IC/2002/141

THE LITHOSPHERE-ASTHENOSPHERESYSTEM IN THE CALABRIAN ARC

AND SURROUNDING SEAS

G.F. Panza

and

A. Pontevivo

Available at: h t tp : //www. i c t p . t r i e s t e . it/~pub_of f IC/2002/141

United Nations Educational Scientific and Cultural Organizationand

International Atomic Energy Agency

THE ABDUS SALAM INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS

THE LITHOSPHERE-ASTHENOSPHERE SYSTEM IN THE CALABRIAN ARCAND SURROUNDING SEAS

G.F. Panza1

Department of Earth Sciences, University of Trieste, Via Weiss, 4, 1-34127, Trieste, Italyand

The Abdus Salam International Centre for Theoretical Physics, SAND Group, Trieste, Italy

and

A. Pontevivo2

Department of Earth Sciences, University of Trieste, Via Weiss, 4, 1-34127, Trieste, Italy,

Abstract

Through the non-linear inversion of Surface-Wave Tomography data, using as a prioriconstraints seismic data from literature, it has been possible to define a fairly detailed structuralmodel of the lithosphere-asthenosphere system (thickness, S-wave and P-wave velocities of thecrust and of the upper mantle layers) in the Calabrian Arc region (Southern Tyrrhenian Sea,Calabria and the Northern-Western part of the Ionian Sea).The main features identified by our study are: (1) a very shallow (less then 10 km deep) crust-mantle transition in the Southern Tyrrhenian Sea and very low S-wave velocities just below avery thin lid in correspondence of the submarine volcanic bodies in the study area; (2) a shallowand very low S-wave velocity layer in the mantle in the areas of Aeolian islands, of Vesuvius,Ischia and Phlegraean Fields, representing their shallow-mantle magma source; (3) a thickenedcontinental crust and lithospheric doubling in Calabria; (4) a crust about 25 km thick and amantle velocity profile versus depth consistent with the presence of a continental rifted, nowthermally relaxed, lithosphere in the investigated part of the Ionian Sea; (5) the subduction ofthe Ionian lithosphere towards NW below the Tyrrhenian Basin; (6) the subduction of theAdriatic lithosphere underneath the Vesuvius and Phlegraean Fields.

MIRAMARE - TRIESTE

October 2002

1 panza@dst.univ.trieste.it2 Corresponding author, pontevia@dst.univ.trieste.it

Introduction.

The description of the Central Mediterranean geodynamic processes and of the

lithosphere structural properties, as, for example, the nature (oceanic, continental or

intermediate) of the Ionian crust, is characterised by several unknown aspects and it is

still debated. In fact, the opening of the Tyrrhenian Basin, the subduction of the Ionian

lithosphere in the Calabrian Arc and the migration of the back arc system associated to

the Apennine chain, that are also associated to significant lateral variations of S-wave

(Vs) and P-wave (VP) velocities, density (p) and thickness (h) of the lithosphere, aren't

processes jet completely understood.

To make a step towards a better knowledge of the area of the Calabrian Arc and

adjacent seas (Southern Tyrrhenian Sea, Calabria and the North-Western part of the

Ionian Sea; see Fig. 1), new structural models of the lithosphere-asthenosphere system

42° N

4G"N

38°N

36 °N10°E 12°E 14°E 16°E 18PE 20"E

Fig. 1, The sketch of the study area with the considered cells. Different patterns indicate different phase and group

velocity measurement errors associated to the cells, accordingly with the path coverage: (1) Vg measurement error

equal to twice p g (bad coverage) and Vph measurement error equal to Pphm (good coverage); (2) measurement

errors equal to twice p E and twice pDh (bad coverage both in group and phase velocities); (3) measurementom r m

errors equal to p g and twice Pp^ (bad coverage in phase velocity); (4) measurement errors equal to p g and

Pph (S00** coverage both in group and phase velocities). The black bold line represents the study section AB.

(elastic properties and thickness of the crust, lid and asthenosphere) are defined for

cells of l°xl°. It is done through the non-linear inversion of local dispersion data,

obtained from surface wave tomography, using as a priori and independent information,

wherever it is available, seismic and geological data, derived from previous studies in

these areas. The interpretation of the obtained Vs ranges is performed considering the

distributions versus depth of the seismic energy release and the proposed geochemical

and petrological models of the lithosphere (e.g. Ahrens, 1973; Bottinga and Steinmetz,

1979; Graham, 1970; Delia Vedova et al., 1991; Ringwood, 1966). The primary aim of

this study is to fill in the gap in the depth range (Moho discontinuity-250 km) where

body wave tomography does not have optimal resolution (Spakman, 1990; Babuska and

Plomerova, 1990; Alessandrini et al., 1995; Piromallo and Morelli, 1997; Bijwaard et

al., 1998; Lucente et al., 1999) as the so far performed surface wave tomography

(Marquering and Snieder, 1996; Ritzwoller and Levshin, 1998). Ritzwoller and Levshin

(1998) present the result of a study of the dispersion characteristics (from 20 s to 200 s

period for Rayleigh waves and from 20 s to 125 s for Love waves) of broadband

fundamental surface waves propagating across Eurasia, with average resolutions

ranging from 5° to 7.5°. The shear-wave velocity model of Marquering and Snieder

(1996) is a regional European model obtained, from waveforms in the time window

starting at the S-wave arrival and ending after the fundamental mode Rayleigh wave

arrival, using a partitioned waveform inversion. The method takes surface wave mode

coupling into account but it neglects the important effect of amplitude differentiation

(Du and Panza, 1999). In the retrieval of their model, Marquering and Snieder (1996)

have used three background velocity models and they have demonstrated that the

velocity perturbation in the upper layer (from the surface down to a depth of about 30

km) of the inverted model is a combination of the effects of using an erroneous crustal

thickness in the background model and of lateral heterogeneity. For this reason they

have focused their attention to the deeper part of the upper mantle, for which the

influence of crustal thicknesses is small. Our study, that combines surface-wave

tomography and non-linear inversion methods, instead, does not require to resort to any

background model.

The second aim is to see if in the depth range investigated there are relevant phenomena

of partial melting, more or less directly related to volcanism.

The main elements that we are going to investigate are: (1) the lithospheric structure in

correspondence of the Marsili and Magnaghi-Vavilov huge volcanic bodies in the

Tyrrhenian Sea; (2) the evaluation of the horizontal extension, at depth, of the sub-

marine volcanic bodies characterizing the Southern Tyrrhenian Sea, in so far as the

resolution of this tomography study allows us; (3) the evidences about the subduction

towards North-West of the Ionian lithosphere below the Southern Tyrrhenian Sea; (4)

the definition of the lithospheric structure in the Ionian Sea; (5) the evidences about the

subduction of the Adriatic lithosphere.

Geological setting.

The Southern Tyrrhenian Sea is a basin characterised by a thinned continental crust,

about 20 km thick, in periphery (Lliboutry, 2000; Pepe et al., 2000), and its thickness

reduces progressively to less than 10 km, going towards the basin centre.

The central crust, that is younger than 6 Ma and even younger (less than 1.9 Ma) in its

eastern part, in correspondence of the Marsili Basin (Kasten et al., 1988; Faccenna et

al., 2001), is oceanic and lies on the top of abnormal (very low Vs) mantle (Panza and

Calcagnile, 1979; Lliboutry, 2000). In fact, in the central part of the basin, below the

oceanic crust, there are shallow anomalous mantle materials (Pasquale et al., 1990;

Catalano et al., 2001; Trua et al., 2001) in correspondence of the huge submarine

seamounts, peculiar of the area (Marsili and Magnaghi-Vavilov). The volcanism in the

Southern Tyrrhenian Sea was characterized by tholeitic magmas, similar to mid-ocean

ridge magmas, which were thrown out by the mantle in Ustica between 7.5 Ma and 2.5

Ma ago (De Astis et al., 1997). Then there were also eruptions of alkali basalts and,

between 1.3 Ma and 0.2 Ma ago, the volcanism moved away from the centre of the

Tyrrhenian Basin becoming calc-alkaline. More recently, lavas from the potassic series

(shoshonite) appeared in the Aeolian Islands (100.000 years ago in Vulcano and 30.000

years ago in Stromboli). This volcanism evolution is different from that of the

"traditional" intra-arc basin and, perhaps, it is due to the contamination of the magma

by continental sediments, which thicken progressively going away from the centre of

the basin (Lliboutry, 2000). The extension of the volcanic bodies in the Southern

Tyrrhenian Sea isn't very clear but it seems to be larger than earlier believed and, in

fact, Marani and Trua (2002) define the Marsili Basin to be nearly circular in shape

with a diameter of the order of 120 km. The Aeolian arc is a volcanic structure

extending for about 200 km along the Northwestern side of the Calabro-Peloritano

block. The arc, for its magma composition and for the evolution of magmatism,

consists of two main sectors separated by the central islands of Vulcano and Lipari

(Calanchi et al., 2002): the western sector is constituted by Alicudi and Filicudi islands

and by the seamounts of Sisifo, Enarete, Eolo, North Alicudi and North Filicudi; the

Eastern sector by Panarea and Stromboli islands and the seamount of Lamenting Salina

island lies at the intersection between the western sector and the Lipari-Vulcano

alignment. The geochemical analysis of Peccerillo (2001) shows that the mantle

sources beneath Vesuvius, Phlegraean Fields and Stromboli consists of a mixture of

intraplate- and slab-derived components.

