3-D velocity structure beneath the crust and upper mantle ... · 26 VNE 27 VPA 28 VSI 29 VSK 30 VTH...

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 and 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

The region o f the Aegean Sea and the su r round ing areas in the Eas te rn

Med i t e r r anean lies on the b o u n d a r y zone between the Euras ian and the Af r ican

plates and consists o f the Aegean and some small plates (MCKENZIE, 1970, 1972;

PAYO, 1976; DEWEY and SENGOR, 1979).

The Afr ican pla te is subduc t ing under the Aegean p la te wi th a d ip o f 300-40 ~

(PAPAZACHOS and COMNINAKIS, 1971; PAPAZACHOS, 1973). Volcanic act ivi ty

assoc ia ted with the subduc t ion is found (NINKOVITCH and HAYS, 1972; FYTIKAS et

al., 1976) a long the Aegean volcanic arc (F igu re 1).

A large posi t ive a n o m a l y o f gravi ty is observed in the Aegean 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,

NORTH ANt-TOLIAN TRANSFORM NOR'fH STRAND ~--~------~'-'~t'--.,---

.~ ,J T H S TR.AI'tO .

40c~N

\ 0 s,~o~

GULF OF

38ch

B6o~

20oE

. d " . K , .

sc~mo.C ~oOF.S

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 (CALCAG-

NILE 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 Velocity Structure 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 = A m + 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 ( 2 A + O) - i (4)

2 is the variance of errors in the data. where, crj


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 tB.o i j3. o

23.0 ":~a. 0 R3.G

~ 8 . 0 -:~"a. 0

'~:].O 3 3 . 0 tS,0

�9 C

~_k c~-~

�9

d 23.0 ZI.0

~. f.glCElflT.l~ 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 Veloc i ty S t r u c t u r e 405

J

N Code

1 APE 2 &RG 3 ATH 4 ITM 5 JAN 6 KZN 7 NPS 8 PLG

1 : PRK PTL

I I RLS 12 VAM 13 VLS 14 GaG 15 KNT 11 L I T 17 OUR 18 PAIG 19 SOH 20 SRS 21 THE 22 VAG 23 VFI 24 VGL 25 VMA 26 VNE 27 VPA 28 VSI 29 VSK 30 VTH 31 ALT

T a b l e 1

Seismological Stations

v 'N ~'E A l t ( m ) N Code y e n ~ ' E

3 7 . 0 7 3 1 . 5 3 S 2 0 32 CIN 3 6 . 2 2 2 8 . 1 3 1 7 0 33 CTT 3 7 . 9 7 2 3 . 7 2 95 34 DST 37.18 21.93 400 35 EDC 39.66 20.85 540 36 ELL 4 0 . 3 1 2 1 . 7 7 9 0 0 !~ 37 EZN 35.26 25.61 370 I 38 GET 4 0 . 3 7 2 3 . 4 5 5 8 0 ';,~ 39 HRT 3 9 . 2 5 2 6 . 2 7 1 0 0 40 ISK 38.05 ' 2 3 . 8 6 500 41 IST 3 8 . 0 6 2 1 . 4 7 100 42 IZM 3 5 . 4 1 2 4 . 2 0 2 2 5 43 MFT 3 8 . 1 8 2 0 . 5 9 3 7 5 44 YER 4 0 . 9 6 2 2 . 4 0 5 6 0 45 DIN 4 1 . 1 6 2 2 . 9 0 3 8 0 46 DMK 4 0 . 1 0 2 2 . 4 9 4 8 0 47 KDZ 4 0 . 3 3 23.98 60 i 48 VTS 3 9 . 9 3 2 3 . 6 8 1 4 0 1 49 BCI 40.82 23.35 670 i 50 KKS 41.12 23,59 4 0 0 ' 51 OHR 40.83 22.96 70 52 PUK 38.32 22.90 760 53 SKO 39.24 22.59 360 54 TTG 3 9 . 4 5 22.88 424 55 VAY 3 8 . 7 1 2 3 . 5 9 4 6 8 56 KBN 3 9 . 3 1 2 3 . 2 3 6 9 2 57 PHP 38.78 22.34 1084 58 SDA 38.88 23.21 448 59 SRN 39.11 23.69 374 60 TIR 38.20 23.36 756 61 VLO 39.06 3 0 . 1 1 1060 ; 62 LCI

