Structure of the Hellenic Subduction Zone from Gravity Gradient Functions and Seismology

Structure of the Hellenic Subduction Zone from Gravity Gradient Functions and Seismology

TOLGA GONENC1 and MUSTAFA AKGUN

1

Abstract—The Normalized Full Gradient (NFG) method has

widespread applications in the analysis of potential fields, espe-

cially the gravity and magnetic fields. This method is used to

identify the lateral and horizontal density variations in the crust and

lithosphere. In this study, the NFG method was applied to the

gravity data of the Cretan Arc and its surroundings. Because of the

tectonic features of the eastern Mediterranean, the Cretan Arc and

the neighboring areas are seismically very active. Especially the

subduction zone and the complicated crustal features have been

defined applying many different geophysical methods. In this

study, first the NFG method is tested with synthetic prisms (two

cubes). After that, the NFG method was applied to the Bouguer

gravity data of the Cretan Arc and its subduction zone (Hellenic

subduction zone) and Hellenic subduction zone was defined with

the foci depth data (USGS) along the south–north direction. Thus,

geometry of the focal depth distribution has been created to

determine probable media depths and their localizations. According

to the NFG results, vertical structural transitions were observed at a

depth ranging between 10 and 180 km. Also, these results were

compared with the foci depth model and the other results of the

related publications. Finally, some considerations in vertical solu-

tion with the NFG method have been presented and locations of the

different structures at horizontally have been defined with appli-

cation of the NFG method.

Key words: Hellenic subduction zone, normalized full gra-

dient method, Cretan Arc, bouguer gravity data, tectonic.

1. Introduction

The Cretan Arc (Hellenic Arc) is a region with

high, still on-going tectonic activity which is seen in

terms of seismicity. This region is under the inter-

action of the Dead Sea Fault Zone (movements of the

African and Arabian plates), North Anatolian Fault

Zone, Greek shear zone and western movement of

west Anatolia (Fig. 1). As a result, the Anatolian

plate is being shaped by the movements of the Afri-

can plate, Greek shear zone and the Arabian plate.

The Cretan Arc, which separates the Mediterra-

nean Sea and the Aegean Sea in the western

Mediterranean, has a bathymetry with variable val-

ues. Ridges and trenches which were formed as a

result of faulting alternate each other. In the south of

the Cretan Arc lie three trenches called Helen, Pliny

and Strabo (with depths of 3,500–4,000 m) which

form the deepest parts of the Mediterranean.

As a result of seismic tomographic studies, it was

determined that the region has a very complex lith-

ographic structure and that it shows very complex

changes in the Aegean Sea in terms of mantle

velocity and continental crust thickness (PAPAZACHOS

et al. 1995; PAPAZACHOS and NOLET 1997).

Using seismic and gravitational methods, MAKRIS

and YEGOROVA (2006) conducted modeling studies in

a total of five profiles in north–south and west–east

direction that describe Crete Island and its peripheral.

Using Nafe Drake and Birch equations (BROCHER

2005) densities were deduced from seismic velocity,

and the crust thickness in the middle of Crete Island

was determined to be 32–34 km.

Using results of seismic velocity, SNOPEK et al.

(2007) calculated crust thicknesses as average 40 km

around Peleponnese, 30 km around Crete, and 20 km

in inner parts of the Aegean Sea. These changes

observed in values of crust thickness stem from active

tectonism that is present in the region.

According to the results of the studies conducted

by MEIER et al. (2004a); DELIBASIS et al. (1999), the

region lying in the south of Cretan Island has great

seismological activity at depths of 20–40 km.

According to the studies by PAPAZACHOS et al.

(2000) and GONENC et al. (2006), the Wadati–Beni-

off zone, which shows an arc-like development

1 Faculty of Engineering, Department of Geophysics, Dokuz

Eylul University, 35160 Tınaztepe, Campus Buca/Izmir, Turkey.

E-mail: [email protected]

Pure Appl. Geophys. 169 (2012), 1231–1255

� 2011 Springer Basel AG

DOI 10.1007/s00024-011-0391-2 Pure and Applied Geophysics

while proceeding northward, reaches depths of

150–200 km.

In seismic refraction and reflection studies con-

ducted by BOHNHOFF et al. (2001) and BRONNER

(2003), the crust was found to be around 35 km thick.