The Calabrian Arc is involved in the subduction of the Ionian Hthosphere towards NW,

below the Tyrrhenian Sea, where the depth of the earthquake sources increases with

increasing distance from the Calabrian Arc (e.g. Caputo et al., 1970; Caputo et al.,

1972; Anderson and Jackson, 1987). This Wadati-Benioff Zone represents an

interesting object of study: from the geodynamics point of view, for example, Doglioni

et al. (1998; 2001) propose a different degree of rollback of the oceanic side of the slab

(Ionian part) with respect to the continental western counterpart (western part of Sicily),

which generated a slab window along the Malta escarpment favouring mantle melting

just below Mt. Etna (Gvirtzman and Nur, 1999). From the sketch representation,

proposed by Panza and Romanelli (2001), of the geometry of the Hthosphere involved

in the subduction, one can identify different volumes, each one characterised by its own

uniform stress distribution (Bruno et al., 1999), and the existence of a window on the

asthenosphere beneath northern Calabria (Marson et al., 1995).

The Ionian Sea, which had an important role in the interaction between Africa and

Europe during Tertiary and Quaternary, represents a key area for the understanding of

the evolution of the Mediterranean geodynamics, both for the Apennines and Hellenic

subduction zones (Doglioni et al., 1998; Catalano et al., 2001). There is still debate

about the evolution of the Ionian Sea and its lithospheric properties. Makris (1981), De

Voogd et al. (1992), Finetti et al. (1996), Stampfli et al. (1998), and Catalano et al.

(2001) consider the Ionian abyssal plain floored by oceanic crust, which is about 14 km

thick and is covered by about 8 km of flat-lying sediments. Catalano et al. (2001) from

a review of the geophysical data of the Ionian basin propose that the Ionian Sea is an

abyssal plain of oceanic nature, bounded by two conjugate passive continental margins

(Apulia and Malta escarpments), and that the disappearance of the original median

ridge is due to thermal cooling and lateral burial by tertiary sediments. On the other

side Weigel (1978), Papazachos and Comninakis (1978), Cloetingh et al. (1980),

Farrugia and Panza (1981), Calcagnile et al. (1982), Ismail-Zadeh et al. (1998) and

Bosellini (2002) propose that the Ionian basin crust is of continental-intermediate type

(thinned and stretched). Bosellini (2002), from the reconstruction of the palaeotectonic

and palaeogeographic history of the Eastern Mediterranean area (taking in account the

presence of large dinosaurs on the Apulia carbonate platform), supports the hypothesis

of the Ionian basin as a stretched attenuated continental crust area, generated during the

Jurassic extensional phase.

The Method.

Surface waves dispersion curves, determined along different paths by the Frequency-

Time Analysis (Levshin et al., 1972; 1992), and tomographic methods applied to these

measurements, are widely used to study lateral variations of the crust and of the upper

mantle (e.g. Yanovskaya et al., 1988; Ritzwoller and Levshin, 1998; Yanovskaya et al.,

1998; Vuan et al., 2000; Yanovskaya and Antonova, 2000; Yanovskaya et al., 2000;

Pasyanos et al., 2001; Karagianni et al., 2002; Pontevivo and Panza, 2002). To

determine the thickness and the vertical velocity distribution of the lithospheric layers

in the area under investigation, represented in Fig. 1, we consider the surface-wave

tomography regional study made by Pontevivo and Panza (2002) and, where pertinent,

the large-scale one by Pasyanos et al. (2001).

In Fig. 2 there are shown some examples of tomography maps of group (a, b) and phase

(c, d) velocity obtained by Pontevivo and Panza (2002), using the two-dimensional

tomography algorithm developed by Ditmar and Yanovskaya (1987) and Yanovskaya

and Ditmar (1990).

The lateral resolving power of the surface-wave tomography made by Pontevivo and

Panza (2002) is of about 200 km. However, fixing some parameters of the uppermost

part of the crust, on the base of a priori independent geological and geophysical

information, may improve the lateral resolving power of each cellular mean dispersion

curve and it justifies the choice to perform the inversion for cells of l°xl°. Each cell of

the grid is characterised by average dispersion curves of group velocity, Vg(T) (in the

period range of 10-35 s), and of phase velocity, Vph(T) (in the period range of 25-100

s). Each mean cellular velocity is the average of the values read from the relevant

tomography maps at the four knots of each l°xl° cell. The dispersion relations

computed in such a way are listed in Table I.

a. c.

b.48°N

40 "N

36'N

-25 -8 -6 -4 -2 -I 1 2 4 6 » 25 -5.0 -3.0 -1.5 -1.0 -0.5 -0.2 0.0 0.2 0.5 1.0 1.5 3.0 5.0

Fig. 2. Percent variation of group velocity, dVg/Vg, and of phase velocity, dVph/Vph, with respect to the reference

value (Ref.): group velocity tomography maps at 15 s (a) and 25 s (b) and phase velocity tomography maps at 25 s

(c) and 100 s (d).

At each period the measurement error for the group velocity is estimated from the

difference in the group velocity values determined along similar paths crossing similar

areas, and it is assumed to be the same, pgm, in all the cells characterized by a good

path-coverage. For the cells with a low path-coverage, the assumed error is twice pg .

The measurement error associated to the phase velocity, pphm , is fixed accordingly to

typical values given in the literature (Biswas and Knopoff, 1974; Caputo et al., 1976) in

cells with a good path-coverage, and it is twice pphm elsewhere. In Fig. 1 the cells are

grouped accordingly with the measurement errors associated to group and phase

velocities.

In the inversion, each single point error (pg , pph ) associated to group and phase

velocity values of a cell, is the 2D Euclidean norm (quadratic sum) of the measurement

error (pg , pph ) and of the standard deviation (ag , crph ) of the average cellular

velocities (Vg(T),Vph(T)). In general, the latter error is negligible with respect to the

former.

Non-linear Inversion.

Average models of the crust and of the upper mantle are retrieved by the non-linear

inversion method called Hedgehog (Valyus et al., 1969; Valyus, 1972; Knopoff, 1972;

Panza, 1981) applied to the l°xl° averaged dispersion relations in the study area. This

inversion method represents an optimized Monte Carlo search. Following this

procedure, the structure is modelled as a stack of N homogeneous isotropic elastic

layers, and each layer is defined by Vs, Vp, p and h. Each parameter of the structure can

be independent (the parameter is variable and can be well resolved by the data),

dependent (the parameter has a fixed relationship with an independent parameter) or

fixed (the parameter is held constant during the inversion, accordingly with

independent geophysical evidences). The adopted parameterization for each cell is

given in Table II. For each structural model, phase and group velocity curves are

computed and if, at each period, the difference, d,, between the computed and the

experimental values is less than the single point error (pg , pph ), and if the r.m.s.

values (two distinct values: one for the phase and one for the group velocity) of the

differences are less than a chosen fraction (usually about 60%) of the average of the

single point errors, the model is accepted (Biswas and Knopoff, 1974; Panza, 1981).

Since the problem is not unique, for each cell, the solution is composed by a set of

models (accepted structures), which depends on the parametrization used.

