shallow structure of the somma–vesuvius volcano from 3d inversion of gravity data

al Research 161 (2007) 303–317www.elsevier.com/locate/jvolgeores

Journal of Volcanology and Geotherm

Shallow structure of the Somma–Vesuvius volcano from3D inversion of gravity data

Federico Cella a,⁎, Maurizio Fedi b, Giovanni Florio b,Marino Grimaldi b, Antonio Rapolla b

a Department of Earth Sciences-University of Calabria-Italyb Department of Earth Sciences-University “Federico II” of Naples-Italy

Received 23 February 2005; received in revised form 8 November 2006; accepted 7 December 2006Available online 19 January 2007

Abstract

A gravity investigation was carried out in the Somma–Vesuvius complex area (Campania, Italy) based on a dataset recentlyenlarged with new measurements. These cover the volcanic top and fill some other important spatial gaps in previous surveys.Besides the new gravity map of the Vesuvius, we also present the results of a 3D inverse modelling, carried out by using constraintsfrom deep well exploration and seismic reflection surveys. The resulting density model provides a complete reconstruction of thetop of the carbonate basement. This is relevant mostly on the western side of the survey area, where no significant information waspreviously available. Other new information regards the Somma–Vesuvius structure. It consists of an annular volume of rocksaround the volcanic vent and that extends down to the carbonate basement. It results to be denser with respect to the surroundingsedimentary cover of the Campanian Plain and to the material located just along the central axis of the volcanic structure. Thecoherence between these features and other geophysical evidences from previous studies, will be discussed together with the otherresults of this research.© 2007 Elsevier B.V. All rights reserved.

Keywords: Mt. Somma–Vesuvius; gravity modelling; 3D data inversion; volcanic structure; Mesozoic carbonate basement

1. Volcanological and geophysical framework

The Phlegrean Fields and the Somma–Vesuviuscomplex are located on the eastern margin of theCampanian Plain (Southern Italy). This Plio-Pleistoceneplain is surrounded to the N, E and S by the mesozoiccarbonate massifs of the Apennine chain and, to the W,by the Tyrrhenian Sea (Fig. 1). It is filled by marine andvolcanic sediments covering the carbonate basement,which has a maximum depth hypothesized to reach some

⁎ Corresponding author.E-mail address: [email protected] (F. Cella).

0377-0273/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.jvolgeores.2006.12.013

thousands of meters (Ippolito et al., 1973). The genesis ofthe Campanian Plain is similar to other peri-Tyrrhenianregions and is related both to tensional stresses andcrustal thinning toward the Tyrrhenian Basin, caused bythe anti-clockwise rotation of the Italian Peninsula, and tothe recent extensional phases following the ApennineChain building. The subsidence in this area wasaccompanied by intense and still active volcanic pheno-mena evidenced in its southern part by the Somma–Vesuvius complex. It is a stratovolcano of moderate sizeformed by an older edifice,Mt. Somma, and by a youngernested cone, the Vesuvius (1276 m a.s.l.). Since theearliest activity, dated to 0.4 yr B.P. (Bocchini et al.,

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304 F. Cella et al. / Journal of Volcanology and Geothermal Research 161 (2007) 303–317