37.60 41.15 39.61 40.35 36.75 39.83 40.11 40.82 41.07 4 1 . 0 4 3 8 . 4 0 40,79

--; 37.13 42.05 41.82 4 1 . 6 4 4 2 , 6 0 42.37 42.08 41.11 42.04 41.97 42.43 41,32 40.62 41.69 42.02 39,88 41,35 4 0 . 4 7 4 0 . 3 3

A l t ( m )

28.09 1 28.43 324 28.63 685 27.86 269 29.91 1230 26,33 49 27.57 590 29.67 645 29.06 132 28.98 65 27.26 631 27.28 924 28.28 729 25,58 1 27.76 315 25.35 329 23.20 2 0 . 0 7 2 0 . 4 1 2 0 . 8 0 739 1 9 . 8 9 2 1 . 4 4 3 4 6 19.26 40 2 2 , 5 7 168 20.81 20.44 19.50 20.00 19.87 197 19.50 18.11

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).


D ~

m

z

sl

Vol. I35, 1991 3-D Velocity Structure 407

~ o

II

m

.a

o

z

..=

O ~ " ~

r

..=

,r


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)

+114.7 -L+'.S I':1..~ -L. l . ! ] - 3 . 4 1"28.2 1"15.7 / " ~9.2 - L . 3 - 5 . 1 ~2.1 - 3 . 3 - 7 . 3 - L2 .1 - 7 . 0 1"2.4 / 9 .4 - t . | - 0 . S - t . O t- t .2 - t 0 . l - 7 . 7 - 3 . 0 1".I.3 1 . t 4 . t / .9 -2.O FL,3 ~3.9 ~0,9 - 4 . 6 - 3 . 4 - 5 . 4 - 0 . 7 1..9.8/

0.9.0 - 4 . 3 - 0 . 5 I 'L .9 1-9.3 - 2 . 5 - 2 . 5 ~3.3 0 .3 .4 / - 2 1 . 9 0.8.8 1-6.2 ~3.6 - 10 .6 -t1+4 1-1.5 1-5.9 1 -3 .5 /

1-6.9 -7.9 - 2 9 . 1 - 6 . 2 - 1 . 2 1-4.8 - 9 . 4 1-26.2/ - 1 7 . 3 : '9.4 ~'2.3 1-7.8 -5+fl ~tO,2 / - 3 . 9 - 9 . 6 - 6 . t 0"3.4 1-27.9 /

~39.2 ~29.2 ~12,1 /

LAYER 1 (b)

- 3 8 . 1 1-L7.7 / / - 6 , 5 - 4 . 0 +1.1 - 11 . ( I +3 .4 - 0 . 0 -13,11 +1.11 /

3 . 0 + 7 . 9 - 4 . Z -11.4 " 1-3.1 -S.O - 0 . 6 - 3 . 2 - L . 3 +3 .8 / / +ll - 0 . 7 t '3 .3 1"3.7 43 .3 - I . O - 7 . 3 - 3 . 6 '-~|.3 + 9 . 6 /

1"33.9 - 3 , 2 ~-2 .$ - 0 . 2 ++2.$ +2.3 -+1.7 - 3 . 4 1 .0 .9 / -10.11 - 1 . 1 +4 .2 1-3.9 +1 .0 - 1 . 9 - 0 . I +3.4 - 1 . 3 / /

1'13.3 ~'10.0 - 1 4 . 9 - 7 . 3 -6.11 1-1.3 -+1.9 + 9 , 4 / - 1 | . | - 0 . 1 +0 .1 +4 .8 - 3 . 7 1-3.11 + 2 . 4 / /

- L 3 , l - L I . I I -11.4 - 1 . 4 1'13.0 / 1-11.1 -0+,I 1-13.1 1-10.4 " /

. L A Y E R 2 (a)

-11.3 +4.1 - 2 . 4 - 0 . 7 .1-2.3 - 9 . 4 - 3 , 3 / /

L.O +L.O - 3 . 7 - 1 . 1 - 1 . 1 - 0 . 7 1-0,1 -,~.1 - 1 w / .1 - Z . I 1"3.9 - 1 . 1 ' , 'L. I +0 .1 - 0 . 1 1-6.1 - 1 , L /