The parametrization of the structure to be inverted is chosen taking into account the

resolving power of the data (Knopoff and Panza, 1977; Panza 1981) and relevant

petrological information (e.g. Ringwood, 1966; Graham, 1970; Ahrens, 1973; Bottinga

and Steinmetz, 1979; Delia Vedova et al., 1991). In such a way we have fixed the upper

limit of Vs, for the sub-Moho mantle, at 4.9 km/s, and the lower limit of Vs at 4.0 km/s

for the asthenosphere at depths greater than about 60 km.

The available interpretations of the seismic profiles that cross most of the peninsula and

adjacent seas and other information available from literature (Calcagnile et al, 1982;

Mantovani et al, 1985; Finetti and Del Ben, 1986; Delia Vedova et al., 1991; Ferrucci et

S-Waye Velocity (km/s)0 1 2 3 4 5 0 1 2 3 4 5

I ' I

0 12 3 4I " 1 ' I '" T T T T T M

0 12 3 4 5

Fig. 3. The set of solutions (thin lines) obtained through the non-linear inversion of the dispersion

relations of each cell. The investigated parameter's space (grey zone) and the chosen solution (bold line)

are shown as well. The Moho is marked with M. Ml and M2, in cell C6, mark the lithospheric doubling.

al., 1991; De Voogd et al., 1992; Scarascia et al., 1994; Marson et al., 1995; Cemobori

et al., 1996; Morelli, 1998; Pepe et al., 2000; Catalano et al., 2001; Finetti et al, 2001)

are used to fix h and Vp of the uppermost crustal layers, assuming that they are formed

by Poissonian solids. The density of all the layers is in agreement with the Nafe-Drake

relation, as reported by Fowler (1995). In most of the cells of the study area, one and

the same relation between Vs, Vp and p has been used for all the dependent parameters

in the inversion. In cells A2, A3, Bl, B2, B3 and B4 in Fig. 1, we have used the value

of Vp/Vs = 2.00 (see Table II), in agreement with the geophysical, geochemical and

petrological model of the sub-marine lithosphere proposed by Bottinga and Steinmetz

(1979). Inversion experiments performed considering values of Vp/Vs smaller than 2.00

supply sets of solutions that do not differ significantly from the ones discussed here.

However, we prefer a high VP/VS also because, all the other elements of the

parametrization remaining unchanged, it gives the largest number of solutions in each

cell. Accordingly with the results of Panza (1981) this means that a high Vp/Vs is more

consistent with the data than a low one. The deeper structure, below the inverted layers,

is the same for all the considered cells and it has been fixed accordingly with already

published data (Du et al., 1998).

Average lithospheric models.

The set of solutions (Vs versus depth) for all the cells, from the surface to the depth of

250 km, are shown in Fig. 3. In each frame, the solutions (thin lines), the explored part

of the parameters space (grey area), the chosen solution (bold line) for each cell are

shown; M stands for Mohorovicic discontinuity, or Moho. At the base of our choice of

the solution for each cell there is a tenet of modern science known as Occam's razor: it

is vain to do with more what can be done with fewer (Russell, 1946). In other words a

simple solution is preferable to one that is unnecessarily complicated (deGroot-Hedlin

and Constable, 1990). Therefore, our choice starts in cells B5 and C5, where the

distribution of the intermediate and deep earthquakes3 is correlated with the largest

seismic velocities, and moves to the neighbouring cells preserving only robust lateral

variations. To reduce the effects of the projection of possible systematic errors into the

3 We consider the ISC catalogue and, to have a gross estimate of the seismic energy, E, released, we use therelation Log E = 11.8+1.5M8, proposed by Gutenberg and Richter (1956). Where necessary, we transform mb

into Ms using Peishan and Haitong (1989) relations, correcting Ms for focal depth as indicated by Herak et al.(2001). The distribution of the energy versus depth is calculated in each cell grouping the events in 15 kmdepth intervals.

10

inverted structural model, the r.m.s. of the chosen solution is as close as possible to the

average value of r.m.s., which is computed from all the solutions.

The structural models chosen for the cells in the Ionian area (B6 and B7, from C6 to C9

and from D5 to D8), the less resolved by our data, satisfy (within the r.m.s. value) the

additional condition to be consistent, at periods greater than 35 s, with the group

velocity values given by Pasyanos et al. (2001). The same condition is satisfied in the

well sampled cells in the Tyrrhenian area (A2-A4, from Bl to B5 and from Cl to C5).

The Crust.

In Table II the fixed Vp, Vs and h of the upper crustal layers are reported for each cell.

The thickness of the water layer is chosen accordingly with standard bathymetric maps.

In cells A2-A3 and B1-B4 (see Fig. 3) the sedimentary layer is very thin or totally

absent and it is followed by a layering consistent with standard schematic oceanic

crustal models (e.g. Panza, 1984). All the other cells are characterized by a thicker

crust, in which the velocity of the sediments increases with increasing depth. In some

cells a low velocity layer appears in the crust, just above the Moho in A4, C4, C9 and

D5 with the Vs in the range 2.75-3.25 km/s, and in the upper crust in B7 (Vs about 2.65

km/s). With the exception of cells C3, C4 and C5, all the cells in the continental region

and in the Tyrrhenian offshore of Sicily, are characterized by a layer just above the

Moho, with Vs in the range 3.60-3.95 km/s. The Vs, just above the Moho, in the Ionian

cells Cl, C8, and D5-D8, instead, falls in the lower range 3.25-3.45 km/s. The location

of the Moho depth in each cell is defined within a few kilometres.

The Moho and the uppermost Upper Mantle.

In the north-western part of the study area (cells A2, A3 and B1-B4), the average Moho

is very shallow (about 7 km deep) and the lid thickness is less than 10 km. Below this

thin and strongly laterally variable lid (Vs in the range 4.00-4.50 km/s), there is a very

well developed low velocity layer (Vs in the range 2.90-3.65 km/s) with variable

thickness (in the range 8-25 km) and centred at a depth of about 20 km (uppermost

asthenosphere). A similar layering is found in the uppermost 30 km of cell A4, the main

difference being a deeper Moho, at a depth of about 15 km. The Vs in the uppermost

upper mantle of this area (cells A2-A4 and B1-B4) is consistent with a high percentage

11

of partial melting (about 10% accordingly with Bottinga and Steinmetz, 1979). This

regional feature, in cells B1-B4, is well compatible with the presence of anomalous

shallow mantle materials reported by Trua et al. (2001) in correspondence of the huge

volcanic structures like the Vavilov-Magnaghi and the Marsili Seamounts; the shallow

partial melting in cells A3 and A4 can be associated to Ischia, Phlegraean Fields and

Vesuvius active volcanoes.

In cells A3 and A4 the top of the high velocity lid (Vs about 4.50 km/s) is at a depth of

about 30-40 km. In the other cells (A2 and B1-B4), the Vs just below the very low

velocity layer in the uppermost asthenosphere, is in the range 4.10-4.15 km/s. In cell

B4, the asthenosphere is perturbed by a layer with relatively high velocity (Vs about

4.40 km/s) and centred at a depth of about 105 km.

Going towards East (cells B5-B7) the Moho is as deep as about 30 km. In cells B6 and

B7 the lid is characterized by high Vs and reaches depths of about 80 km and 100 km,

respectively. In B5 the lid velocity increases with increasing depth and the lithosphere

is about 100 km thick.

The structural models of cells C1-C5 are markedly different from those of cells B1-B4.

In fact, the Moho depth varies from about 13 km to 35 km and the smaller value is

reached in correspondence of the active Aeolian volcanic islands (cells C4 and C5). In

cell C3 the Vs gradient in the lid, which is about 90 km thick, is stronger than the

gradient that characterizes the sub-Moho materials in cells Cl and C2. In C4, the Vs

immediately below the Moho can be as low as 3.85 km/s (about 3-4% of partial

melting, accordingly with Bottinga and Steinmetz, 1979), while the high velocity lid is

found at a depth larger than about 50 km. In C5 the relatively soft lid extends only to

about 17 km of depth and it is on top of a low velocity (Vs about 3.50 km/s) layer, the

asthenospheric wedge (about 10% of partial melting, accordingly with Bottinga and

Steinmetz, 1979), which overlies a lithospheric root (Vs around 4.60 km/s) reaching

about 140 km of depth. The large amount of partial melting (where the S-wave velocity

in the upper mantle is less than 4.00 km/s) in C4 and C5 correlates well with the

presence of the Aeolian volcanic islands (Lipari and Vulcano in C4 and Stromboli in

C5).