As a result of the studies conducted by KNAPMEYER

and HARJES (2000) and LI et al. (2003) in the north of

the island, Moho depth was found to be between 44

and 69 km. In another study by STIROS (2000), large

earthquakes, volcanic disasters and tsunamis that

occurred in the region were examined in the light of

archeological data. BOHNHOFF et al. (2001) con-

structed the model of the subduction zone basing his

study on the acoustic impedance differences, while

CASTEN and SNOPEK (2006) tried modeling only in the

light of gravity data.

In his study, MEIER et al. (2004b) found that the

intermediate boundary between African lithosphere

and Aegean lithosphere was in the depth of

20–40 km. Meier modeled Crete Island, where the

Ptolemy trench exists, as African Oceanic Mantle

Lithosphere. He also modeled the point which is

nearly at the depth of 80–100 km under the Island

Santoroni as the terminal point of the African Oce-

anic Crust and as the zone where African Oceanic

Mantle lithosphere exists (T. MEIER et al. 2004b).

According to MEIER’s study (2004b), segments up

to the depth of 25 km under Crete were described as

Aegean crust, depths between 25–40 km as Aegean

mantle lithosphere, depths between 40–60 km as

African oceanic crust and depths between 0–15 km as

sediment accumulation area.

Figure 1General tectonic of Eastern Mediterranean. (modified from MC CLUSKY et al. 2000; HUGUEN 2001, MAKRIS and STOBBE. 1984; PAMUKCU et al.

2007)

1232 T. Gonenc, M. Akgun Pure Appl. Geophys.

When USGS earthquake epicenters in the eastern

Mediterranean are examined, it is observed that

earthquakes concentrate mainly along the Cretan Arc.

On the other hand, the fact that the earthquake epi-

centers get deeper towards the north of the Cretan

Arc points to the great possibility that they stem from

the active subduction zone (GONENC 2008).

In the current study, the Normalized Full Gradient

(NFG) method will be applied to gravity data and the

depth and position of density inhomogeneities

belonging to the Hellenic Arc and its circumference

will be obtained. In the second stage, distributions of

earthquake epicenter depths covering 32 years from

1973 to 2004 which were taken from USGS and the

geometric relation between bathymetry–topography

will be examined and subduction zone will be

described. In the final stage, the results of the current

study will be evaluated together with the results

obtained from previous studies in the region in

general.

Working area is between 23�000–28�000 east

longitude and 33�000–38�300 north latitudes (Fig. 2).

The data of gravity, magnetic and bathymetry were

obtained from IOC (Intergovernmental Oceano-

graphic Commission) (UNESCO) 1988–1989, IBCM

(International Bathymetric Chart of the

Mediterranean-1981/1987) and the offshore bathy-

metric data set were obtained from Topex data.

2. The NFG (Normalized Full Gradient) Method

The Normalized Full Gradient method is one of

the most successful methods used for the determi-

nation of the lateral and vertical localization of single

density source points generating a potential field

(OZYALIN 2003). Thus, the localization of structures

can be determined and studies of modeling are sup-

ported. The basic concept of the method is the

downward continuation of the normalized full gra-

dient values of the geophysical data. In the downward

continuation of the potential field data, continuity

down past the anomaly source usually causes unsta-

ble results. Because the derivatives are used in the

computation of the Normalized Full Gradient opera-

tor instead of the anomaly itself, the Normalized Full

Gradient method eliminates these effects which exist

near the anomaly source during downward continu-

ation (BEREZKIN 1988). Therefore, arithmetical basis

of the normalized full gradient method is constituted

by G xi; zj

� �:

G xi; zj

� �¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffioV xi;zjð Þ

ox

� �2

þ oV x;zð Þoz

� �2

" #vvuut

1N

PNi¼1

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffioV xi;zjð Þ

ox

� �2

þ oV xi;zjð Þoz

� �2" #v

vuut

ð1Þ

BEREZKIN (1973).