+ l l . l + l l . l +O.t - I , I +0 .9 - I , S - I I , I 1 '1,I / 1'0.5 +2.11 - 0 . 6 ~L.L 1-2.0 1-3.1 - I . 0 1'3.2. /

- 4 . 1 +3 .1 +3 .1 - 0 . 7 +1 .7 +4 .1 +6 .9 / - 6 . 0 - I . I I +1 .3 +1 .0 1-3+0 -1 . I I *7 .2 /

- 4 0 . 7 - 1 1 . 0 +11.0 + l . l +1 .7 /

/ LAT~ 2 (b)

- 9 . 0 - 7 . 5 ~'S.9 1"14.3 - 9 . 0 - 1 . 9 +2.9 / r 7 . 9 - 1 . 4 I . l .S - 4 , g - 0 . 1 - 1 . 4 1-0.3 - 3 . 2 /

- ) . 9 . 9 1-2,1 - 2 , 0 - 1 . 9 1-1.4 - 0 . 0 -(1.9 - 0 . 4 - 3 . 5 /

Figure 4(a)


LAYER 3, (a)

/ ~6{ . .6 411.2 -17.8 + 0 . 7 + t , 9 -33.6 +5.1 ,2.8 - t . 4 -1.1 -0.0 e2.6 +3.5

+ 9 . 6 ~ 9 . 7 + 2 . 7 + 4 . 5 - 1 , 0 - 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.! J

48207 -14.8 +X.3 +2..* +6.4 ~ /

/ LAYE~ 3 (b)

/ 415.5 -0.8 -8 .9 /

-67.6 -15.7 ~0.7 -6.0 -2.6 +0.2 -1.6 / �9 i ' 6 .2 ~'8,8 + 3 , 1 + 4 . 2 - 0 . 4 + 2 . 0 ~-0.6 -3.0 +8.3 +2.8 45.4 1-2.5 -0.8

-61,0 -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 q . ( a )

- 6 . z + 9 . 7 - z . 7 + 5 . 4 +z 4 // +0-.91 ~-4.6 -0.6 ~4,9 r

44.5 +0.7 42.5 +2,7

/ LAYER ~ ( b )

-3.3 / + 5 . 6 + 2 , 0 ~ 2 . 6 V l . 8 + 3 . 6

- 1 5 . 1 + 0 . 3 +0.{ . + 5 . t , 3 . 9 -8.7 +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.


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 6 8 0 . 9 9 0 , 9 9 0 .99 0 . 9 9 0 , 9 9 0 . 9 9 0 .94 / 94 0 , 9 9 0 . 9 9 0 .99 0 9 9 0 99 0 . 9 9 0 .99 0 .99 0 . 4 4 / 8 0 . 9 8 0 .99 0 . 9 9 0 .99 0 .99 0 .99 0 . 9 9 0 .98 0 . 9 0 /

0 .97 0 . 9 9 0 . 9 9 0 .99 0 . 9 9 0 . 9 9 0 . 9 9 0 .98 0 . 9 0 / 0 .63 0 . 9 9 0 . 9 9 0 .99 0 ,99 0 . 9 9 0 . 9 9 0 .98 0 . 8 6 /

0 . 7 9 0 . 9 8 0 .99 0 .99 0 . 9 9 0 . 9 9 0 .98 0 . 8 7 , / 0 . 9 0 0 . 9 9 0 .99 0 9 8 0 .96 0 .21 /

0 .66 0 .9? 0 .93 0 .96 0 . 1 9 / 0 .36 0 . 4 4 0.02 /

/ o1"7" 065 / / 0 . 9 8 0 . 9 9 0 ,99 0 ,9? 0 .9? 0 . 9 9 0 .99 0 .84 / / 0 . 6 8 0 .95 0 , 9 9 0 .99 0 .99 0 . 9 9 0 . 9 9 0 .99 0 .99 0 . 8 4 /

