the four layers for the
first (a) and second (b) block configurations. Positive ( + ) and negative ( - ) values correspond to lower
and higher values of the initial velocity. The unit is 10 -2 sec/km.
410
G. Drakatos and J. Drakopoulos
PAGEOPH,
layers, for the first (a) and second (b) block configurations. The solution is given as
slowness perturbation. The unit is 10-2sec/km. Positive ( + ) and negative ( - )
values correspond to lower and higher velocity from the initial value.
In Figure 5, the diagonal elements of resolution matrix are shown for each block
of the four layers, for the first (a) and second (b) block configurations. A large
~AYE8 I
(i)
.62 0 . 9 ?
0.93
0.87
0.96
0.87
0.74
J
68
0.99
0,99
0.99 0.99
0,99
0.99
0.94
/
94 0 , 9 9
0.99
0.99 099
0 99 0 . 9 9
0.99 0.99 0.44/
8 0.98
0.99
0.99
0.99
0.99 0.99
0.99
0.98 0.90/
0.97
0.99
0.99
0.99 0.99
0.99
0.99
0.98
0.90/
0.63
0.99
0.99
0.99
0,99 0.99
0.99
0.98
0.86/
0.79
0.98
0.99 0.99 0.99
0.99
0.98
0.87,/
0.90
0.99
0.99
098
0.96
0.21
/
0.66
0.9?
0.93 0.96
0.19
/
0.36
0.44
0.02
/
/
o1"7"
/
/0.68
/0.63
/
/
/
/
065
0.98
0.99
0,99 0,9?
0.9?
0.99
0.99 0.84
/
0.95
0,99
0.99 0.99 0.99
0.99
0.99
0.99
0.84/
0.95
0.99
0.99
0.99 0.99
0.99
0.99
0.99 0.93/
0.82
0.99
0.99
0.99 0.99
0.99
0.99
0.99 0.98/
0.04 0.69
0.99
0.99 0.99
0.99
0.99
0.99 0.93/
0.29
0.99
0.99 0.99
0.99
0.99
0.99 ~.98/
o.9o
0.70
/
0.97
o.99
o.99
0.95 0.98 0,98
0.930.500.890.26
LAVE9 2
/
/0.03
/0.37
/
/
/
/
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/
o.99
o.96
/
0.64/
0.84
/
(a)
0.96
0.98 0.96
0.97 0.92
0.94
0-59
/
0.98
0,99 0.99 0.99
0.99 0.99
0.98
0.65
/
0.98
0,99 0.99 0.99 0.99
0.99
0.98 0,91
/
0.93
0.99 0.99 0.99 0.99
0.99
0.98 0.93
/
0.34
0.99 0.99
0,99 0,99
0.90
0.99
0.94
/
0.96
0 . 9 9 0 . 9 9 O. 99 0 . 9 9
0.99
O. 90
/
0.65
0.98 0.99 0.99
0.99
0.98
0.36
/
0.910.900.960.940.39
/
LATE9 2
(b)
0.64
0.60 0.69 0.62
0.69
o.37
0.20
/
0.90
0.99 0.99 0.99
0.96
0.98
0.98 0.77
/
0.99
0.9~ 0.90 0.99
0.99
0.93
0.99 0.95
0.54./
0.68
0.99 0.99 0.99
0.99
0.99
0.99
0,96 0.65/
0.02
0.98 0.99 0.90
0,99
0.99 -0.99
0.97
0.68/
0.8? 0.99 0.99
0,99
0.99
0.99
0.97 0.49/
0.96
0,09 0.99 0.98
0.99
0.94 0.04/
0.83 0,96 0.99
0.98
0.98
0.75
/
0.22
0.30
0.70
/
Figure 5(a)
411
3-D Velocity Structure
Vol. 135, 1991
LAYER
/
0.70
0.87
0.71
3
002
(a)
0.10
/
0 52
0 62 0 97 0,94 0.82
0.70 0 5 5
0.09
0.63 0.98 0 98 0.94 0 8 9
0.76
094
0.95 0.96 0 . 9 8
093
0.92
0.77
/
0.03
0.97 0.97 0.97 0.87
0.97 0.79
0,78 0.90 0.90 0.97 0.63
/
0.43
/
LAYER 3
0.78
0.53
0.30
0.89
0.90
0.10
0.8~
(b)
0.66
/
0.33
/
0.10
0,20
0,95 0 98 0.95
0.82
O.flO
0.71
0.90 0 9 0
0.98 0.91
0.92
0.71
0.43 0.93 0.98 0,97 0.97 0 . 9 7 0.96
0.91 0 . 9 7
0.93
0.98 0.95 0 . 6 7
0.05 0 . 3 l
0.69 0.99
0.68
0.34
LAYE~
/
/
/
4 (a)
/
0.711 0.88 0 . 4 6 0.30 0 . 0 3
0.12 0.90 0,91
0.94 0 . 7 9
0 . 8 0 0.95 0 . 8 2
0.42
LAYZI 8 ( b )
O. 42
0.~14 0 . 0 0
0.36
0.00
/
0.00
0.00 0.32
0.06
0.05
0.04
0.08
0.74
Figure 5(b)
Figure 5
Diagonal elements of resolution matrix for path parameters, in each block of the four layers, for the first
(a) and second (b) block configurations.