V(xi, zj) : Function of the geophysical field

N : Number of the observation points

v : Degree of the NFG operator. v is the

degree of the NFG operator which controls

the peak amplitude value and the peak

anomaly width of the normalized full

gradient sections. Although v can be taken

as 1, 2, 4, 8, etc., v = 1 is generally used for

the potential field data (AYDIN 2007)

zj : Depth of interest

The NFG sections are composed of the downward

continued values of the observed field, calculated at

24 25 26 27 28 29 30

32

33

34

35

36

37

38

39

40

-5000

-4000

-3000

-2000

-1000

0

1000

2000

METER

CRETEHELEN

PLINY TRENCH

STRABO TRENCH

RHODOS

LONGITUDES

LA

TIT

UD

ES

TURKEY

N

Figure 2Bathymetric map of the Cretan Arc

Vol. 169, (2012) Structure of the Hellenic Subduction Zone 1233

several depth horizons between surface (z = 0) and a

maximum depth to which the downward continuation

of the anomaly field is computed (z = zm) with cer-

tain Dz intervals. Because using the sum of the

horizontal and vertical derivatives, is called the full

gradient method. The denominator of Eq. (1) is the

mean value of the full gradient computed over N

observation points, and this normalization makes the

full gradient value dimensionless.

In his study, OZYALIN (2003) magnetic data, and

in the study of SINDIRGI et al. (2008) results per-

taining the determination of the location of the

structures were set for by using a two-dimensional

normalized full gradient method. Separately, in his

study, AYDIN (2009) applied the NFG method on

resistivity data.

The calculation of the NFG operator is accom-

plished by utilizing a Fourier series approach in

which the V(x, z) function along the x axis between

(-L/2, L/2) interval is given as (BRACEWELL 1984;

BLAKELY 1995)

Vðx; zÞ ¼X1

n¼0

An cospn

L

� �xþ Bn sin

pn

L

� �x

h ie

pnzLð Þ;

ð2Þ

where the exponential term e pnz=Lð Þ corresponds to

change in V(x, z) along the z axis, An, Bn are Fourier

coefficients and n is the harmonic number.

– If the data are definitive in (0, L) interval, then only

the sine or cosine can be used (RIKITAKE et al.

1976).

– If the data have zero values at both end points in

this interval, a faster approach can be obtained to

the Fourier sine series. Therefore, (a ? bx) linear

trend is subtracted from the data values in (0,

L) interval. Here a is the beginning value of the

V(x) function and b = (V(L)-V(0))/L (SıNDıRGı

et al. 2008).

According to this definition, the function is

defined as (JUNG 1961)

V x; zð Þ ¼XN

n¼1

Bn sinpnx

L

� �h ie

pnzL ð3Þ

Bn : The Fourier sine coefficient

n : Harmonic number (n-th harmonic of the

Fourier series)

N : Number of terms of series

z : Differences in height

L : Length of profile

Bn ¼1

L

ZL

�L

Vðx; 0Þ sinpnx

L

� �dx ð4Þ

The function V (x, z) means field of gravity along

x axis and downward analytic extensions are made

along z axis. The number of points on N profile, the

functions of Vx (x, z) are the derivatives of V (x, z)

along x and z axis. The derivatives are obtained as

oV x; zð Þox

¼ pL

XN

n¼1

nBn cospn

Lx

h i� e

nzpL ð5Þ

oV x; zð Þoz

¼ pL

XN

n¼1

nBn sinpn

Lx

h i� e

nzpL ð6Þ

The q (smoothing) factor, however, modifies the

frequency characteristic of the NFG operator by

transforming its shape into a band pass filter, which

becomes asymmetrical with increasing depth (AYDIN

A 2007).

q ¼sin pn

NpnN

l

ð7Þ

l : Degree of smoothing. It controls the curvature

of the q function.

In general, l = 1 or 2 for reasonable results.

Moreover, the harmonic interval in 3, 5 and 6

equations is restricted to a lower limit of N1 and an

Figure 3Synthetic model parameters of the structures and their localizations


Figure 4Anomaly map of the same cubes and the cross section (A–B)


Figure 5a Different NFG harmonics (N = 5, 10, 15, 20) of the first synthetic model. b Different NFG harmonics (N = 25, 30, 40) of the first synthetic

model


Figure 5continued


upper limit of N2. N1 and N2 harmonic limits gener-

ally are determined by a trial-and-error method, N1 is

generally selected 1 in potential fields (SıNDıRGı et al.

2008).