/ 0 . 6 3 0 .95 0 . 9 9 0 . 9 9 0 .99 0 . 9 9 0 . 9 9 0 .99 0 .99 0 . 9 3 / / 0 .82 0 . 9 9 0 . 9 9 0 .99 0 . 9 9 0 .99 0 . 9 9 0 .99 0 . 9 8 /

/ 0 .04 0 . 6 9 0 . 9 9 0 .99 0 . 9 9 0 .99 0 . 9 9 0 .99 0 . 9 3 / / 0 . 2 9 0 . 9 9 0 .99 0 . 9 9 0 . 9 9 0 . 9 9 0 .99 ~ . 9 8 /

/ o.9o 0.97 o.99 o.99 o.99 o.96 0.64/ 0 .70 0 .95 0 .98 0 ,98 0 .84

/ 0 . 9 3 0 . 5 0 0 . 8 9 0 . 2 6 /

LAVE9 2 ( a )

/ 0 . 9 6 0 .98 0 .96 0 .97 0 .92 0 .94 0 -59 / / 0 . 0 3 0 .98 0 ,99 0 .99 0 . 9 9 0 .99 0 . 9 9 0 .98 0 .65 /

/ 0 . 3 7 0 .98 0 ,99 0 .99 0 . 9 9 0 . 9 9 0 . 9 9 0 .98 0 ,91 / / 0 .93 0 .99 0 . 9 9 0 . 9 9 0 . 9 9 0 . 9 9 0 .98 0 .93 /

/ 0 .34 0 . 9 9 0 . 9 9 0 ,99 0 ,99 0 . 9 0 0 . 9 9 0 .94 / / 0 .96 0 .99 0 .99 O. 99 0 .99 0 . 9 9 O. 90 /

/ 0 .65 0 .98 0 .99 0 . 9 9 0 . 9 9 0 .98 0 .36 / / 0 . 9 1 0 . 9 0 0 . 9 6 0 . 9 4 0 . 3 9 /

/ LATE9 2 (b )

0 .64 0 .60 0 .69 0 . 6 2 0 . 6 9 o . 3 7 0 .20 / 0 . 9 0 0 . 9 9 0 .99 0 . 9 9 0 . 9 6 0 . 9 8 0 .98 0 .77 /

0 .99 0 .9~ 0 . 9 0 0 . 9 9 0 . 9 9 0 . 9 3 0 . 9 9 0 .95 0 .54 . / 0 . 6 8 0 . 9 9 0 . 9 9 0 . 9 9 0 . 9 9 0 . 9 9 0 . 9 9 0 ,96 0 . 6 5 /

0 . 0 2 0 . 9 8 0 . 9 9 0 . 9 0 0 , 9 9 0 . 9 9 - 0 . 9 9 0 .97 0 . 6 8 / 0 .8? 0 . 9 9 0 . 9 9 0 , 9 9 0 . 9 9 0 . 9 9 0 .97 0 . 4 9 /

0 .96 0 , 0 9 0 . 9 9 0 . 9 8 0 . 9 9 0 .94 0 . 0 4 / 0 .83 0 ,96 0 . 9 9 0 . 9 8 0 . 9 8 0 .75 /

0 . 2 2 0 .30 0 . 7 0 /

Figure 5(a)

Vol. 135, 1991

/ 3-D Velocity Structure

LAYER 3 (a)

/ 0 . 7 0 0 . 8 7 0 . 7 1 0 0 2 0 . 1 0 /

/ 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

0 9 4 0.95 0.96 0 . 9 8 0 9 3 0 . 9 2 0 . 7 7 / / 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 . 4 3

411

/ LAYER 3 ( b )

0 . 5 3 0 . 8 9 0 . 1 0 0 . 7 8 0 . 3 0 0 . 9 0 0 . 8 ~ 0 . 6 6 0 . 3 3 0 . 1 0 / 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 . 7 1 0.43 0.93 0.98 0,97 0.97 0.97 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 0.46 0.30 0.03 0.12 0.90 0,91 0.94 0.79

0.80 0.95 0.82 0 . 4 2

/

/ LAYZI 8 (b )

O. 42 / 0.~14 0 . 0 0 0 . 0 0 0.00 0.32

0 . 3 6 0 . 0 0 0 . 0 6 0 . 0 5 0 . 0 4 0 . 0 8 0 . 7 4

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.


