3D crustal structure of a neotectonic basin (Saronikos gulf, Greece)

3D crustal structure in the neotectonic basin

of the Gulf of Saronikos (Greece)

G. Drakatosa,T, V. Karastathisa, J. Makrisb,1, J. Papouliac,2, G. Stavrakakisa

aNational Observatory of Athens, Institute of Geodynamics, P.O. Box 20048, GR 118 10 Athens, GreecebGEOPRO GmbH, St. Annenufer 2, 20457, Hamburg, Germany

cHellenic Center of Marine Research, Institute of Oceanography, P.O. Box 712, 19013 Anavissos, Attiki, Greece

Received 18 March 2003; accepted 9 February 2005

Available online 29 March 2005

Abstract

An on-/offshore seismic network consisting of 36 three-component stand-alone digital stations was deployed in the area of

the Saronikos Gulf, in the vicinity of Athens (Greece), in the fall of 2001. In the present study, from an initial set of more than

1000 micro-earthquakes, 374 were selected and 6666 P- and S-wave arrivals were inverted, based on a 3D linearized

tomography algorithm, in order to determine the 3D velocity structure of the region.

The resulting 3D velocity distribution, in agreement to the micro-seismicity distribution, reflects the Saronikos structure

down to a depth of 12 km. So, the neotectonic basin of the Saronikos Gulf is divided in two parts by a central platform, which

implies the existence of a NNE–SSW-trending rupture zone. This zone is probably the offshore extension of a large thrust belt

dominating the adjacent onshore areas. Due to their different structure, the two basins are dominated by different velocity values

in comparison to the central platform.

The western part is characterised by higher seismic activity than the eastern one. Furthermore, the western Saronikos Gulf is

divided in a northern and a southern part by a well-defined rupture zone trending E–W. This seems to be the extension of the

Corinthiakos Gulf fault zone. At the depth of 17 km, the velocity increases considerably and the crustal thickness is restricted

down to 20 km. This dunexpectedT low thickness in the region of Saronikos Gulf seems to be the result of the extensional stress

field, which dominates the region, as well as of the emergence of the mantle material along the volcanic arc, which clearly

appears at the depth of 12 km. Yet the lack of deep events and, hence, the poor resolution below the depth of 17 km does not

support a definite conclusion about the crust–mantle boundary in this region.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Velocity structure; Tomography; Saronikos Gulf; Greece

0040-1951/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.tecto.2005.02.004

T Corresponding author. Tel.: +30 10 3490164; fax: +30 10 3490180.

E-mail addresses: [email protected] (G. Drakatos), [email protected] (J. Makris), [email protected] (J. Papoulia).1 Tel.: +49 40 30399576; fax: +49 40 30399578.2 Tel.: +30 22910 76370; fax: +30 22910 76323.

Tectonophysics 400 (2005) 55–65

www.elsevier.com/locate/tecto

1. Introduction

The Saronikos Gulf is situated in the northwestern

part of the Hellenic volcanic arc, between the Pliocene

volcano of Aegina and the Pleistocene volcanoes of

Methana and Sousaki (Fig. 1). The area is in generally

characterised by low seismicity. However, at its north-

ern and western borders, strong historical as well as

recent earthquakes have occurred (Fig. 1), all associ-

ated with a roughly N–S extensional tectonic field. In

the vicinity of the Saronikos Gulf, the existence of the

highly populated Attiki region and particularly the city

of Athens gives to the seismicity of the area a major

significance, from the social and economic point of

view (see also Papadopoulos et al., 2000).

During the instrumental observation period of

Greece, in the broader area of Athens and particularly

in the Saronikos Gulf area, no considerable seismic

activity had been recorded prior to the event of

September 7, 1999 (Ms=5.9). Therefore, it is not

surprising that no systematic micro-seismicity study

has been carried out in the past. After the event of 1999,

the need of mapping the active faults in the Attiki

offshore area was reconsidered. Specifically, the

understanding of the active deformation of the Sar-

onikos Gulf area is a must in order to estimate the

seismic potential and seismic hazard of the Attiki

region. The acquisition of a local velocity model is also

judged essential for understanding the seismogenic

processes.