2001), the volcanic complex has erupted about 200–300 km3 (Andronico et al., 1995) of potassic magmas,ranging from leucitic tephrite to leucitic phonolite(Santacroce et al., 1995). After a long period ofquiescence, that began 0.3 myr B.P. (Bocchini et al.,2001), the activity re-started after 37,000 yr B.P. andcontinued with a variable kind of volcanism (both quietlava emissions and strong explosive eruptions) (Santa-croce, 1987; De Vivo et al., 1993; Rolandi andMcGeehin, 1995; Rolandi et al., 1998). Several eruptivecycles were distinguished (Carta et al., 1981) (‘Basal’:18,500 yr B.P.; ‘Mercato’: 8800 yr B.P.; ‘Avellino’:3800 yr B.P.; ‘Pompei: 79 A.D.). They were correlated tothe break-up of the upper part of the Somma cone byinternal collapse (Andronico et al., 1995; Rolandi et al.,2004), and to the failure of the W and SW flank of theSomma edifice (‘Avellino’ eruption) and of the SE flank(‘Pompei’ 79 A.D. eruption) (Rolandi et al., 2004). TheVesuvius cone grew up during the interplinian phase ofthe Middle Age, (Rolandi et al., 1998) even though someminor collapses are recognized (i.e. 472, 1139 and 1631).The subsequent cyclic strombolian activity (1631–1944)(Arno' et al., 1987; Scandone et al., 1993) wasinterpreted as the cyclic emptying of the shallowplumbing system (Civetta and Santacroce, 1992; Santa-croce et al., 1994). Presently, only moderate fumarolicemissions and a low level of seismicity are observed.

The first geophysical investigations carried out on theSomma–Vesuvius volcanic complex consisted in agravity survey (Ciani et al., 1960; Tribalto and Maino,1962; Maino et al., 1963). A seismic reflection survey(Finetti and Morelli, 1974; Oliveri del Castillo, 1966;Carrara et al., 1973) in the offshore area of the Gulf ofNaples revealed the existence of a strong reflector inter-preted as the top of the mesozoic carbonate basement.

Petrological and isotopic studies (Barberi et al.,1981; Rosi et al., 1987a,b; Civetta and Santacroce,1992) identified the Vesuvius magma chamber at depthranging 3–6 km. The studies of the gas inclusions inejected nodules by Belkin et al. (1993) suggest a fluidtrapping pressure at 4–10 km depth. The existence of anextended and deep (top at 8 km) magmatic body wasfirst proposed by Fedi and Rapolla (1987) to interpret alarge gravity low in the Campanian plain. Recent studiesbased on seismic data (Zollo et al., 1998; Auger et al.,2001) confirm the gravity result and identify a lowvelocity layer possibly representing the top of themagma chamber at depths (8–10 km).

A regional aeromagnetic survey carried out by AGIP(1981) revealed a nearly symmetric, isolated and strongdipolar anomaly, in correspondence of the volcanicrelief. The occurrence of highly magnetized rocks within

the volcanic complex (Cassano and La Torre, 1987) alsoresults from measurements on lavas and pyroclasticrocks sampled in the Trecase 1 well (Principe et al.,1987). Fedi et al. (1998) interpreted the magnetic field bya model showing high magnetisations from the emergedcentral part of the volcano down to a depth of 2 km, thehighest values occurring in the shallowest part. The lackof magnetization below 2 km was interpreted as due tothe presence of the carbonate basement.

Geoelectric and magnetotelluric surveys over theVesuvian area (Di Maio et al., 1998), found largeconductive volumes extending towards the Tyrrheniansea below the Somma caldera, and high resistivity at600–2200 m b.g.l. beneath a former caldera (Avellinoeruption). These may be attributed to altered andmineralised blocks of cemented volcanic breccias.

A complete reprocessing of several seismic reflectionlines provided a bathymetry map of the top of thecarbonate basement (Bruno et al, 1998; Bruno andRapolla, 1999). A high rigidity core beneath the axis ofthe volcano, surrounded by rocks with lower seismicvelocities was suggested by De Matteis et al. (2000) andLomax et al. (2001). Accordingly, Zollo et al. (2002)interpreted a shallow high velocity zone beneath theMount Somma caldera as a palaeovolcanic structure.

2. Gravity survey

Previous gravity investigations at Mt. Vesuvius(Tribalto and Maino, 1962; Cassano and La Torre,1987; Italian Geologic Survey, unpubl. data) providedlow resolution data in the area above the volcanic relief.Hence, new gravity measurements were collected duringtwo surveys at the end of the ‘90s (Bruno et al., 1994;Rapolla et al., 1996; Bruno et al., 1997). Gravity wasmeasured at 436 stations covering an area of more than12 km2 (Fig. 1). Most of the gravity stations were locatedat elevations higher than 600 m a.s.l., both on circularprofiles around the crater and on radial profiles along itsslopes. Their positions strongly depend on the morphol-ogy of the area, where some locations were inaccessible.