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 �9 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 0 7

1 ,6 0.1 0 .0 0.0 0.0 0.1 0 , 0 O, l 0.9 1 .4 0,2 0.1 0.0 0.1 0.1 0.2 0.9

0 . 7 0 , I 0 . 1 0 . I 0 . 3 1 4

17 i?/ 0~o?i, ' ~

LAYER i (b)

0.9 1,9 / 0 . 1 ( , . 0 0 . 1 0.2 0 .2 0 . 1 0 . 1 1 .1

8 0 . 4 0,0 0 .0 0 ,0 0.1 0.1 0.1 0,1 0 ,9 s 0 .3 0 .0 0.0 0 .0 0.0 0 .0 O 0 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 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 ,

LAYEI T ( I )

0.3 0 .2 0.3 0.2 0 .9 O.tl 1 9 / /

/ 0 . 2 O.l+ 0 . 0 0 , 1 0 . I 0 . I 0 . I O.Z 1 . 8 / / z .e 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 / / , . e 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 , , / 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 / 0 . 7 0 . 1 0 , 0 o . 0 o . 0 0 . 1 0 . 1 o . 3 1 . 1 1 /

o . t o . t 0 . o o . 0 0 . 0 0 . 0 0 . 1 O.Z 1 . 8 / 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 /

�9 1 . 3 1 . 8 1 . 8

Figure 6(a)


/ LAYER 3 (a)

l . S 0 . 7 1 .7 0 . 2 0 . 6 /

m

2 . 0 1 . 9 0 . 3 0 . 4 1 .1 I . . 7 1 . 9 0 . 7 / 1 . 9 0 . 2 0 . 1 O.S 0 , 7 1 , 2 /

0 . 4 0 . 4 0 . 2 0 . 2 0 . 5 0 . 6 1. , i / 0 . 3 0 . 2 0 . 3 0 . 3 0 . 3 0 , 2 1 .1 11_3_7 O 7 0 . 7 0 . 2 1 . 7

/ LAYER 3 (b)

/ 2 r l 0 , 8 0".'7 / g

1 ,2 1 ,7 0 . 1 1 .1 1 . 9 1 .6 0 . 7 " / 1 3 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 0 3 0 . 6 0 . 2 0 . 5 0 . 2 0 . 4 l . ~

0 . 4 1 , 7 1 . 7 [ . .1 t . 6 t . 4

/ LAYER 4 ( a )

/ /

I . .4 0 . 8 2 .1 1 .5 0 . 2 / "0 .9 0 . 7 0 . 7 0 , 4 1 .1 / 0 , 7 0 . 3 1 .1

1 . 8

/ LAYER q ( b )

~176 / 1.3 0 . 8 1 .4 0125 | . 9 I . O 0 . 3 0 . 3 .

1 . 0 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.


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 (PAPAZACHOS et 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).

43.0 N LA .~ER 1

38.0 l~

33.Ol'

lg.O E 23.Q E 2 8 . 0 E

Figure 7(a)

Vol. 135, 1991 415

43.0 N

38 .0N-

"D

_C

33,0t' 18.0E

3-D Velocity Structure

L A':'~ R 2

I I A ~ 8'

A .B

:13.0 E

b

Figure 7(b)

28.0 E

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 large- scale 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


43.0 N

38.0 N-

LA'f E R 3 .

AJ B'

H,/,

A B 33.0N 1 I I i

la ,OE 23.OE 2a.OE

0

r

F igure 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) sup- ports 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.


43.0 N

3 8 . 0 N

-D

--C

33 .0 I~ 18 .0E

LAYER

J L, A'

c ' -

A B t 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 (AA' 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

418

40

8O

ZOOM) 120

180

G. Drakatos and J. Drakopoulos

A

W

M

PAGEOPH,

m

40

80

1 2 0

160

B Ro

4 0

r

0

8 0

1 2 0

160

r

D

0

4 0

80

1 2 0

160

D"

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.


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.

REFERENCES

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CALCAGNILE, G., D'INGEO, F., FARRUGIA, P., and PANZA, G. F. (1982), The Lithosphere in the Central-eastern Mediterranean Area, Pure Appl. Geophys. 120, 389-406.