The 2D crustal structure of the Saronikos–eastern

Corinthiakos basins was investigated in the past,

suggesting an intense crustal thinning below the

volcanic area of the Saronikos Gulf (Makris et al.,

2004a). Following the passive seismic observations of

the fall of 2001, a 3Dactive seismic experimentwas also

conducted, the results of which are under evaluation

(Makris, personal communication). Some work has

been also done in the adjacent onshore area of Attiki,

after September 7, 1999 earthquake, with 3D passive

seismic tomography based on local seismological net-

works (Drakatos et al., 2002). The existing information

Fig. 1. A simplified tectonic map of the study region is shown. The black lines represent faults, whereas the black broken line represents the

thrust belt zone dominating Attiki region. The diamonds represent the historical earthquakes from 450 B.C. to 1900 (MN6.0, after Papadopoulos

et al., 2000). The stars show the epicentre distribution from 1901 to 1964 (MN5.0, after Makropoulos et al., 1989; Papazachos and Papazachou,

1997). The black circles show the epicentre distribution from 1965 to 2000 (MN6.0, Monthly Bulletins of Institute of Geodynamics, NOA).

G. Drakatos et al. / Tectonophysics 400 (2005) 55–6556

for the velocity structure, provided by large-scale

regional passive seismic tomography studies for the

wholeGreek territory, is not of the required resolution to

be effectively used for a detailed study of the active

deformation (e.g., Spakman, 1986; Spakman et al.,

1988; Drakatos and Drakopoulos, 1991; Ligdas and

Main, 1991; Papazachos et al., 1995;Alessandrini et al.,

1997; Drakatos et al., 1997).

The aim of the present study is to derive the 3D

velocity structure of the Saronikos Gulf area, based

on the recorded micro-seismic activity during the

field experiment of 2001, that involved the instal-

lation of eight ocean bottom seismographs (OBS)

and 28 stand-alone land stations (Fig. 2). Both

marine and land stations were three-component

digital stations, used in the above passive experi-

ment the SEDIS III seismic recorder of GeoPro,

Hamburg (Makris and Moeller, 1990).

2. Regional geological and tectonic setting

The proposed geodynamic models for the con-

struction of the Aegean neotectonic basins consider, in

general, a tensional regime with the formation of

tectonic grabens by normal faulting in the back-arc

area of the Hellenic trench (McKenzie, 1978; Dewey

and Sengor, 1979; Le Pichon and Angelier, 1979;

Brooks et al., 1988). Within this process, the

Saronikos Gulf lies along the Hellenic volcanic arc,

within the Pliocene volcano of Aegina and the

Pleistocene volcanoes of Methana and Sousaki

(Papanikolaou et al., 1988). It is divided into a

western and an eastern part by a very shallow N–S-

trending platform, part of which emerges at the islands

of Methana, Angistri, Aegina and Salamina (Fig. 1).

To the west, the Gulf includes two basins, the WNW–

ESE-oriented Epidaurus basin to the south, with a

depth greater than 400 m and the E–W-oriented

Megara basin to the north, which is relatively shallow

(less than 250 m). The WNW–ESE marginal faults of

the Epidaurus basin in the southwest have about 350

m of throw and create a rather symmetric tectonic

graben with significant volcanic intrusions within its

eastern part. The Megara basin is a graben formed by

E–W to ENE–WSW marginal faults having a throw of

400–500 m (Papanikolaou et al., 1988).

In the eastern Saronikos Gulf, NW–SE faults with

relatively smaller throw control the alternation of

basins and plateaux of the sea bottom morphology.

Fig. 2. The distribution of the on-/offshore seismic network is shown (Makris et al., 2004b).

G. Drakatos et al. / Tectonophysics 400 (2005) 55–65 57

Judging by the fault displacements and sediment

distribution, we concluded that the western part is

obviously much more active than the eastern one. The

presence of recent volcanoes in the central part leads

to an even more complicated structure and it separates

the active western part from the relatively inactive

eastern one.