The reference base station was located at the site ofthe Vesuvian Observatory and was linked to that ofNaples, belonging to the Italian Zero Order Gravity Net:Lat.=40° 50′ 48″, Long.=14° 15′ 31″, h=24 m,g=980 258.066±0.005 mGal, (Berrino, 1995).

The elevation of each station was measured withreference to the geodetic networks installed by theVesuvian Observatory for the volcanic surveillance. Thegravity was measured by a La Coste and Romberggravimeter mod. D-137 and corrected with standardprocedures (tidal and instrumental drift, free air,

Fig. 1. Sketch map of the investigated area; small crosses: gravity stations; dashed lines: profiles of the sections A, B, C, D, E.; dark contour lines:altitude levels (equidistance=200 m). The Somma–Vesuvius complex is located within the Campanian Plain, filled by marine and volcanic sedimentsof Plio-Pleistocenic age and formed by subsidence of the Mesozoic carbonate basement, due to the crustal thinning toward the Tyrrhenian Basinand to the Apennine Chain building.

305F. Cella et al. / Journal of Volcanology and Geothermal Research 161 (2007) 303–317

Fig. 2. Bouguer gravity anomaly map in the investigated area. Equidistance: 1 mGal; black crosses: gravity stations. The north-westward extendedgravity low represents the south-eastern side of a large gravity low centred north of the investigated area (near Acerra). Two less intense negativeanomalies are shown near Pompei and in correspondence of the Somma–Vesuvius complex; they are presumably due, respectively, to a tectonicdepression of the carbonate basement and to the shallow volcanic structure. Two gravity highs are visible, respectively, at the southern and easternends of the investigated area, where the carbonate series of the Apennine Chain outcrops.


Bouguer and terrain corrections). The gravity anomalieswere computed by using the International formula of theGeodetic Reference System 1980 (Moritz, 1984). A

Fig. 3. Enlarged Bouguer anomaly map in the region including the investigatecut-off wavelength. This value provided the best threshold among several tregional trend due to deeper sources.

reference density of 2.4 g/cm3 was assumed for theBouguer and terrain corrections allowing integrationwith existing datasets.

d area (magenta square). The numerical filtering was set up by a 45 kmests to separate the shallow effects (upper crustal structure) from the

Fig. 4. Regional trend of the gravity anomaly field in the investigated area, NNE-SSWoriented and presumably due to deep crustal and sub-crustalsources. Equidistance: 1 mGal; black crosses: gravity stations. Low pass filter with cut-off wavelength: 45 km.

Fig. 5. Residual gravity anomaly field in the investigated area, related to density contrasts at shallow crustal depths. Equidistance: 1 mGal; blackcrosses: gravity stations. The negative anomalies, in correspondence of the volcanic complex and east of Pompei, are clearly visible.



3. Gravity anomaly field

The Bouguer anomaly field of the studied area(Fig. 2) shows a gravity low extended toward the north-westward side of the studied area. It represents thesouth-eastern part of a long-wavelength anomaly lowcentred near Acerra (north of the investigated area).

A second, less intense, main anomaly low is locatedbetween Poggiomarino and Pompei, whereas a third oneis centred just in correspondence of the Somma–Vesuvius complex, presumably due to a shallowvolcanic structure. Some local relative highs with veryshort wavelength and low amplitude are located aroundthe Vesuvius crater and surround the above mentionedanomaly low.

Two anomaly highs occur in correspondence of thecarbonate outcrops, south of Castellammare and east ofPalma Campania (respectively, southern and easternends of the investigated area).