As a result of these, the function V(x, z) and its

derivatives are defined as (BEREZKIN 1988),

Vðx; zÞ ¼XN2

n¼N1

Bn sinpnx

L

� �e

pnzL

sin pnN

� �

lnN

l

ð8Þ

oV x; zð Þox

¼ pL

XN2

n¼N1

nBn cospn

Lx

h i� e

nzpL

sin pnN

� �

lnN

l

ð9Þ

oV x; zð Þoz

¼ pL

XN2

n¼N1

nBn sinpn

Lx

h i� e

nzpL

sin pnN

� �

lnN

l

ð10Þ

Areas of which normalized full gradient values

are higher than 1, determine the localization of the

structure (DONDURUR 2005; detailed in DONDURUR

2005; AYDIN 2007)

In order to test the efficiency of the NFG method

was applied to gravity effects of two cubes. For all of

the three simple theoretical models, the profile length

is chosen as 500 m while the structural locations of

the models correspond to the midpoints of the pro-

files. The gravity anomaly of a cube model is given

by GRANT and WEST (1965).

In the first condition (Fig. 3) both cubes are

located at same depths and distance is 250 m between

the two cubes.

The gravity anomaly map was created (Fig. 4)

and the A-B cross section defined along the

maximum peak of the map (Fig. 4). The presence of

the cubes can be observed clearly in the anomaly map

and in the A-B cross section (Fig. 4). It is observed

that the NFG method produces closed contours

around the anomalous body for all harmonic inter-

vals. In this study, it is proposed that the center of the

fully closed contours belonging to NFG harmonics

corresponds to the actual burial depth of the structure

(Cubes). Therefore, 30 m depth of the cubes is

visually identified from the maximal enclosure loca-

tion for n = 1–25 and 1–30 harmonic interval. The

NFG method also determines the surface projection

point of the structures precisely (x1 = 125,

x2 = 375 m) for all harmonic intervals (Fig. 5a, b). It

can be noticed that the value of the NFG increases

while it approaches the correct position of the cube.

In the second example, the first cube is at the

same depth and the second cube has been localized

deeper than the first cube. Also, different densities are

defined for each cube (q1 = 1 g/cm3 and q2 = 5 g/

cm3; Fig. 6). Sign of the existence of cubes can be

observed clearly in anomaly map and A-B cross

section (Fig. 7). The first cube, which has lower

density defined as low amplitude, and the second

cube, the deeper one, was defined as high amplitude

and can be seen in Fig. 7. According to these con-

ditions, depth of the first cube (30 m) was identifies

from the maximal enclosure location for N = 1–40

and the depth of the second cube (50 m) was iden-

tifies from the maximal enclosure location for

N = 1–35/N = 1–40 harmonic interval. Surface

projection point of the structures precisely (x1 = 125,

x2 = 375 m) for all harmonic intervals (Fig. 8a, b).

In the third condition, the first cube is at the same

depth. The second cube has been localized deeper

than the first cube. On the contrary, their densities are

the same (q1 = q2) but distance is closer (80 m)

(Fig. 9). Sign of the existence of cubes can not be

observed clearly on the anomaly map and A-B cross

section (Fig. 10). Anomaly is almost acting like a

single structure (Fig. 10) cause of the distance. The

amplitude of the first cube is greater than the second

cube on the anomaly map (Fig. 10) and total anomaly

field shape is presented as an interference zone along

the cross section. Therefore, the 50 m depth of the

combined effect of cubes as one structure is visually

identified from the maximal enclosure location for

Figure 6Second condition; Synthetic model parameters of the structures and

their localizations


Figure 7Anomaly map of the different cubes and the cross section (A–B)


Figure 8a Different NFG harmonics (N = 5, 10, 15, 20) of the second synthetic model. b Different NFG harmonics (N = 30, 35, 40) of the second

synthetic model


Figure 8continued


n = 1–40 and 1–50 harmonic interval. In this case,

the two cubes cannot be resolved. Effects of the two

cubes were resolved as one body because they are

very close to each other. The NFG method also

determines the surface projection point of the struc-

ture precisely (x1 ? x2 = 275–300 m) for all

harmonic intervals (Fig. 11a, b).

3. The NFG Application

For application, three gravity profiles with south–

north direction were determined from the Bouguer

anomaly map (Fig. 12). Depending on the aim of the

study (structural model of the Hellenic Arc and its

circumference), general tectonic features of the

region were taken into consideration in selecting the

profiles. The Normalized Full Gradient application

was conducted for three profiles. Results of applica-

tion: obtained for different harmonics from the profile

are given below respectively (Figs. 13, 14, 15, 16).