The structural properties change abruptly in cell C6. Below a crust about 25 km thick,

and a very thin lid, there is a low velocity (Vs around 3.60 km/s) layer about 10 km

thick, on top of a layer with Vs around 4.60 km/s that reaches a depth of about 100 km.

12

These features (not imposed a priori in the inversion scheme) can be interpreted as a

lithospheric doubling and the deeper Moho can be seen about 38 km deep.

Going towards East, the cells C7-C9 and all the cells D5-D8, in the Ionian zone of the

study area, show a Moho depth in the range between about 22 km and 30 km. In cells

C8, C9, and D6 the velocity in the lid, in general, increases with increasing depth and

the lithospheric thickness exceeds 100 km. In cells C7, D7 and D8, on the top of the

layers characterizing the lid of cells C8 and D6, there is a high velocity layer (in the

range of 4.55-4.75 km/s) that can be as thick as about 25 km. This high velocity layer,

with Vs around 4.60 km/s, thickens going from cell C7 to the West (cell C5). The lid

terminates at a depth of about 140-175 km in C7-C8 and D6-D8.

Starting from C8 and going towards the West (cell C5), the soft lid, with Vs around

4.30 km/s, deepens and its thickness increases from 15 km to 70 km. This soft lid layer

overlies a high velocity layer that reaches a depth greater than 250 km in C5-C6. In cell

D5 an asthenospheric wedge (Vs about 4.10 km/s) is detected just below the Moho and

it overlies a high velocity lid (Vs about 4.50 km/s) that reaches a depth of about 90 km.

The Vs around 4.10 km/s implies that, in the mantle wedge, the partial melting is

around 1-2% (Bottinga and Steinmetz, 1979). The structural setting depicted by the

models for cells C5-C8 can be consistent with the subduction of a rifted continental

Ionian Hthosphere, formed during the Jurassic extensional phase.

The Asthenosphere.

The properties below the uppermost upper mantle change significantly going from the

Tyrrhenian Sea to the Sicily-Calabria offshore and to the Ionian Sea. In fact, the depth

of the top of the asthenosphere is very variable, while its bottom is detected, in general,

at about 200 km of depth. In the Tyrrhenian Sea the asthenosphere is as shallow as

about 10 km, while it starts below 120 km in the Ionian Sea: between the two basins,

the asthenosphere is interrupted by a seismogenic high velocity body dipping towards

NW (cells C5 and C6).

In cells A3 and A4 the high velocity lid is surmounted by an asthenospheric wedge (Vs

about 3.35-4.25 km/s) originally detected by Calcagnile and Panza (1981) and Panza et

al. (1980) and confirmed by the geochemical analysis by Locardi (1986). A similar

situation is seen in cells C4 and C5, where the Vs in the wedge is in the range 3.50-4.35

km/s. In the cells, A2 and B1-B3, Vs reaches about 4.30 km/s in the lower

13

asthenosphere. In general, in this part of the study area, the asthenosphere is

characterized by a layering of Vs that increases with increasing depth. The lower bound

of the asthenosphere is at a depth of about 185 km in A3, 140 km in A4, and deeper

than 200 km in A2 and B1-B3.

In B4 and B5 there is a well developed low velocity asthenospheric layer with a

thickness of about 80 km and a Vs in the range 4,00-4.10 km/s, centred at 150-180 km

of depth.

In B6 and B7, as in Cl and C2, the asthenosphere, with Vs of about 4.40 km/s, starts at

a depth larger than about 50 km and extends to a depth exceeding 200 km, as in

standard stable continental structures (Panza, 1980). In cells C3 and C4 the Vs of the

asthenosphere is about 4.20 km/s. The asthenospheric low velocity layer is well

developed in cells C7, C8 and D6-D8 (Vs about 4.00 km/s). The deep structure of C9 is

very different with respect to that of the neighbouring cells, and resembles that of cells

B6-B7 and C1-C2 where the asthenospheric velocities are relatively high. In cell D5 the

top of the asthenosphere, Vs around 4.00 km/s, is deeper than about 200 km since a

relatively high velocity body (Vs in the range 4.40-4.50 km/s) separates it from the

shallow mantle wedge with Vs around 4.10 km/s.

Discussion.

A new feature, common to the models of most of the cells, is the layering (fine structure)

of the lithospheric mantle. This important finding has been made possible by the improved

resolving power of the data of Pontevivo and Panza (2002) with respect to the one of the

data used to depict the main features of the 3D shear-wave velocity structure beneath the

European area by Marquering and Snieder (1966) and Du et al. (1998).

In the following we will interpret our findings, comparing the structure of each cell in the

study area with pertinent independent models (e.g. Mueller, 1978; Bottinga and Steinmetz,

1979; Panza, 1980; Zhang and Tanimoto, 1992; Keith, 2001).

The Tyrrhenian Basin: Magnaghi-Vavilov and Marsili Seamounts.

The generalized presence of low seismic velocity in the Tyrrhenian Basin is consistent

with the mantle plume hypothesis formulated by Zhang and Tanimoto (1992): our results

are very similar to the upper mantle Vs model of the mid-ocean ridge, associated with

14

shallow and passive upwelling, that is characterized by regions of low seismic velocity at

depths shallower than 50 km (East Pacific Rise, Pacific-Antartic Ridge, Southeast Indian

Ridge). Analogue models, with low velocities in the upper mantle, are found in Afar and

western Saudi Arabia by Makris and Ginzburg (1987), Knox et al. (1998), Hazier et al.

(2001). The East African Rift model, proposed by Knox et al. (1998), shows an extensive

low velocity zone in the upper mantle with a Vs retardation of about 4% attributed to

elevated temperature and partial melting. The presence of a very low Vs below a thin lid

(Fig. 3) makes cells B1-B4 some how similar to the models formulated by Bottinga and

Steinmetz (1979) for the oceanic lithosphere within few tens of kilometres from the ridge

axis. The identification of oceanic ridge structural properties in these cells is in agreement

with the compilations by Mueller (1978) and Panza (1980) about the average crust and

uppermost mantle velocity models that represent the various phases of the evolutionary

process from the break-up of a continent to the generation of new oceanic crust/lithosphere

in the ridge. In the central Tyrrhenian area (cells A2-A3, B1-B4, C5), a relatively high

velocity lid is always present, even if sometime it is very thin. The cell B4 is characterized

by the thinnest lid, and this well correlates with the presence of the Marsili Basin, the

youngest seamount in the Southern Tyrrhenian Sea, while cell A2 has the thickest lid and

no volcanic activity. All the Tyrrhenian submarine volcanic complexes, in cells B1-B4,

seem to be fed by the shallow-mantle magma source represented by the low velocity

mantle material present at depths not exceeding 30 km. This is well in agreement with the

findings of Marani and Trua (2002) who assign to the sub-marine volcanic bodies in the

Southern Tyrrhenian Sea an extension much larger than previously reported and

comparable to that of cells B1-B4.

The Aeolian Islands.

In cell C5 the lid is very thin, while it seems to be missing in cell C4. The Vs model of

these two cells well correlates with the presence, in C5, of the Stromboli volcano and, in

C4, of Lipari and Vulcano islands. In spite of their closeness, the Vs distributions versus

depth under Stromboli and Vulcano-Lipari are different and a significant difference in the

geochemical signatures of the volcanoes is reported by Peccerillo (2001). The clear

difference in Vs distribution, between the Eastern (containing Stromboli island) and the

Central-Western (containing Vulcano and Lipari islands) sectors of the arc, could have

15

been masked by a different positioning of the l°xl° cells, therefore the geochemical data is

a strong support for our findings.

Zanon et al. (2002) in their model of the Vulcano island magmatic evolution give

indication on the depths at which magma reservoirs appear and suggest the existence of a

deep accumulation reservoir, at about 18-21 km, and of a shallow accumulation level in the

upper crust, at a depth of 1.5-5.5 km. Petrological and geochemical data of Gauthier and

Condomines (1999) and Francalanci et al. (1999) show that the magma of the volcano has

properties that are characteristic of sources located at different depths. It is possible to

associate the layer with Vs at about 3.95 km/s, just below the Moho, in C4 to the deepest

magma reservoir found by Zanon et al. (2002), while our data resolution does not allow us

to sort out the uppermost (crustal) accumulation zone. The presence of a deep (about 20-30

km) magma reservoir, possibly correlated with an asthenospheric wedge, is a common

feature to all active volcanoes in the area (see triangles in the insert of Fig. 5).