A significant correlation probably exists between theabove described gravity pattern and the morphology ofthe top of the Mesozoic carbonate platform. As inferred

Fig. 6. Map of the isobaths of the top of the carbonate basement from seisminformation just beneath the volcanic relief and within the western and Southeinformation as far as the buried structures. However, a generalized deepeningplain. Black line: boundary delimiting the western area where no constraints

by previous works (Fedi and Rapolla, 1987), the gravityanomalies due to the Mesozoic carbonates are partiallymasked by a regional trend, NNE oriented, and relatedto deep crustal and sub-crustal sources.

The most suitable cut-off wavelengths (from 55 to20 km) were tested to separate by numerical filtering theshallow effects (upper crustal structure) from theregional trend due to deeper sources. To this aim, adata set covering an area much larger than that relativeto this research (Long.(°): 13.8–14.8; Lat.(°): 40.58–41.3) was processed. The data matrix was previouslytapered by means of a maximum entropy predictivefilter to prevent the appearance of fictitious anomaliesalong its edges. Finally, a cut-off wavelength of 45 kmwas chosen for high-pass numerical filtering (Fig. 3). Inhigher detail, the residual gravity map obtained bysuppressing the regional contribution to the gravity field(Fig. 4), shows a gravity pattern similar to the originaldata set, but with some noticeable difference (Fig. 5): theanomaly low of the Somma–Vesuvius and, even more,the Pompei anomaly low are increased in amplitude,whereas the anomaly high is attenuated southward.

ic data (modified from Bruno et al., 1998). The seismic data set lacksrn sectors of the investigated area and, therefore, provides a few reliableof the carbonate basement is inferable beneath the western side of theare available.


4. 3D gravity modelling

The residual gravity anomaly field was interpreted bymeans of a 3D inversion method (‘GRAV3D’) developedby Li and Oldenburg (1995). Their software was inte-grated with a custom program with new tools, in order tomore easily manage large 3D input/output data sets (seeAppendix). In the present case, the shallow crustalstructure of the Campanian Plain around the Somma–Vesuvius complex (with a volume of about 7500 km3)was discretized by means of a mesh model that extended26,200 m eastward, 25,000 m northward and 10,700 mdownward, for a total number of 371,864 finite elements.The demand of high resolution within the central and

Fig. 7. 3D density model of the shallow crust beneath the Somma–Vesuvius v0.95 km, 1.2 km, 1.55 km, 1.9 km. The gradual north-westward increase in dlight blue: marine-volcanic sediments).(For interpretation of the references tothis article.)

shallower sectors of the investigated volume was satisfiedby gradually decreasing the size of the finite elementstoward the inner zone of the model (see Appendix).

A crucial phase of the gravity data interpretation wasestablishing constraints based on the available geologi-cal and geophysical data. They are represented by:

(1) The individuation of massive Mesozoic dolomitesrepresenting the top of the carbonate basement at adepth of about 1665 m below sea level incorrespondence of the deep well “Trecase I”(Bernasconi et al., 1981; Calducci et al., 1985);

(2) The outcrop of the carbonate basement borderingthe boundaries of the Campanian Plain, along the

olcanic complex: horizontal sections. Depths: 0.2 km, 0.45 km, 0.7 km,epth of the carbonate basement is clearly pointed out (red: carbonate;colour in this figure legend, the reader is referred to the web version of

Fig. 8. Map of the isobaths of the top of the carbonate basement from 3D density model of the shallow crust beneath the Somma–Vesuvius volcaniccomplex. Equidistance: 125 m; blue line: boundary delimiting the eastern area where the top of the carbonate series is partially constrained by seismicdata (Bruno et al., 1998) from the western area where no constraints are available. Several structural lineaments can be inferred from the morphologyof the top carbonate basement. Red solid lines: faults verified by previous studies; dashed red lines: faults conjectured by other studies; dotted redlines: fault inferred by this investigation and uncorrelated with previous evidences.


southern and north-eastern side of the investigatedarea (near Castellammare and Palma Campania),and along the coastline from Castellammare toTorre Annunziata (Rovigliano Islet).

(3) The map of the isobaths of the top of the carbonateprovided by Bruno et al. (1998) by interpolatingresults from seismic reflection prospecting.