412
9
G. Drakatos and J. Drakopoulos
PAGEOPH,
number of unknown parameters causes nonunique and unstable solutions but the
main good measure of the uniqueness of the ith component of the solution
parameters is the ith diagonal element of resolution matrix. Considering as reliable
solutions those with the diagonal elements of resolution matrix higher than 0.5
(HORIE, 1980), it is possible to estimate that the resolution for most blocks is good.
The standard errors of the solution (Figure 6) are very low. The unit is
10 -2 sec/km.
LAYER
/
1
(a)
11.7 0 . 2 0 . 5 0 . 9 0 . 3 0 . 9 1 . 6
/
.7
0.1
0.0
0.1
0.1
0.0
0.1
0.1
0.5
0.5
0.1
0,0
0.0
0.0
0.1
0.1
0.1
0.1
2 1 /
1,4
0.1
0.0
0.0
0.0
0.0
0.0
O.I
0,1
07
/
1,6
0.1
0.0
0.0
0.0
0.1
0,0
O,l
1.4
0,2
0.1
0.0
0.1
0.1
0.2
0.7
0,I
0.1
0.I
0.3
14
17 i?/ 0~o?i,
LAYER
i
0.9
0.9
'~
(b)
0.9
1,9
/
0.1
(,.0
0.1
0.2
0.2
0.1
0.1
1.1
0.4
0,0
0.0
0,0
0.1
0.1
0.1
0,1
0,9
0.3
0.0
0.0 0.0
0.0
0.0
O0
0.i
0.5/
1.1
0.0
0.0
0.0
0.0
0.0
0.0
0.1
0.2/
0.3
0.1
0.0
0.0
0.0
0.1
0.0
0 1 0.5
1.7
0.1
0.1 '0.1
0.0
0.I
0,1
0.I
0.7
0.2
0.1
0.1
0.1
0.2
1 . 9 /
/
1.6
0.3
0.1
O.I
0.7
0.4
2.1
0."/
1.1
/i ,
8
s
5/
LAYEI T ( I )
/
/0.2
/ z.e
/
/
/
/
/ , . e
0.3
0.2
0.3
0.2
0.9
O.tl 1 9
O.l+
0.0
0,1
0.I
0.I
0.I
O.Z
1.8
o.9
o.o
o.o
o.o
o.o
o+t
o,z
o.9
o.4
o.o
o+o
o.o
o.o
o o o.t
o,5
o.z
o.o
o.o
o.o
o.t
o.1
/
/
/
/
o.s
o.3 o.1 o.o o.o o.o o.t o.+
1.9 o.z o.z o.z 0.1 o., 1.,
o.s o., o., o., t . ,
/
/
/
/
LAYER 2 ( b )
, . o . 1 9 , , 1 . 1 ,
9
,
0.2
0.1
0.1
0.1
0.1
0.1
O.Z
1.4
"0,1
0.1
0.0
0.0
0.0
0.1
0.1
0.4
1
0.7
0.1
0,0
o.0
o.0
0.1
0.1
o.3
1 .
o.t
o.t
0.o
o.0
0.0
0.0
0.1
O.Z
1 .