The Campanian Province.

Cells A3 and A4 contain Ischia island and Vesuvius-Phlegraean Fields, respectively, that

constitute the Campanian Province (Peccerillo and Panza, 1999). The crust is remarkably

different in the two cells, while a low Vs layer (as low as about 3.35 km/s) characterizes

the uppermost mantle of both cells.

The Vs distributions versus depth in cells A4 (Phlegraean Fields and Vesuvius) and C5

(Stromboli volcano) are remarkably similar, and a recent geochemical investigation

(Peccerillo, 2001) reveals close compositional affinities between the Phlegraean Fields,

Vesuvius and Stromboli. Thus there seems to be a good correlation between this volcanic

activity and the subducting Adriatic (Vesuvius and Phlegraean Fields) and Ionian

(Stromboli) slabs proposed by Gvirtzman and Nur (1999).

The transitional and continental area.

Cell B5 can be classified as a continental margin area (e.g. Pasquale et al., 2002), a

transition zone between the Tyrrhenian Basin (oceanic structure) and the Calabria region

with its Ionian offshore (cells B6 and B7 appear as continental structures). In the offshore

of Sicily, cell Cl can be classified as a continental-graben structure and cell C2 as

continental. In cell C3, which contains the, now inactive, Pleistocene volcanic island of

16

Ustica, there is no evidence of an asthenospheric wedge and the lid velocity is quite high

(Vs about 4.50 km/s) allowing to define C3 a continental-like region.

The subduction of the Ionian Lithosphere.

All the cells in the Ionian area cannot be easily associated to one of the models proposed

by Mueller (1978), but, since they miss the high velocity (Vs > 3.6 km/s) lower crust, it is

possible to define them as continental margin structures. Along the section crossing cells

C8-C5, it is possible to identify the presence of a high velocity body dipping below the

Tyrrhenian Sea. To focus on such a feature, in Fig. 4 we have plotted Vs values, from the

surface to the depth of 250 km, of a balanced cross section through cells Bl, B2, B3, B4,

C5, C6, C7 and D7, from the Tyrrhenian Sea to the Ionian Sea, along the line AB shown in

Fig. 1. The line AB follows the palaeogeographic prolongation of the Ionian Ocean

towards the North-West, proposed by Catalano et al. (2001).

WOO] IWaterj > 'U.6o\ IWap (

25 -_

50 -_

75 -j

^ •

•5 125 -

H 150 H

175 H

200 -j

225 -

4.35 - - /350 - =Hf f_*_*>

===ibrz=sc=SE2£=:r̂-/.J5

r̂.30 4.30

4.30

4.40

4.00

4.60

4.25

4.254.80

4.00

4.60

250

4.60 4 (joBl (W- B3 B4

4.60C 5

4.60C6 "J€7

4.30

4.60

4.00

4.60D 7

150 200 250 300 350 400 430 300 550 600 650 700

Distance (km)

Fig. 4. Vs (km/s) versus depth for the uppermost 250 km for the section along AB, crossing cells Bl, B2, B3, B4, C5,

C6, C7 and D7 (with balanced distances), from the Tyrrhenian Sea to the Ionian Sea (see Fig. 1). The section follows the

line of the paleogeographic prolongation of the Ionian Ocean towards the northwest proposed by Catalano et al. (2001).

The Vs values of likely crustal layers are framed.

Some of the distinctive features shown in Fig. 4 are in good agreement with the model

proposed by Doglioni et al. (2001). The main features in Fig. 4 are: the low Vs layers

below the thin lid, corresponding to the shallow-mantle magma sources in the Tyrrhenian

area (Magnaghi-Vavilov and Marsili Basins; cells B1-B4); the very low velocity layer

17

below a thin lid in correspondence of the Stromboli area (cell C5); the stratification in the

uppermost upper mantle of cell C6, representing the lithospheric doubling in Calabria; the

high velocity body of the Ionian subducting lithosphere towards North-West and below the

Southern Tyrrhenian Sea (cells C5 and C6); the thermal effect, due to the subduction, on

the material around the slab and the mechanical interaction between the subducting Ionian

lithosphere and the hot Tyrrhenian upper mantle (cell B4). In this cell, the layer with Vs

around 4.00 km/s, extending from about 140 km to 220 km of depth, can be explained by

dehydration processes and melting along the downgoing slab. In fact, the presence of water

can significantly alter seismic velocities: formation of even small amounts of free water

through the dehydration of hydrous minerals is known to significantly lower seismic

velocities in crustal rocks and the water reduces mantle seismic velocities through

enhanced anaelasticity (e.g. Goes et al., 2000 and references therein).

Going from the Ionian area towards the Tyrrhenian Sea, in cells D7 and C7 the crustal

thickness is less or equal to about 30 km and the lithospheric upper mantle is characterized

by a layering where a relatively low velocity body (Vs about 4.30 km/s) lies between two

fast ones. At depths grater than about 150 km, a very well developed low velocity (Vs

about 4.00 km/s) asthenospheric layer is present in these two cells. In cells C6 and C5,

instead, the low velocity asthenospheric layer is absent and the relatively low velocity

body (Vs about 4.25 km/s) in the lithospheric mantle becomes deeper and thicker going

towards West (Fig. 4).

The layering in the cells D7-C5 seems to be consistent with the subduction of

serpentinized and attenuated continental lithosphere formed in response to the Jurassic

extensional phase. In fact, in zones of lithospheric extension, due to either laterally

directed horizontal forces and/or an upwelling mantle plume (e.g. see Brown and Musset,

1995), a continental lithosphere starts to reduce its thickness, allowing hot asthenospheric

mantle to reach much shallower depths. If the stresses are maintained, a new ocean basin

may form, as in the Tyrrhenian basin. If the strain rate starts to diminish, the lithosphere

approaches its original thickness but with a thinner crust, as could be occurred in the

Mesozoic Ionian lithosphere (e.g. cells C7, C8 and D6-D8). During the tensional phase, the

relatively low velocity (Vs in the range 4.25-4.30 km/s) layer could be formed as result of

the serpentinization of peridotites. Such processes produces Vs retardations of a few

percent, as shown by the experiments of Christensen (1966), and a relevant expansion of

rocks that could have supplied the uplift required by the palaeodynamic and

palaeogeographic review of Bosellini (2002). The presence of a low velocity layer of

18

chemical and not of thermal origin is consistent with the low heat flow in the Ionian Sea

(Delia Vedova et al., 1991). This layer, when subducted (cells C7, C6, C5), gets thicker,

consistently with the dehydration of serpentinite, that is responsible of the weakening of

the neighbouring material.

Conclusion.

The high percentage of partial melting in the upper mantle of the Tyrrhenian Sea produces

Vs values that are usually observed only in crustal layers (Vs < 3.90 km/s). This fact may

make it difficult the seismic definition of the thickness of the crust and consequently of its

nature (e.g. see cells C5 and C6 and their layer with Vs about 3.50 km/s and 3.60 km/s,

respectively, below a high velocity mantle layer).

In cell C5 a detailed analysis of the seismicity leads to two distinct interpretations of the

layer (two-faced "Janus") with Vs around 3.50 km/s. As can be seen in Fig.5, if we plot

only the hypocenters that fall into the band 100 km wide (i.e. of width similar to that of a

cell), along the line AB, the "Janus" layer is totally aseismic and therefore it can be

reasonably assigned to the mantle, where it may represent the deeper magma source for

Stromboli (Zanon et al., 2002). On the other side, if all the earthquakes falling in the cells

crossed by the line AB are plotted, the "Janus" layer is occupied only by hypocenters of

events located either in Sicily and Calabria (continental area) or close to their shoreline. In

such a case the layer can be reasonably assigned to the brittle crust, thus the lithospheric

doubling is not limited to C6 but it can be extended to the continental side of C5. Keeping

this in mind, in the interpretation of the section along AB (Fig. 4), shown in Fig. 5, the red

dashed line has been traced to sketch the subducting Ionian slab, even if the differentiation

between the Tyrrhenian basin and the Ionian Basin is outstanding. Cell B4 represents the

"transition" zone between the very different Ionian and the Tyrrhenian domains.