The sedimentary cover filling the Campanian Plainexhibits a broad range of densities depending on the natureof the sediments (marine or volcanic, originating silts ortuffites). Zollo et al. (1998), chose a 0.3 g/cm3 densitycontrast between the sedimentary fill and the carbonaterocks for simple 2D gravity modelling. Cassano and LaTorre (1987) interpreted a profile crossing the Somma–

Vesuvius complex and the whole Campanian Plain andused the constraint of the density sample from theexploration well Trecase I. Here, the average density ofthe volcanic-sedimentary cover (2.26 g/cm3) is computedbasing on 18 samples and is considered by the authors as“fairly indicative of the rocks filling the Campanian basinin the Vesuvian sector”. The shallower layer of thebasement (about 300 m thick) is made of a carbonatematrix conglomerate with a 2.54 g/cm3 density, whereasthe underlying dolomites have a density of approximately2.81 g/cm3. Thus, an average value slightly higher than2.6 g/cm3 can reasonably describe the density of thebasement, leading to a 0.35 g/cm3 density contrast. On theother hand, the authors set a 2.3 g/cm3 density for thecover and 2.6 g/cm3 for the basement, but along another


section they use a 0.5 g/cm3 density contrast. Therefore,the density contrast initially set in the reference modelbetween the carbonate basement and the upper shallowcover (0.4 g/cm3) seems a good compromise between thedata reported by the above mentioned studies and thedensity determinations from the Trecase I well.

As far as the goodness of fit is concerned, theinversion was set up to provide output data affected by amaximum absolute error smaller than 5% for each gravitymeasurement. In fact, trying to fit for less than this valueled to very unstable solutions, magnifying the data errors,while allowing for a bigger error in the solution resultedin the loss of resolution. Hence, a 5% misfit wasempirically judged as a reasonable value assuring ameaningful and detailed character to the solution.

5. Discussion

5.1. The carbonate basement

The map of the isobaths of the top of the carbonatecomputed by Bruno et al. (1998) (Fig. 6) provides themain information about the basement. It howevertakes into consideration only the north-eastern, easternand south-eastern side of the investigated area. Themorphology of the basement in the remaining area,including the sector immediately beneath the Somma–Vesuvius complex, was still debated because of thescarcity of complete and reliable data. Therefore, the 3Dgravity modelling was aimed to focus on the recon-struction of the top of the carbonate sequence withinthose sectors (the western and the southern ones,including the area beneath the volcano) where informa-tion is lacking.

It is important to point out that the massivedolostones found at the base of the sedimentarysequence in the “Trecase I” well are covered by a300 m thick layer consisting of a carbonate matrixconglomerate with clasts made by limestone/dolostoneand by a 100 m thick layer of limestone conglomerates.Their origin is due to the erosional phases following thenormal faulting of the Tyrrhenian margin of theApennines during the Early Pleistocene (Bocchiniet al., 2001) and its widespread presence within thesouthern Campanian Plain is highly presumable. Thedensities of these conglomerates and of the massivecarbonate are almost similar and therefore their gravityeffect cannot be distinguished. The shallowest layer(limestone) marks the carbonate top reconstructed byBruno et al. (1998), used in this case as the constraint.Therefore it is contained within the high densitybasement provided by gravity data inversion.

The inverted 3D gravity model is summarized byhorizontal density sections (Fig. 7). The morphologicalanalysis of the top of the carbonate basement, mapped inFig. 8, supports the existence of several escarpments bynormal faulting, locally evidenced within the carbonateseries by seismic data (Bruno et al., 1998). Manytectonic lineaments with prevalent Apennine (NW) andanti-Apennine (NE) directions can be singled out; someof these are quite coincident with the fault linespreviously extrapolated from seismic data by Brunoet al. (1998) and by other authors (Finetti and Morelli,Cassano and La Torre, 1987; Rosi et al., 1987a,b;Bianco et al. 1998). This is the case of the normal faultsbordering the western side of the carbonate series incorrespondence with the Sorrento Peninsula (southernpart of the map) and the fault at the south-westernboundary of the Sarno Mountains.