The onshore areas of Attiki (as mapped and

published in the 1:50,000 scale geological maps of

the Institute of Geology and Mineral Exploration of

Greece, IGME, 1989) belong to an autochthonous

metamorphic basement of Palaeozoic–Mesozoic age.

It consists of marbles and schists (Papanastassiou et

al., 2000), with remnants of a tectonic cover identified

in the same localities. A major tectonic boundary,

trending ENE, separates the mountains of Penteli and

Himmitos to the southeast, from the mountains of

Parnis and Aegaleo to the northwest (broken line, Fig.

1). Normal faults along the Parnis Mountain are

considered as the eastward extension of the Corin-

thiakos Gulf fault system of almost E–W orientation.

3. Data–method–modelling

More than 1000 micro-earthquakes were recorded

during the passive seismic experiment of 2001

(Makris et al., 2004b). Four hundred forty five (445)

earthquakes (0.3bMLb3.8) were determined in the

investigated region (37.48N–38.28N, 22.88E–24.28E),recorded at least at five stations. The locations of the

hypocenters were obtained using the HYPOINVERSE

software (Klein, 1989), which allows the application

of spatially varying local velocity models for the

hypocenter location. Magnitudes were defined by the

coda length of the seismograms, calibrated by using

earthquakes also recorded by the Seismograph Net-

work of the National Observatory of Athens. As far as

the magnitude estimates are concerned, a mean

function from four stations having similar site

response was derived, and this was applied to the full

temporary network. More details about RMS, ERH,

ERZ, magnitude and depth distribution are given in

Makris et al., 2004b.

The inverse problem of three-dimensional local

earthquake tomography is formulated as a linear

approximation of a non-linear function (Pavlis and

Booker, 1983). Solutions are generally obtained by

linearization with respect to a reference earth model

(Aki and Lee, 1976; Nolet, 1978). The solutions

obtained and the reliability estimates depend thus on

the initial velocity model. Unrealistic initial condi-

tions may result in artefacts of significant amplitude.

The used tomography technique was initially

introduced by Thurber (1983) and further developed

by Eberhart-Phillips (1986, 1990). The method

performs an iterative simultaneous inversion for 3D

velocity structure and hypocenter parameters using

travel time residuals from local earthquakes. The

velocity of the medium is parameterised by assigning

velocity values at the intersections of a non-uniform,

three-dimensional grid. The spacing within the grid is

selected in such a way as to have enough ray paths

near each grid point so that its velocity may be

adequately resolved. The spacing does not need to be

uniform throughout the study area. The velocity for a

point along a ray path and the velocity partial

derivatives are computed by linear interpolation

between the surrounding grid points. Thus, the

velocity solution will show progressive changes in

velocity rather than the sharp discontinuities shown in

typical wide-angle reflection models or block-type

parameterisations. The ray-tracing technique used is

the bART+Pseudo-bendingQ (Approximate Ray Trac-

ing, proposed by Thurber, 1983).

As far as the modelling stage is concerned, we set

the grid of the nodes at the specified area from

37.408N to 38.208N and 22.808E–24.208E. The nodesare well distributed in seven horizontal layers (slices)

at depths of 1, 3.5, 7, 12, 17, 22 and 30 km,

respectively. Each layer includes 12 nodes in the E–

W direction and 9 nodes in the N–S direction. The

distance between two consequent nodes varies from

10 km to 20 km in both directions. In order to

constrain better the 3D inversion, initially based on

the velocity model (Table 1) proposed by Makris et al.

(2004a), we used the Vp and Vs models shown in

Table 2. Vp values between two consequent nodes

Table 1

Velocity model in Saronikos Gulf (Makris et al., 2004a)

Velocity (km/s) Depth (km)

4.70 0.0

5.70 3.0

6.80 7.0

8.10 17.0


were computed by linear interpolation. From the

initial set of earthquakes, 374 events were selected

(1.6bMb2.6), which gave a good coverage distribu-

tion. Some of the events were located outside the

modelled area. The inclusion of earthquakes and/or

stations outside the modelled area generally improves

the ray path distribution within it. In order to optimize

the best depth determination, we included in the

relocation process not only P- but also S-wave

arrivals. In total, 6666 arrivals (3402 P-wave and

3264 S-wave readings) were considered. The data set

has been inverted five times to get a stable solution.