Our data do not resolve at depths greater than about 250 km, therefore, below this depth,

the subducting slab in Fig. 5 is outlined only on the base of the hypocenters of the

intermediate and deep focus earthquakes (e.g. Caputo et al., 1972; Bruno et al., 1999). The

shallow, intermediate and deep seismicity, with the depth error bars, as given in the ISC

catalogue, is plotted for the time interval 1964-2000. Owing to the three-dimensionality of

the seismogenic body, we plot only the events that fall into a band, about 100 km wide

along the line AB. The seismicity is distributed along the slab and it seems to decrease, but

19

it is not absent, in correspondence of the serpentinized peridotites in the Ionian subducting

lithosphere.

The heat flow values (Delia Vedova et al., 1991; Mongelli et al., 1991) and the mean

Bouguer (Marson et al., 1995) and Airy (Bowin et al., 1981) gravity anomalies well

correlate with the lateral variations of the crustal thickness and of the sub-Moho Vs. In

particular, the Airy anomaly, about -20 mGal in the Ionian Sea and up to 60 mGal in the

B

m Bouguer Anomaly (mGaS)Heat Flow (mW/m**2)

* Airy Anomaly (mGal)

r 60

T 40

~ 20

0

-20

- -40

- -60

-SO

100• " " " I " '

300 3 SO 400

Distance (km)

-rpnr,

450

-i-ef

500 530 700

Fig. 5. Interpretation of section along AB, of Fig. 4, extended to 500 km. The shallow, intermediate and deep seismicity,

with the depth error bar, as given in the ISC catalogue, is plotted. The events fall into a band (the light-blue one in the

insert), about 100 km wide, along the line AB. The red dashed line outlines the downgoing slab. The mean values of the

heat flow (Delia Vedova et al., 1991; Mongelli et al., 1991), of the Bouguer anomaly (Marson et al., 1995) and of the

Airy anomaly (Bowin et al., 1981) are plotted in the upper part of the figure (the italic scale, on the right, is for the Airy

anomaly). From East to West the triangles delineate, in the order, the position of the active volcanic edifices of

Magnaghi-Vavilov, Marsili and Stromboli along the section. In the insert, the location of all the active volcanoes in the

Thyrrenian area are plotted (INGV web page: http://www.ingv.iL/vulcani/vulcani-mappa.html): Ischia in A3, Phlegraean

Fields and Vesuvius in A4, Lipari and Vulcano in C4, Stromboli in C5, Etna in D5.

20

Tyrrhenian Sea, reflects the difference in the lithosphere in the two domains. In the studted

part of the Ionian Sea, the lithosphere is attenuated continental, thermally relaxed after the

Jurassic extensional phase, while in the Southern Tyrrhenian Sea it is very young oceanic.

The seismicity in cell B5 is shallower than 25 km and deeper than 240 km, i.e. the

asthenospheric layer with Vs around 4.10 km/s is totally aseismic. The Vs structure and the

earthquake distribution in B5 could be consistent with the detachment of the Adriatic slab,

in the depth range from about 120 km to 190 km, surmounted by the lithosphere of the

overriding Tyrrhenian Plate, as indicated in the three-dimensional sketch of the south

Tyrrhenian subduction zone proposed by Gvirtzman and Nur (1999). The slab detachment

seems to contradict the P-wave tomography of De Gori et al. (2001) and it is not seen in

our model of cell A4, that contains Vesuvius and Phlegraean Fields. Therefore we interpret

the low velocity layer (Vs about 4.10 km/s) in B5 as a window on the asthenosphere and,

in agreement with Panza and Romanelli (2001), we assign the intermediate depth

seismicity of the cells C5-C6 to the Ionian slab. A much more indisputable similarity

between the sketch of the south Tyrrhenian subduction zone of Gvirtzman and Nur (1999)

and our models is seen in cell D5. In this cell the shallow asthenospheric mantle wedge

between the base of the Tyrrhenian plate and the top of the Ionian slab, well corresponds to

the layer about 20 km thick, with a Vs around 4.10 km/s, centred at about 35 km of depth,

that can well be the deep magma feeder of Etna volcano.

The models of Fig. 3 and the section of Fig. 5 are consistent with the presence of the

asthenospheric window feeding Etna volcano (Gvirtzman and Nur, 1999) and support the

hypothesis, formulated from geochemical investigations (Peccerillo, 2001), that the

intraplate material is provided by inflow of asthenospheric mantle into the wedge above

the subducting plates (Adriatic and Ionian) either through the lithospheric window of the

Adriatic lithosphere and/or from the Tyrrhenian Sea region.

Acknowledgements.

We are grateful to, G.V., Dal Piaz, C , Doglioni, A., Peccerillo, and, S., Sinigoi for their

very fruitful comments about the geological, geochemical and petrological aspects of

our results. This work has been supported by the projects: CNRC007AF8, MURST-

COFTN prot. 9904261312_006 (1999), MURST-COFIN prot. MM04158184_002

(2000), MURST-COFIN prot. 2001045878_007 (2001).

21

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29

Tab. I. Group (Vg) and phase (Vph) velocity values at different periods (from 10 s to 35 s and from 25 s to 100 s,

respectively) with the single point error ( p g , p p l l ) and the r.m.s. values for each cell.

Period

(s)

10

15

20

25

30

35

50

80

100

r.m.s.

CeUA2 (12.5; 40.5)

vE(km/s)

2.60

2.99

3.14

3.17

3.36

3.51

P»(km/s)

0.10

0.08

0.08

0.08

0.08

0.16

0.07

Vph

(km/s)

3.59

3.66

3.71

3.84

3.94

3.98

Pph

(km/s)

0.09

0.06

0.07

0.06

0.06

0.06

0.04

Cell A3 (13.5; 40.5)

(km/s)

2.55

2.74

2.89

2.96

3.22

3.37

Ps(km/s)

0.11

0.12

0.11

0.10

0.10

0.17

0.07

vph(km/s)

3.60

3.66

3.71

3.84

3.94

3.98

PpS

(km/s)

0.09

0.06

0.06

0.06

0.06

0.06

0.04

Cell A4 (14.5; 40.5)

v?(km/s)

2.36

2.45

2.63

2.76

3.06

3.20

Pi!

(km/s)

0.20

0.17

0.15

0.15

0.16

0.32

0.12

Vp,

(km/s)

3.61

3.68

3.72

3.85

3.94

3.98

Pph

(km/s)

0.09

0.06

0.06

0.06

0.06

0.06

0.04

Period

(s)

10

15

20

25

30

35

50

80

100

r.m.s.

Cell Bl (11.5; 39.5)

V,

(km/s)

2.51

2.95

3.16

3.28

3.40

3.52

Ps

(km/s)

0.10

0.10

0.10

0.08

0.09

0.16

0.07

vph(km/s)

3.63

3.68

3.74

3.84

3.94

3.99

Pph

(km/s)

0.09

0.06

0.06

0.06

0.06

0.06

0.04

Cell B2 (12.5; 39.5)

v,(km/s)

2.53

2.85

3.02

3.16

3.30

3.44

Ps(km/s)

0.10

0.11

0.10

0.08

0.08

0.16

0.07

vph(km/s)

3.63

3.68

3.73

3.84

3.94

3.99

Pph

(km/s)

0.09

0.06

0.06

0.06

0.06

0.06

0.04

Cell B3 (13.5; 39.5)

Vg

(km/s)

2.47

2.69

2.85

3.02

3.21

3.36

Ps(km/s)

0.11

0.10

0.09

0.08

0.09

0.16

0.07

Vi*

(km/s)

3.63

3.68

3.73

3.84

3.93

3.99

Pf*(km/s)

0.09

0.06

0.06

0.06

0.06

0.06

0.04

Period

(s)

10

15

20

25

30

35

50

80

100

r.m.s.