It must be pointed out that the fault lines traced inFig. 8 do not often correspond to abrupt dislocations ofthe Mesozoic basement, as one could expect. This isprobably due to a) a lack of vertical resolution caused bytoo large vertical cell sizes, b) the occurrence of a systemof step escarpments rather than a single fault, and c) thepresence of an oblique slope generated by the fault scarpretreating due to old sub-aerial erosional processes. Inthis last case the original fault scarp lines were tracedalong their presumed true position that is coincidentwith the lower base of the oblique slope.

Note that the existence of other tectonic trends, onlyconjectured by previous studies, is confirmed here. Asan example, a fault system has to be mentioned withENE-WSW, NNW-SSE and NW-SE directions, causinga localized subsidence of the carbonate basement incorrespondence of Pompei (South-East of the Somma–Vesuvius). The resulting trough is clearly related tothe corresponding anomaly low mentioned in SectionIII and probably reaches a maximum depth of 2000 mb.s.l., even though in a narrow area (Figs. 7, 8, 10and 11).

Another example is a system of normal faults, orientedNE and NW, identified just beneath the Somma–Vesuviusand probably related with the emplacement of the feedingsystem of the volcanic complex.

Finally, a third class of structural trends has to bementioned, including features that have no correlationwith evidence from previous studies. Fault lines havingprevailing NNE and ENE orientation may be noted,which mark the escarpments displacing downward to thecarbonate basement on the Tyrrhenian side at depth ofabout 2000 m below sea level. The resulting depressionhas a flat morphology and covers the whole westernsector of the studied area (Fig. 8 and western side of the

Fig. 11. 3D density model of the shallow crust beneath the Somma–Vesuvius volcanic complex: Section C (see Fig. 1) located to the east of thevolcanic relief; southward, the section shows the small carbonate trough beneath Pompei; no vertical exaggeration.

Fig. 10. 3D density model of the shallow crust beneath the Somma–Vesuvius volcanic complex: Section E (see Fig. 1) located south of the volcanicrelief; eastward, the section shows the small carbonate trough beneath Pompei; no vertical exaggeration.

Fig. 9. 3D density model of the shallow crust beneath the Somma–Vesuvius volcanic complex: Section D (see Fig. 1) located north of the volcanicrelief; the section shows an increase in depth of the top of carbonate basement to the west. The gradual deepening probably reveals a verticaldislocation along a system of normal faults; no vertical exaggeration.


Fig. 13. 3D density model of the shallow crust beneath the Somma–Vesuvius volcanic complex: Section A (see Fig. 1) running beneath the volcanicrelief; the density contrast between the central axis of the volcano (shattered wall rocks) and its lateral structures (lavas and tuffs) is shown; no verticalexaggeration.

Fig. 12. 3D density model of the shallow crust beneath the Somma–Vesuvius volcanic complex: Section B (see Fig. 1) running beneath the volcanicrelief; north of the volcanic complex, the depth of the carbonate basement does not change significantly. The density contrast between the central axisof the volcano (shattered wall rocks) and its lateral structures (lavas and tuffs) is shown; no vertical exaggeration.

Fig. 14. 3D model of the shallow crust beneath the Somma–Vesuvius volcanic complex from magnetic data (from Fedi et al., 1998); highmagnetisation sources are identified from the outcropping central part of the volcano down to a 2 km depth. Highest values occur in the shallowestpart and abrupt decrease below 2 km, presumably because of the presence of the carbonate basement.



section in Fig. 9). It presumably extends northward,toward the graben structure causing the anomaly low ofAcerra (Fedi and Rapolla, 1987). This deepening of thecarbonate basement is a local effect of the regionalsubsidence by normal faulting caused by the stretchingand thinning of the continental crust connected to theopening of the Tyrrhenian basin.