In this method, an NFG value, where the contour

value is greater than 1, is the location of the body

(PAMUKCU 2010). Along the profiles formed, various

structural locations at different places and depths

were described.

On Profile 1, there is one body at lat 33� with

depth between 60 and 160 km (Fig. 13), and a second

one at lat 36� with depth between 140 and 170 km

below the volcanic arc (Fig. 13). A body below the

trench at 40–60 km (Fig. 14) and another body

slightly north of the volcanic arc (lat 37�) at a depth

of 60 km (Fig. 14). On Profile 2, there is the presence

of bodies, below the volcanic arc (lat 36�–37�), with

depth between 20 and 80 km (Figs. 15, 16). At Pro-

file 3 ,which lies east of Crete, there is one body at lat

36� with depth of 160 km a second one at between lat

35� and lat 36� below Crete Island, the third body at

lat 37� below the volcanic arc.

4. The Relationship Between Earthquake Epicenter

Depths and Subduction Zone

Seismicity is concentrated in the south of the

Island of Crete, and this system has continued with a

broad range to southwest Turkey (Fig. 17). South-

westward movement of the Anatolian plate is met

with the African plate along the Hellenic Arc sub-

duction zone. The presence of the subduction zone

and structural geometry can be seen clearly in Fig. 18

with obtained data from USGS. When depths of

80–120 km are investigated, it can be observed in

Fig. 18 that dipping underwent a second bending.

Similarly, this result was examined in detail in the

study by PAPAZACHOS et al. (2000) and dipping angles

of 30�, and secondary 45�, were described. As far as

180 km depth from the surface of this dipping seis-

micity was observed but there is no data about deeper

sections (PAPAZACHOS et al. 2000).

For the purpose of conducting examination toge-

ther with the gravity results on the map obtained

according to focal depths, it has been observed that

the seismic activity along the Hellenic Arc is con-

centrated along a line in the south of Crete (Fig. 17).

To examine the relationship between topography,

geometry of subduction zone and the arc of volcanic

islands in the north, two profiles along the S–N

direction were formed (Figs. 18, 19).

Bathymetry–topography values and focal depths

on the map were drawn into graphics according to

longitude–latitude values (Figs. 18, 19). When the

changes on the graphics are examined, it is observed

that there is dense seismicity to the south of Crete.

This situation is most probably a sign of the begin-

ning of the subduction zone. According to this

situation, dipping starts at latitude 34�.

Figure 9Third condition; Synthetic model parameters of the structures and

their localizations


Figure 10Anomaly map of the same cubes in different depths and the cross section (A–B)


Figure 11a Different NFG harmonics (N = 5, 10, 15, 20) of the third synthetic model. b Different NFG harmonics (N = 25, 30, 40, 50) of the third

synthetic model


Figure 11continued


Again, from epicenter distributions, it is observed

that dipping constitutes a secondary slope between

depths of 80–120 km and after the latitude 36�(Fig. 18). The projection of this secondary slope on

the Earth surfaces with the area of volcanic island arc

(Santoroni Island, etc.).

As seen in Figs. 18 and 19, focal depths reach

nearly 180 km in this area, and Santoroni Island,

which has active volcanism in our day and lies within

the area, is bordered by latitudes 36� and 38� where

the secondary slope begins.

All these structural variations have different

localizations and their own physical specification

cause of the tectonic movements and density differ-

ences. Therefore, application of the NFG method to

the Bouguer gravity data of the Cretan arc and its

periphery must be support the seismological

approach. In all NFG profiles, harmonic interval

where fully closured contours were well-matched

with the foci depth cross section. Especially where

the depth of the changing structural specifications or

deformation of their shapes (subduction zone and

slopes) was defined by the NFG method.

5. Discussion

The shape of the potential anomalies in the data

processing and modeling phase is important. Con-

vergence of structures laterally affects the shape of

the potential field anomaly. According to the results

of synthetic model studies, if the potential fields of

the bodies do not dominate each other (Figs. 4, 5, 6,

7), presence of the bodies can be defined horizontally

and vertically. On the contrary, if there is domination

(interference zone) between the two potential field

anomalies and total potential field anomaly shape

(Fig. 10), presence of the two bodies can not be

Figure 12Bouguer gravity anomaly map (mGal) and bathymetry of the Hellenic Arc and profiles


Figure 13Different NFG harmonics (N = 6, 7, 8, 9) of Profile 1


resolved because the maximum and minimum peaks

of the potential field of two cubes (bodies) are not

clear. As a result, the solution is designed as a single

body. Therefore, in view of the NFG analysis on the

field data set, total effect of the potential field which

is caused by tectonic structures is important. At dif-

ferent harmonics, for each contour greater than one,

the maxima can not be defined as a different body or

structure. It can be the same object, body or structural

element vertically or laterally along the profile.