Cell B4 (14.5; 39.5)

vg(km/s)

2.34

2.50

2.68

2.89

3.11

3.28

Ps(km/s)

0.11

0.09

0.08

0.08

0.08

0.16

0.07

vph(km/s)

3.64

3.69

3.73

3.84

3.93

3.98

Pph

(km/s)

0.09

0.06

0.06

0.06

0.06

0.06

0.04

CellBS (15.5; 39.5)

v»(km/s)

2.19

2.36

2.56

2.78

3.05

3.24

PE

(km/s)

0.10

0.09

0.08

0.08

0.08

0.16

0.07

Vpt,

(km/s)

3.64

3.69

3.74

3.85

3.93

3.98

Pph

(km/s)

0.09

0.06

0.06

0.06

0.06

0.06

0.04

Cell B6 (16.5; 39.5)

(km/s)

2.11

2.27

2.47

2.73

3.02

3.25

Ps(km/s)

0.10

0.08

0.07

0.07

0.08

0.16

0.07

vph(km/s)

3.65

3.70

3.75

3.85

3.92

3.98

Pph

(km/s)

0.18

0.12

0.12

0.12

0.12

0.12

0.07

Table I: continuing

30

Table I: continuation

Period

(s)

10

15

20

25

30

35

50

80

100

r.m.s.

Cell B7 (17.5; 39.5)

Vg

(fcm/s)

2.18

2.29

2.44

2.80

3.07

3.32

Pi-

(km/s)

0.11

0.09

0.07

0.09

0.09

0.16

0.07

Vph

(km/s)

3.65

3.70

3.75

3.85

3.92

3.98

PPh

(km/s)

0.18

0.12

0.12

0.12

0.12

0.12

0.07

Cell Cl (11.5; 38.5)

vB(km/s)

2.44

2.73

2.91

3.19

3.34

3.48

Ps(km/s)

0.20

0.17

0.15

0.14

0.16

0.32

0,12

vp!l(km/s)

3.66

3.69

3.74

3.84

3.94

3.99

PPh

(km/s)

0.09

0.06

0.06

0.06

0.06

0.06

0.04

Cell C2 (12.5; 38.5)

vs(km/s)

2.41

2.63

2.80

3.10

3.25

3,41

Ps(km/s)

0.20

0.17

0.15

0.14

0.16

0.32

0.12

vph(km/s)

3.65

3.69

3.74

3.84

3.93

3.99

Pph

(fcm/s)

0.09

0.06

0.06

0.06

0.06

0.06

0.04

Period

(s)

10

15

20

25

30

35

50

80

100

r.m.s.

Cell C3 (13.5; 38.5)

v6(km/s)

2.34

2.52

2.71

3.01

3.17

3.34

Pi

(km/s)

0.11

0.10

0.08

0.08

0.08

0.16

0.07

V,*

(km/s)

3.66

3.70

3.74

3.84

3.93

3.99

PPI.

(km/s)

0.09

0.06

0.06

0.06

0.06

0.06

0.04

Cell C4 (14.5; 38.5)

vg(km/s)

2.25

2.45

2.66

2.93

3.11

3.28

Pg

(km/s)

0.11

0.08

0.07

0.07

0.08

0.16

0.07

(km/s)

3.66

3.70

3.75

3.84

3.92

3.98

PT*

(km/s)

0.09

0.06

0.06

0.06

0.06

0.06

0.04

Cell C5 (155; 38.5)

vE(km/s)

2.16

2.39

2.62

2.87

3.07

3.26

PE

(km/s)

0.10

0.08

0.07

0.07

0.08

0.16

0.07

V*

(km/s)

3.66

3.70

3.75

3.85

3,92

3.98

PPI.

(km/s)

0.18

0.12

0.12

0.12

0.12

0.12

0.07

Period

(s)

10

15

20

25

30

35

50

80

100

r.m.s.

Cell C6 (16.5; 38.5)

vB(km/s)

2.12

2.32

2.54

2.80

3.03

3.25

Pg

(km/s)

0,10

0.08

0.07

0.07

0.08

0.16

0.07

vph(km/s)

3.66

3.70

3.75

3.85

3.92

3.98

PPh

(km/s)

0.18

0.12

0.12

0.12

0.12

0.12

0.07

Cell C7 (175; 385)

vs(fcm/s)

2,16

2.29

2.46

2.76

3.04

3.28

Pe(fcm/s)

0.10

0.08

0.07

0.07

0.08

0.16

0.07

Vph

(km/s)

3.65

3.70

3.75

3.85

3.92

3.98

Pi*

(km/s)

0.18

0.12

0.12

0.12

0.12

0.12

0,07

Cell C8 (18.5; 38.5)

Vg

(km/s)

2.25

2.33

2.45

2.79

3.11

3.36

P«(km/s)

0.11

0.09

0.07

0.08

0.08

0.16

0.07

vph(km/s)

3.65

3.70

3.75

3.85

3.92

3.98

PP*

(km/s)

0.18

0.12

0.12

0.12

0.12

0.12

0.07

Table I: continuing

31

Table I: continuation

Period

(s)

10

15

20

25

30

35

50

80

100

r.m.s.

Cell C9 (19.5; 38.5)

ve(km/s)

2.35

2.41

2.52

2.86

3.20

3.46

Pi

(km/s)

0.11

0.10

0.08

0.09

0.09

0.16

0.07

Vp,,

(km/s)

3.64

3.69

3.74

3.85

3.92

3.98

Pph

(km/s)

0.18

0.12

0.12

0.12

0.12

0.12

0.07

Cell D5(15.S; 37.5)

vs(km/s)

2.18

2.45

2.71

2.99

3.16

3.31

Ps(km/s)

0.20

0.16

0.14

0.15

0.16

0.32

0.12

V,*

(km/s)

3.66

3.70

3.75

3.85

3.92

3.98

PPh

(km/s)

0.18

0.12

0.12

0.12

0.12

0.12

0.07

Cell D6 (16.5; 37.5)

v,(km/s)

2.18

2.43

2.67

2.93

3.12

3.30

Pg

(km/s)

0.10

0.09

0.08

0.09

0.09

0.16

0.07

Vph

(km/s)

3.66

3.70

3.75

3.85

3.92

3.98

Pph

(km/s)

0.18

0.12

0.12

0.12

0.12

0.12

0.07

Period

(s)

10

15

20

25

30

35

50

80

100

r.m.s.

Cell I>7 (17.5; 37.5)

ve(km/s)

2.20

2.39

2.61

2.87

3.10

3.32

Pe(km/s)

0.10

0.09

0.09

0.09

0.08

0.16

0.07

vph(km/s)

3.65

3.69

3.75

3.85

3.92

3.98

Pph

(km/s)

0.18

0.12

0.12

0.12

0.12

0.12

0.07

Cell D8 (18.5; 37.5)

Vg

(km/s)

2.24

2.36

2.56

2.82

3.11

3.37

Pe(km/s)

0.10

0.09

0.09

0.08

0.08

0.16

0.07

VPh

(km/s)

3.65

3.69

3.75

3.85

3.92

3.98

Pph

(km/s)

0.18

0.12

0.12

0.12

0.12

0.12

0.07

32

Tab. II. Parametrization used in the non-linear inversion. Grey area: h (thickness), Vs and VP of each layer. The

uppermost layers are fixed on the base of available literature (Calcagnile et al, 1982; Mantovani et al, 1985; Finetti

and Del Ben, 1986; Delia Vedova et al., 1991; Ferracci et al., 1991; De Voogd et al., 1992; Scarascia et al., 1994;

Marson et al., 1995; Cemobori et al., 1996; Morelli, 1998; Pepe et al., 2000; Catalano et al., 2001; Finetti et al,

2001). The variable parameters are Pi, with i= 1, ... 5 for thickness and i= 6, ... 10 for Vs. White area: step and

variability range for each parameter Pi.

h

(km)1 .

&.?

2,

n

P5

h

(km)

PI

P2

P3

P4

P5

vs(km/s)

P6

P7

P8

P9

P10

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v , ) vP

(krK/s)

0.00

1.20

3,45

4.00.

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flO

Step

(km)

4

10

40

50

70

Step

(km/s)

0.30

0.40

0.40

0.15

0.30

(knife)

\S2 '2.05

6.00

.6.90-

PfkLBP7x2.00

1*8x2.00

.F9x2.00

P?0x2.OO

Range

(km)

2-14

10-30

10-50

55-10570-140

Range

(km/s)

3.404.50

2.804.40

3.354.554.004.75

4.004.90

' Cell A3 (13^40.5)

& •

<kns):

15

2.2 •

I

Z

Pi• F 2 •

f>3P i

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h

(km)

PI

P2

P3

P4

P5

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P6

P7

P8

P9

P10

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1.203.45

•. M [ -

;pr.. .PS- •

PiO

Step

(km)

4

7.5

15

45

60

Step

(km/s)

0.40

0.25

0.40

0.25

0.20

Vi>:

' (fcirs/s)

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Z Q S •

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:. P?x2M.