The above mentioned tectonic depressions are sepa-rated by a sort of carbonate “ridge” (central part of thesection in Fig. 10). It runs slightly southward with respectto the Somma–Vesuvius volcanic relief, and is interceptedby the Trecase I well at a depth of about 1350 m. The axisof the volcano is located in correspondence with thewestern flank of this ridge, where the top of the carbonatesequence is at about 1400–1500 m of depth b.s.l.. Thisrange is very similar to that suggested by recent seismicstudies (1300–1600 m b.s.l.; De Matteis et al., 2000).

The basement outcropping on the southern side of theinvestigated area represents the emerging part of theSorrento Peninsula carbonate horst (southern end of thesection in Fig. 11). On its western side a S-N ridge ispresent rising at shallower depths (southern part of thesection in Fig. 12). The second carbonate outcrop marksthe north-eastern side of the investigated area (PalmaCampania) and represents the external slope of theApennine chain (Fig. 9).

5.2. The volcanic structure

The density model provided by 3D gravity datainversion indicates an average contrast between base-ment and sedimentary cover (∼0.4 g/cm3) slightlysmaller than that suggested in previous work (Cassano

Fig. 15. Morphology, reconstructed by gravity modelling, of the carbonsurrounding regions.

and La Torre, 1987). However, sediments slightlydenser in average than the surrounding Campanianseries are evidenced beneath the Somma–Vesuviusrelief. Immediately beneath the volcanic relief andalong its vertical axis a mass deficiency has been alsosingled out (section b in Fig. 12; section a in Fig. 13).The less dense material is located from the top of thevolcano down to the carbonate basement and showsdensity contrasts ranging from −0.15 g/cm3 to −0.2 g/cm3 with respect to the surrounding rocks.

As already stated, recent modelling of aeromagneticdata (Fedi et al., 1998) linked the intense magneticanomaly centred on Vesuvius to high magnetizationsources (∼5 A/m) located at shallow depths in thevolcanic complex (Fig. 14), mainly between thetopographic surface and down to about 2000 m b.s.l.(1 A/m). This magnetic source hence accords with theabove mentioned low-density body. The hypocentres ofthe Vesuvian earthquakes are concentrated along avertical belt just beneath the crater, in correspondenceto the expected source of gravity and magneticanomalies. We may interpret the above seismic,magnetic and gravity evidence with a brittle, highlymagnetized and less dense body, which should beexpected at levels located between the carbonatebasement and the surface. This shallow source, justbeneath the topographic relief, would fill a collapsedzone made by intensely broken lava dykes. Such anintense fracturing of volcanic rocks could explain boththe negative density contrast and the seismic activity inthis area but, at the same time, this body source shouldstill keep enough high volume magnetization to generatean intense magnetic anomaly at the surface.

ate basement beneath the Somma–Vesuvius volcanic complex and


6. Conclusions

To constrain the 3D inversion of the gravity datasetof the Vesuvian region, information from reflectionseismic data was used to define the depth to the car-bonate basement within the studied area in the referencemodel. Other constraints came from wells and surfacegeology. Our interpretation of the gravity low centred onMt. Vesuvius seems to be in agreement with the hypo-thesis of a collapse within a narrow, funnel-shaped zonein the center of the volcanic edifice. This wascharacterized also by the repeated break-up of theupper part of the Somma cone, that occurred mainly byexplosive eruptions that produced internal collapses(Escher, 1929; Rolandi et al., 2004). The result is a low-density caldera filled not only by pyroclastics but alsoby shattered lava rocks.

It is inferred from the inverted density model thatdenser material surrounds the central axis of the volcanicstructure and forms a ring-shaped volume directlybeneath the flanks of the volcano. Results from theinverted density model justify expectations that the areaconsist of more compact rocks, mainly made by lavaflows, tuffs and tuffites, like those sampled at severaldepths in the Trecase I well (Bocchini et al., 2001).

This interpretation agrees with evidence fromprevious seismic studies (Bruno and Rapolla, 1999)pointing out a ring fault system around the volcano andindicating a high lateral heterogeneity with a largenumber of fractures.