According to these approaches, Profile1, Profile2

and Profile 3 can be analyzed as shown below.

5.1. Profile 1

According to the contours which are observed in

the Normalized Full Gradient values and are higher

than 1, structures

1. at horizontal between 36� and 37� latitudes of the

profile and at seventh and eighth harmonics, at

depths averaging between 130 and 170 km

(Fig. 13),

2. at 25th harmonics of the profile, between hori-

zontally between 34�–35� latitudes and vertically

at a depth of 100 km,

3. at 30th harmonics, at a depth of 75 km (34�–35�latitudes),

4. at 40th harmonics, at a depth of 30 km (34�–35�latitudes), and at the 50th harmonics, at depths of

nearly 20–25 km (36�–37�),

were caught up (Fig. 14).

5.2. Profile 2

According to the anomaly contours higher than 1

in values of normalized full gradient for this profile

that pass through the middle of Crete, shallow

(\30 km) and deep structures (100–130 km) are

described in the following schedule.

Shallow structures are encountered

1. at sixth harmonics, horizontally between 36�–37�latitudes and vertically at around 20–30 km

2. According to 25th, 30th and 40th harmonics,

between 34� and 36� latitudes, structures shal-

lower than 30 km are encountered (Fig. 16).Figure 14

Different NFG harmonics (N = 25, 30, 40, 50) of Profile 1




Deep structures are observed;

3. according to their fifth harmonics, horizontally

between 36� and 37� latitudes and vertically at

depths of about 50–100 km (Fig. 16)

4. according to 25th, 30th and 40th harmonics,

horizontally between 34�–36� latitudes and 37�–

38� latitudes, and vertically between 50 and

100 km (Fig. 16).

5.3. Profile 3

At this profile, which lies to the east of Crete

(Profile 3), according to the changes observed in

normalized full gradient values that are higher than 1,

1. at the seventh harmonics of the profile, horizon-

tally between 36� and 37� latitudes, vertically

approximately 160–180 km (Fig. 20),

2. at 25th harmonics, horizontally at 250 km, at a

depth of about 40 km (35�–36�),

Figure 17Epicenter distribution of the Hellenic Arc (From USGS

1973–2004)



3. at 50th harmonics, horizontally at 50 km, and

vertically at a depth of 20–30 km (35�–36�),

4. again, horizontally between 36� and 37� latitudes

of the 50th harmonics and at a depth of nearly

50 km, the existence of structures was observed

(Fig. 21).

As a result of the application of the normalized

full gradient on the cross section taken from Profile 1,

a structural location has been described between 36�and 37� latitudes of profile in the seventh and eighth

harmonics between depths of 130 and 170 km

(Fig. 13). In MEIER’s study (2004b), this point was

described as the terminus of the African oceanic

mantle lithosphere and African oceanic crust, and in

Snopek et al. (2007) as the terminus of oceanic

Mantle and oceanic crust.

In studies of MEIER et al. (2004b) and SNOPEK

et al. (2007), the structure caught in 25th harmonics

of Profile 1 (Fig. 14), at 34�–35� latitudes in a depth

of 100 km was described as astonophere; counters

caught in 30th harmonic, at a depth of 75 km were

called African oceanic mantle lithosphere; those

caught at 40th harmonic, at a depth of 30 km were

described as African oceanic crust; and those caught

at 50th harmonic, at depths of approximately

20–25 km were described as sediments. At the

counter caught at between 37� and 38� latitudes and

approximate at a depth of 50 km of the 50th

harmonic was described as Aegean mantle litho-

sphere in the study of Meier et al. (2004b) and SNOPEK

et al. (2007) (Fig. 14).