The final epicentre distribution is shown in Fig. 3.

The final calculation of the RMS values was improved

significantly and most of the events have RMS values

varying from 0.1 to 0.3 s. In Fig. 4, the depth

distribution of the events is shown.

The results of the inversion, in terms of velocity

variation, are shown in Fig. 5. Due to the lack of deep

events, the two bottom layers (22 and 30 km) were

poorly resolved. Therefore, the last one was discarded

and not included in the presentation of the final

velocity structure. Fig. 6 shows the velocity distribu-

tion along selected cross sections.

Fig. 3. Epicentral distribution is shown, after the 3D simultaneous inversion.

Table 2

Grid geometry

Depth

(km)

Nodes

(E–W)

Nodes

(N–S)

Initial P-velocity

(kms�1)

Initial S-velocity

(kms�1)

1.0 12 9 4.0 2.2

3.5 12 9 5.6 3.1

7.0 12 9 5.9 3.3

12.0 12 9 6.0 3.4

17.0 12 9 7.0 3.9

22.0 12 9 7.5 4.2

30.0 12 9 8.1 4.5


Several parameters, such as the number of hits at

each block, the sum of derivatives, etc., can be used

to verify the reliability and stability of the solution.

The resolution and, hence, the reliability and the

stability of the solution strongly depend on the

degree of intersection of crossing rays. For the latter,

a dense station network of homogeneous distribution

over the area under investigation is usually required.

In Fig. 7, we present the resolution matrix of the

diagonal elements. It is obvious that the solution we

obtained is stable for the major part of the velocity

model.

4. Discussion and conclusions

The micro-seismicity study of the Saronikos Gulf

in 2001 (Makris et al., 2004b) produced reliable and

accurately located seismic data because of the

homogeneous and densely spaced on-/offshore seis-

mic array. A 3D seismic passive tomography based on

this data set evaluated for the first time the 3D

velocity model for the complete Saronikos area of

60�60 km2. As the first evaluation showed (Makris et

al., 2004b), the microearthquake activity is associated

with the tectonic regime rather than the volcanic

activity of the area. The relocation of the foci after the

simultaneous inversion was improved and exhibits

now an RMS of 0.1–0.3 s. This, however, had no

significant influence on the results already published

in 2004, where the main active faults were located

north of the Aegina island striking E–W and continu-

ing onshore west Peloponnese and to the east of

Aegina striking NNE–SSW (see Fig. 3). From west to

east, the micro-seismicity is restricted along a narrow

zone trending NNE–SSW (Aegina and Salamina

islands) that divides the Saronikos in two regions

(Figs. 1 and 3). The shallow micro-seismicity under-

lines the existence of a shallow platform, part of

which emerges at the islands of Methana, Angistri,

Aegina and Salamina (Papanikolaou et al., 1988).

This feature, as obtained from the velocity structure,

seems to continued up to the depth of 12 km (Figs. 5

and 6). This is probably an offshore continuation of

the thrust belt, which is dominant in the Attiki region

(Fig. 1). This result is also supported by the onshore

epicentre distribution of the aftershock sequence of

the September 7, 1999 earthquake, which was

restricted to the west of the thrust belt (Papanastassiou

et al., 2000; Drakatos et al., 2002; Pavlides et al.,

Fig. 4. A 3D view of the depth distribution is shown.


2002). This could explain the similarity of the

seismotectonic regime of western Saronikos to that

of the Corinthiakos Gulf, while the eastern Saronikos

has a more simple structure.

The western Saronikos Gulf is divided in two parts,

a northern and a southern one, by a well-defined

seismic zone of E–W direction (Fig. 3), which is

shown between the northeastern part of Peloponnisos

Fig. 5. The P-wave velocity distribution is shown, after the 3D simultaneous inversion. Depth denotes the layer (slice), where the grid nodes

(white crosses) were applied.


and Aegina Island. These two parts are actually the

Megara and Epidaurus basins, respectively, but they

are not well defined in terms of velocity variations

(Figs. 5 and 6).