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•P1OX2.0O

Range

(km)

3-23

5.5-28

20-50

15-105

70-130

Range

(km/s)

3.20-4.40

3.10-4.60

3.254.45

4.00-4.75

4.00-4.80

CetiA4(145:

• h •

• (km) ;

• 0.5/ .'

• • ! . - . -

1,5 . •

." sv:

• w-::.

• • re ••

• • P 4 •:

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h

(km)

PI

P2

P3

P4

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P6

P7

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(km)

4

4

12.5

55

70

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(km/s)

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0.25

0.25

0.25

0.25

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Range

(km)

5-25

6-22

10-47.5

50-105

60-130

Range

(km/s)

2.554.15

3.504.50

3.354.60

4.004.75

4.00-4.75

CcB 05 (Il-f:

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3 . •

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2 1 •

: . : f 2 : . ; : •

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• : : ^ : ' . : -

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(km)

PI

P2

P3

P4

P5

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(km/s)

P6

P7

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P9

P10

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0.00.

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Step

(km)

4.5

10

40

60

70

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(km/s)

0.30

0.30

0.25

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Range

(km)

6.5-20

6-31

11-5115-105

60-130

Range(km/s)

3.104.602.804.60

3.404.65

4.004.70

4.004.90

Table II: continuing

33

Table II: continuation

; Cell B2<1Z$; 39S)• b i Vs

fkm) i (k:a/s)

3 • | 0,00

0.7 I 1-20

2-

Pi •

n •, I ' l i n i ' ' , i

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(km)

PI

P2

P3

P4

P5

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P8

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P10

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(km)

4

7

20

50

80

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(km/s)

0.20

0.25

0.40

0.30

0.30

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2,05"':

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6,00 .

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PlOx2.O0

Range

(km)

2-18

8-29

20-60

45-95

55-135

Range

(km/s)

3.10-4.50

2.80^.55

3.30-4.50

4.00-4.90

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P7

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Step

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3

3

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70

70

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(km/s)

0.20

0.20

0.20

0.25

0.30

Range

(km)

3.5-12.5

4-19

30-70

35-105

60-130

Range

(km/s)

3.50-4.70

2.50-4.10

3.70-4.70

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0.30

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Range

(km)

5-21

10-30

25-65

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(km/s)

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2

4

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0.05

0.05

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Range

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6-30

3-31

9-57

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Range

(km/s)

2.42^.61

3.15-4.75

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4.00-4.75

4.00-4.80

Table II: continuing

34

Table II: continuation

( CcB m (145s 39S)

\ &

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50

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(km)

7-22

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3

4

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6-21

12-28

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70-130

40-110

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Range

(km)

12-36

15-40

25-60

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45-105

Range

(km/s)

3.15-4.55

3.15-4.35

3.85-4.65

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(km)

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(km)

9-39

10-40

20-50

50-100

60-110

Range

(km/s)

2.75^.55

3.20-4.55

3.95-4.75

4.00-4.75

4.00-4.80

Table II: continuing

35

Table II: continuation

•«un> •

2

' 0.4 -

•0.4

32 •

• • P J ' . ' . . "

P2.

, F 3 "

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1*5

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5

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7.5-22.5

5-55

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(km/s)

2,55-4.35

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3.60-4.80

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(km)

5-19

5-37

20-50

25-100

25-125

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(km/s)

2.45-4.55

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3,30-4.70

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2-22

4-28

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70-130

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(km/s)

2.30-4.25

3.15-4.75

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60

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(km)

4-16

2.5-32.5

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65-125

60-130

Range

(km/s)

2.70-4.50

3.40-4.40

3.10-4.60

4.00-4.80

4.00-4.75

Table II: continuing

36

Table II: continuation

CeB C? iiySi MSI •:-h .

f ( fen) •

1 2-4f ^ t

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P 2 ; v

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5

15

20

50

60

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(km/s)

0.15

0.20

0.25

0.40

0.40

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Range

(km)

5-30

10-40

15-55

50-100

45-105

Range

(km/s)

2.404.20

3.15-4.55

3.80-4.55

4.00-4.80

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2

15

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50

60

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(km/s)

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0.40

0.45

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0.80

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:mo3'•PIO-si.73

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(km)

4^28

15-30

25-50

50-100

65-125

Range

(km/s)

2.45-4.55

3.10-4.70

3.65-5.00

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4-13

8-24

10-50

70-130

60-120

Range

(km/s)

3.15-4.35

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2.35 '

•::135.-

. - , ! ! « : • • • • .

• : : p9 : :• . . •

Step

(km)

8

13

20

50

60

Step

(km/s)

0.30

0.50

0.35

0.40

0.80

7-:v?,• (ksv's)1.

1.52-. .-

• 3.90-- •

;••.;•• 1 8 0 ; " :

•Mxl.Ti

••??J*h7$

:'-;P8i:I.73::;P9xt73

. • .

Range

(km)

7-23

7-33

25-45

60-110

65-125

Range

(km/s)

2.95-4.45

3.60-4.60

3.45-4.85

4.00-4.80

4.00-4.80

Table II: continuing

37

Table II: continuation

CeRB6{1^5;

.- '.by .

2.7

.lA :

.1-1

•2 .8 •

• w . ••

P2" '

• P3- ::

• " p 4 : - " .

• P S - \

h

(km)

PI

P2

P3

P4

P5

Vs

(km/s)

P6

P7

P8

P9

P10

Vs

(fonfe)

0:0p;

Z2S

•-255:

•3.35 •

• P 6 '

• • • ; • & : : •

: ? 9 : :

•pio:-

Step

(km)

4

20

20

45

60

Step

(km/s)

0.10

0.30

0.50

0.40

0.70

37.5) "

" .V, .

.ikreJ$)

': ^ 2: • 3 . 9 0 :

• 4 . 0 s ' •:

• • S - . S O •

- Pfet.73.:P7xh73

; Hxi.n', p?xi:?3..

'Pioxi.73

Range

(km)

5.5-25.5

10-30

20-40

20-110

65-125

Range

(km/s)

2.35-4.55

3.40-4.60

3.35-4.75

4.00-4.80

4.00-4.70

. . h: : .

' • < & » ' • .

• • 3 • •

: 1-1

, u :.'ZS.--

• • ? i . :

• • • • P S • • . .

; . ; ; . « . : • • :

: . , P 4 . -

•" es 1 '

h

(km)

PI

P2

P3

P4

P5

Vs

(km/s)

P6

P7

P8

P9

P10

• v s -

(JtlStfS)-

• 0 . 0 0 '

2.25-

.235'-

'.ssi~:H'.

P 7 .

• ; p s y

' . • • & .

•pio""

Step

(km)

2

20

40

40

60

Step

(km/s)

0.10

0.20

0.40

0.30

0.40

" -Vy

':••&¥*>:

'•• • \52- ,

•: Aos-

.SM:

•F6xl,?3

F7xl.73

:PSxi.73-

Range

(km)

6-24

10-30

20-60

50-90

65-125

Range

(km/s)

2.45^.45

3.40-4.60

3.50-4.70

4.00-4.90

4.00-4.80

Ceill

ft

(km)-;

.. ! . 2 • •

v 1.1 •:

• 2 . 8 . . :

•, PI:. :

•'.-, P i - : '

'•:'ny::

•:.: P 4 •'••'•:•

':•• P 5 - - .

h

(km)

PI

P2

P3

P4

P5

Vs

(km/s)

P6

P7

P8

P9

P10

• V s

dkm/s)

o.oo-2.25

. ••3.35-.

•:>:**?.;-•

y-n/-

':-M':,yw

Step

(km)

2.5

15

20

50

60

Step

(km/s)

0.10

0.25

0.40

0.30

0.35

"SiSi.

v?• (taWs)

152

3.90 j

- • 4JQ5. j

• •P6x! ,"3 .

:P7x1;73'

•PSxl,73'

PiOxl.73

Range

(km)

5-30

20-35

20-40

55-105

65-125

Range

(km/s)

2.70-4.60

3.50-4.75

3.45-4.65

4.00-4.90

4.00-4.70

38