0.8
o.1
o.0
0.o
0.0
0.1
o,z
1 1
0.3
O.t
0,0
0.0
0.1
0.4
0 . 3
O.I
0.3
0.1
0,1
0.3
1.4
/
1.3
1.8
1.8
Figure 6(a)
/
/
. 0 /
1 1 /
8 /
/
/
Vol. 135, 1991
3-D Velocity Structure
/
LAYER
3
/
(a)
m
l.S
2.0
0.7
1.9
1.9
0.4
0.3
1.7
0.2
0.3
0.4
0.2
0.4
0.1
0.6
1.1
O.S
/
I..7
1.9
1,2
0,7
0.2
0.2
0.5
0.6
0.2
0.3
0.3
0.3
0,2
11_3_7 O 7 0 . 7
0.2
1.7
3
LAYER
/
413
0.7
/
/
/
1.,i
1.1
(b)
/
/
2rl
0,8
0".'7
/
1,2
1,7
0.1
1.1
1.9
1.6
0.7"
13
0.4
0.I
0,4
1.1
1.1 '1.4
0.8
O,1
0,2
0.7
0.1
1.~
1.9
0.5
0.2
0.2
0.2
0,2
03
0.6
0.2
0.5
0.2
0.4
l.~
0.4
t.4
1,7
1.7
[..1
LAYER
t.6
g
4 (a)
/
I..4
"0.9
0.8
0.7
/
/
2.1
1.5
0.2
0.7
0,4
1.1
0,7
0.3
1.1
1.8
/
LAYER q ( b )
/
~176 1 . 3
|.9
I.O
0.8
0.3
1.0
1.4
0.3
0125
/
.
L.5
Figure 6(b)
Figure 6
The standard errors of the solution are shown, in each block of the four layers, for the first (a) and
second (b) block configurations. The unit is 10-2 sec/km.
414
G. Drakatos and J. Drakopoulos
PAGEOPH,
Discussions
A remarkable contrast appears in the velocity structure of the top crustal layer
(Figure 7a). Low velocity is predominant in western Turkey, in the central and
southern Aegean Sea and in Peloponnesus, while high velocities are predominant in
western Greece, in the southwestern Aegean Sea and in the north Aegean trench.
The low velocities in the central and southeastern Aegean Sea nearly correspond
to the distribution of the Neogene-Quaternary grabens (Corinthian Gulf, Sporades,
Kerme, Carpathos). The crust of these regions consists of soft materials of relatively
high temperature. This fact is supported from heat flow data in these regions
(JONGSMA, 1974; MAKRIS, 1978a), from the low Q-value (HASHIDA et al., 1988)
and from the thinning of the crust in the Aegean region (PAPAZACHOSet al., 1986).
Low velocities are also predominant along the Aegean volcanic arc. An exception is the high velocity spot in the geothermal area of Milos island. The same
exception was pointed out by HASHIDA et al. (1988).
LA .~ER
1
43.0 N
38.0 l~
33.Ol'
lg.O E
23.Q E
Figure 7(a)
28.0E
Vol. 135, 1991
3-D Velocity Structure
L A':'~ R
43.0 N
I
I
A~
8'
A
.B
415
2
38.0N-
"D
_C
33,0t'
18.0E
28.0 E
:13.0 E
b
Figure 7(b)
The low velocity zone in western Turkey corresponds to the extensive fault
zones of the region, where every year numerous shallow earthquakes occur.
The appearance of high velocities in the region of the north Aegean trench is a
remarkable fact, as was also pointed out by CHRISTODOULOU and HATZFELD
(1988). They interpreted this fact as the result of the crust thinning, due to a largescale extension which occurs in the region (LYBERm and DESCHAMPS, 1982).
The appearance of high velocity spots in the region of the Rhodes and
Karpathos islands should be pointed out. The same spots were determined by
HASHIDA et al. (1988). This fact is supported by the existence of evidence of
African plate subduction in the region (WYsS and BAER, 1981).