An important feature is shown at the depth of 12

km. In general, we could say that higher velocities

appear underneath the old volcanic regions of

Methana, Sousaki and Salamina. This dring of high

23.0 23.5 24.0

23.0 23.5 24.0

37.5

38.0

37.5

38.0

Location map of cross-sections

A B

E F

DC

G

H

J

K

L

M

37.4 37.6 37.8 38.0 38.2

-20

-10

3 4 5 6 7 8

Vertical Section (LON 23.67)

Dep

th(k

m)

Distance (degrees)L M

Vp (km/s)

22.8 23.0 23.2 23.4 23.6 23.8 24.0 24.2

-20

-10

3 4 5 6 7 8

Vertical section (LAT 37.8)

C D

Vp (km/s)

Distance (degrees)

Dep

th(k

m)

22.8 23.0 23.2 23.4 23.6 23.8 24.0 24.2

-20

-10

3 4 5 6 7 8

Vertical section (LAT 37.98)

A B

Vp (km/s)

Distance (degrees)

Dep

th(k

m)

22.8 23.0 23.2 23.4 23.6 23.8 24.0 24.2

-20

-10

3 4 5 6 7 8

Vertical section (LAT 37.62)E F

Vp (km/s)

Distance (degrees)

Dep

th(k

m)

37.4 37.6 37.8 38.0 38.2

-20

-10

3 4 5 6 7 8

Vertical section (LON-23.1)G H

Dep

th(k

m)

Distance (degrees)

Vp (km/s)

37.4 37.6 37.8 38.0 38.2

-20

-10

3 4 5 6 7 8

Vertical Section (LON 23.33)

Dep

th(k

m)

Distance (degrees)J K

Vp (km/s)

A

B

Fig. 6. The P-wave velocity distribution is shown (B), after the 3D simultaneous inversion, along the cross sections shown in (A).


velocitiesT indicates upraised mantle material due to

the palaeo-volcanic activity. This result is in agree-

ment with the very low resistivity values determined

at depths below 10 km during a magnetotelluric

exploration of Sousaki geothermal area (Tzanis and

Lagios, 1993). The presence of this mantle material

and the extensional stress field are probably the main

causes of the thin crustal thickness in the Saronikos

region observed by Makris et al. (2004a).

At the depth of 17 km, the velocity increases

considerably. As it is shown in the cross sections of

Fig. 6, the thickness of the crust is restricted down

0.0 0.3 0.5 0.8

0.0 0.3 0.5 0.8

0.0 0.3 0.5 0.8 1.0

0.0 0.3 0.5 0.8 1.0

0.0 0.3 0.5 0.8 1.0

0.0 0.3 0.5 0.8 1.0Depth 1.0 km

Depth 3.5 km

Depth 7 km

Depth 12 km

Depth 17 km

Depth 22 km

22.823

23.223.4

23.6 23.824 24.2

37.5

38

22.823

23.223.4

23.6 23.824 24.2

37.5

38

22.823

23.223.4

23.6 23.824 24.2

37.5

38

22.823

23.223.4

23.6 23.824 24.2

37.5

38

22.823

23.223.4

23.6 23.824

24.2

37.5

38

22.823

23.223.4

23.6 23.824 24.2

37.5

38

Fig. 7. The diagonal elements of the Resolution matrix are shown, corresponding to the layers where the grid was applied.


to 20 km. Despite the slight influence of the initial

velocity model in the results of the tomography, the

low thickness in the region of Saronikos Gulf is

also supported by active seismic observations. This

could be the result of the extensional stress field,

which dominates in the region at present, as well as

of the intrusion of mantle material in the volcanic

arc.

Acknowledgements

The authors are grateful to Dr. F. Haslinger and Dr.

C. Chiarabba for their constructive comments and

suggestions. This study was partially supported by the

Earthquake Planning and Protection Organisation and

the General Secretariat for Research and Technology

of Greece.

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Documents

3D crustal structure of a neotectonic basin (Saronikos gulf, Greece)