CHRlSTODOULOU, A., and HATZFELD, D. (1988), Inversion of the Crustal and Upper Mantle Structure Beneath Chalkidiki ~orth Greece), Earth Plan. Sci. Lett. 88, 153-168.

DELIBASlS, N. D. (1982), Seismic Wave Attenuation in the Upper Mantle Beneath the Aegean Region, Pure Appl. Geophys. 120, 820-839.

DEWEY, J. F., and SENOOR, A. M. C. (1979), Aegean and Surrounding Region: Complex Multiplate and Continuum Tectonics in a Convergent Zone, Bull. Geol. Soc. Am. 90, 84-92.

DRAKOPOULOS, J. (1978), Attenuation of Intensities with Distance for Shallow Earthquakes in the Area of Greece, Boll. Geof. Teor. Appl. 20, 114-130.

FYTIKAS, M., GIULIANI, O., INNOCENTI, F., MARINELLI, G., and MAZUOLLL R. (1976), Geochrono- logical Data on Recent Magmatism of the Aegean Sea, Tectonophysics 31, T29-T34.

HASHIDA, T., STAVRAKAKIS, G., and SHIMAZAKI, K. (1988), Three-dimensional Seismic Attenuation Structure Beneath the Aegean Region and its Tectonic Implication, Tectonophysics 145, 43-54.

HORIE, A. (1980), Three-dimensional Seismic Velocity Structure Beneath the Kanto District by Inversion of P-wave Arrival Times, Thesis Geophys. Inst. Fac. Sci., University of Tokyo.

JONOSMA, D. (1974), Heat Flow in the Aegean Sea, Geophys. J. R. Astr. Soc. 37, 337-346. LEVENBER~, K. (1944), A Method for the Solution of Certain Nonlinear Problems in Least Squares,

Quart. Appl. Math. 2, 164-168. LYaER1S, N., and DESCHAMPS, A. (1982), Sismo-tectonique du Fosse Nord Egeen: Relations avec la faille

Nord-Anatolienne, C. R. Acad. Sci. Paris 295, 625-628. MAKRIS, J. (1973), Some Geophysical Aspects of the Evolution of the Hellenides, Bull. Geol. Soc. Greece

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MCKENZIE, D. P. (1972), Active Tectonics of the Mediterranean Region, Geophys. J. Roy. Astr. Soc. 30, 109-185.

NINKOVlTCH, D., and HAYS, J. D. (1972), Mediterranean lsland Arcs and Origin of High Potash Volcanoes, Earth Plan. Sci. Lett. 16, 331-345.

PAPAZACHOS, B. C. (1973), Distribution of Seismic Foci in the Mediterranean and Surrounding Area and its Tectonic Implication, Geophys. J. Roy. Astr. Soc. 33, 421-430.

PAPAZACHOS, B. C., and COMNINAKlS, P. E. (1971), Geophysical and Tectonic Features of the Aegean Arc, J. Geophys. Res. 76, 8517-8533.

PAPAZACHOS, B. C., KIRATZI, A. A., HATZIDIMITRIOU, P. M., and KARACOSTAS, B. G. (1986), Seismotectonic Properties of the Aegean Area that Restrict Valid Geodynamie Models, 2nd Wegener Conference, Dionysos, Greece 14-16 May 1986, pp. 1-16.

PAYO, G. (1976), Surface Wave and Seismotectonics Studies in the Mediterranean Area, Pure Appl. Geophys. 114, 791-796.

SPAKMAN, W., WORTEL, M. J. R., and VLAAR, N. J. (1988), The Hellenic Subduction Zone: A Tomographie Image and its Geodynamic Implications, Geophys. Res. Lett. 15, 60-63.

TASSOS, T. S. (1984), Static and Dynamic Properties of Upper Mantle in Southern Aegean Sea, Ph.D. Thesis, Univ. of Thessaloniki (in Greek).

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(Received April 19, 1990, revised November 1, 1990, accepted November 22, 1990)

3-D velocity structure beneath the crust and upper mantle ... · 26 VNE 27 VPA 28 VSI 29 VSK 30 VTH...

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