As in the crustal layer, the boundaries between velocity zones are well defined in
the second (Figure 7b) and third layer (Figure 7c), due to the strong lateral
variation of velocity.
In the southwestern part of the region, a high velocity zone is determined, which
corresponds to the African slab. This is also supported from heat flow data and
416
G. Drakatos and J. Drakopoulos
PAGEOPH,
LA'fE R 3 .
43.0 N
AJ
B'
38.0 N-
0
r
H,/,
33.0N
la,OE
A
1 I
23.OE
B
I
i
2a.OE
F i g u r e 7(c)
from the high Q-value in the region. High velocities are also predominant in the
margin of the Eurasian plate.
An extensive low velocity zone is predominant in the Ionian Sea, in western
Turkey and in the Aegean Sea. This suggests a large-scale absorption of seismic
energy in the region. The distribution of macroseismic intensities in the central
Aegean Sea suggests a high attenuation in the inner part of the volcanic arc
(DRAKOPOULOS,1978). Based on attenuation of S-waves, DELIBASIS (1982) supports the existence of magmatic material immediately beneath Moho. A high
viscosity zone exists in the area, which corresponds well to the aseismic region of
the Aegean Sea. The high attenuation zone exists beneath the isodepth of 100 km,
while beneath the volcanic arc this zone appears immediately beneath the Moho
(TASSOS, 1984). This is also supported from the low Q-value in the region, which
is about 50-60 (TASSOS, 1984; HASHIDA et aL, 1988).
The high velocity zones in the southern and western part of the region
correspond well with the African slab.
Vol. 135, 1991
3-D Velocity Structure
417
LAYER
J
43.0 N
A'
L,
38.0N
-D
c'-
--C
3 3 . 0 I~
18.0E
t
A
B
I
I
23.0 E
2 8.0 E
Figure 7(d)
Figure 7
Velocity zones determined by the inversion. Shaded and blank areas indicate zones of high (H) and low
(L) velocity, respectively. Heavy lines include blocks in which the diagonal elements of resolution matrix
are greater than 0.5. Broken lines indicate boundaries between velocity zones. (a) Layer 1 (depth:
0-40km); (b) Layer 2 (depth: 40-80km); (c) Layer 3 (depth: 80-120km); (d) Layer 4 (depth:
120-160 kin).
Although there is a small number of used events in the fourth layer, two velocity
zones are determined (Figure 7d), as follows: i) a low velocity zone and ii) a high
velocity zone, which indicates the African slab.
In order to determine the vertical distribution of velocity, four cross-sections
(Figure 8) are drawn, two of N - S direction ( A A ' and BB') and two of E - W
direction (CC" and DD'). In the cross-sections AA" and BB' a high velocity
zone dominates in the northern part of the eastern Mediterranean basin. A second
high velocity zone of N - S direction is determined and coincides with the African
slab. The same zone appears in cross-sections CC' and DD' and is determined
also by HASHIDA et al. (1988) and SPAKMAN et al. (1988). In cross-section CC' a
G. Drakatos and J. Drakopoulos
418
PAGEOPH,
m
A
40
M
8O
ZOOM)
120
W
180
B
Ro
40
80
120
160
r
r
D
D"
0
40
80
120
160
0
40
80
120
160
Figure 8
Four cross-sections of the velocity structure, whose locations are shown in Figure 7. Heavy lines include
only the well resolved regions. Shaded and blank areas indicate zones of high and low velocity.
Vol. 135, 1991
3-D Velocity Structure
419
low velocity zone is shown which dominates the inner part of the Aegean volcanic
arc and absorbs the greater part of seismic energy.
Conclusions
Three-dimensional velocity structure was determined by the inversion of travel
time residuals beneath the Aegean Sea and the surrounding regions. The results
suggest the existence of three main velocity zones, as follows: i) a high velocity zone,
in the western part of the region, which corresponds to the subducting African
plate, ii) a high velocity zone, in the margin of the Eurasian plate and iii) a low
velocity zone in the region of the Aegean Sea, where a large-scale extension and
volcanic activity are taking place.
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(Received April 19, 1990, revised November 1, 1990, accepted November 22, 1990)