Profile 2 crosses through mid Crete. According to

the fifth and sixth harmonics of the profile laterally

between 36� and 37� latitudes, a marked structure

was observed (Fig. 15). At harmonic 5, at a depth of

approximately 100 km, it describes the location of

Figure 18Cross section of the foci depth and bathymetry along the 24.5�

longitudes of Hellenic Arc (from USGS 1973–2004

Figure 19Cross section of the foci depth and bathymetry along the 25.5�

longitudes of the Hellenic Arc (from USGS 1973–2004)


Figure 20Different NFG harmonics (N = 4, 5, 6, 7) of the profile

Figure 21Different NFG harmonics (N = 25, 30, 40, 50) of the Profile 3


oceanic crust, and around 20–30 km depths, very

near to the surface, a structural location was

described (Fig. 15). This area was modeled as

Aegean Crust in the study of SNOPEK et al. (2007).

At the 25th harmonic of the same profile, the area

located at 37�–38� latitudes, between the depths of 50

and 100 km was similarly described as Aegean

Mantle Lithosphere, again at 37�N, at a much more

shallow location, signs of Aegean Crust were caught

up (Fig. 16).

Profile 3 passes by in the east of Crete. A new

structure is observed between the 36� and 37�latitudes of the profile, at about the depths of

160–180 km, at the seventh harmonic (Fig. 20). This

described structure lies at the same place that was

described in the study of PAPAZACHOS et al. (2000) as

the area where the Wadati–Benioff zone terminates.

On the other hand, at the 25th harmonic, at an area

between 35�N and 36�N of Profile 3, at about a depth

of 40 km; at the 50th harmonic, at 20–25 km

shallower depths, structural places are observed. At

the 50th harmonic of the Profile 3, between 36� and

38� latitudes of the harmonic, and at the depth of

nearly 50 km, a presence of a structure is observed

(Fig. 21). This is a place which is the projection of

volcanic island of Santoroni has a secondary slope of

the Wadati–Benioff zone, as seen in Fig. 18, and is

where seismic signs are terminated.

Analogies with previous studies provide all the

evidence but if there is domination between shapes of

the potential fields anomaly of bodies, a vertical

solution can be problematic. Hence, the presence of

potential structures can be clearly defined laterally

along the profile, but some remarks of the seismolog-

ical foci distribution of the area (Figs. 17, 18, 19) are

evident. Therefore, vertical solutions can be more

trustworthy with seismological evidence. In all NFG

profiles, harmonic intervals having fully closed con-

tours were well-matched with the foci depth cross

sections, especially where the depth of the changing

structural specifications or deformation of their shapes

(subduction zone and slopes) was defined by the NFG

method. The main structures between 34�–36� lati-

tudes (below the trenches) and 36�–37� latitudes

(below the volcanic arc) are consistent in north–south

direction along the profile laterally. Also, the other

two profiles show the same logical consistency; for

this reason, the structures below the volcanic arc and

the structures below the trenches of all profiles

extends laterally along the east–west direction.

6. Results

In this study, the performance and reliability of

the NFG method are tested on theoretical and field

data. The Normalized Full Gradient Method was

applied to the gravity values belonging to the Hel-

lenic Arc and its periphery and locations and depths

of the structure with different densities. Separately,

with the help of studies based on seismological foci

depths, structural parameters (locations and depths of

the structures) were obtained. The parameters

obtained from all these studies are consistent with the

results of studies by Meier et al. (2004a, b); SNOPEK

et al. (2007), and PAPAZACHOS et al. (2000). Particu-

larly, as a result of NFG applications to multi-

structure models, the method is able to identify

structures properly. Generally, deep structures can be

identified at shorter harmonic intervals whereas

shallow ones are identified at higher harmonics. It

was determined that the position of the anomaly

source (cubes) affects the shape of NFG contours. For

example, in case the anomaly has a low amplitude

and large wavelength, the shape of the NFG contours

shows similar features. The NFG method can be used

for the determining the locations of the different

structures at horizontally. On the contrary, defining

vertical locations of the different structures with NFG

method may not be clear if there are not any other

approaches which have done before.

Acknowledgments

We are thankful to Prof. Dr. Mustafa ERGUN, Dokuz

Eylul University, Faculty of Engineering, for the data

sets and the view point he brought in.

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(Received March 30, 2010, revised July 11, 2011, accepted July 13, 2011, Published online July 31, 2011)


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Structure of the Hellenic Subduction Zone from Gravity Gradient Functions and Seismology

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