On the other hand, the gravity low observed on theSomma–Vesuvius relief makes other geophysical inter-pretations difficult to justify. Among them the hypoth-esis of a central, quasi spherical high rigidity body,located beneath the axis of the volcano at depths startingfrom 1.5 km, and surrounded by rocks with lowerseismic velocities (De Natale et al, 1998; De Matteiset al., 2000). Such a model is hardly compatible with theabsence of a high density source, since a correlationbetween high rigidity/velocity and high density sourcesshould be expected.

Another problem is given by the depression of thecarbonate basement located by Lomax et al. (2001), downto a depth of 3 km beneath the northern side of theSomma–Vesuvius complex. This structural low is notconfirmed by seismic reflection (Bruno et al., 1998) norseismic first arrival inversion (DeMatteis et al., 2000), norby the densitymodel suggested in the present work. Such astructural feature should in fact cause a significant gravitylow detectable at the surface, but the anomaly field doesnot exhibit any local effect justifying the existence of adeepening of the carbonate basement in this area.

It must be noticed that the eastern side of the abovementioned ring-shaped structure is slightly less densethan the other sides, especially the western one (centralpart of section a in Fig. 12). This feature could be relatedto some specific structural elements of the volcanicrelief, like the asymmetric shape of the Somma caldera,probably caused by the collapse of the W-SW sectorduring the Avellino eruption (Bruno and Rapolla, 1999;Rolandi et al., 2004).

Finally, the hypothesis can be confirmed that theweakness of the south-western side of the volcano(Bruno and Rapolla, 1999) could be one of the effects ofthe subsidence of the Mesozoic carbonate basementalong NW faults toward the Gulf of Naples (Fusi, 1996).This hypothesis seems to agree with the densitydistribution and the pattern of regional faults deducibleby the morphology of the top of the carbonate basement,as reconstructed by our gravity 3D inverse model(Fig. 8)(Fig. 15).

Appendix

The ‘GRAV3D’ 3D inversion method by Li andOldenburg (1995) allows the estimation of the 3Ddensity model from the observed data. The forwardproblem of the gravity field is defined as follows:

FZ rið Þ ¼ g

ZVq rð Þ z−zi

r−rið Þ dv ð1Þ

where FZ (ri) is the vertical component of the gravityfield at the observation point ri, ρ(r) is the anomalousdensity distribution and γ is the gravity constant. Theinverse problem is linear as the mathematical relationbetween unknown function and data. The forwardproblem is discretized in the following form:

Yd ¼ G¼Yq ð2Þ

where Yd is the observed data vector and G¼ is the kernelcalculated by means of the following integral, expres-sing the gravity field due to the mth element with unitdensity at the nth measurement point:

Gnm ¼ g

ZKvm

z−zijr−rj3 dv ð3Þ

whereΔvm is the relative volume of the mth element andYq is the unknown density vector.

Unfortunately, the inversion of potential fields is af-fected by an unavoidable ambiguity. Therefore, the foundmodel tends to be concentrated near the surface, despite ofits actual depth. In order to avoid this, a depth weightingwas proposed by Li and Oldenburg (1995), giving to cells


at different depths an equal probability of entering thesolution. A depth weighting consistent with the decay of asingle pole source was suggested by the authors.

The technique by Oldenburg and Li was chosen sinceit warranted a large-scale 3D inversion of gravity data.But, unfortunately, it does not allow an error-resolutionanalysis (personal communication of the authors). This isbecause while the algorithm efficiently handles large-scale 3D inversions, it solves a nonlinear problem(because of nonnegativity constraints). This means thatalthough one can – in principle – compute the covariancematrix for the solution, it is such an extremely large taskthat cannot be done in practice.

Nevertheless the algorithm is efficient and specifi-cally used (Dufresne, 1996; Li and Oldenburg, 1996; Liand Oldenburg, 1998; Phillips, 2002) for construction oflarge-scale solutions, such as the one of this paper(94×92×43 cells). A-priori information (from geologyand other geophysical information) may be easilyincorporated to produce a valid source model estimate.Based on this information, single parts of a discretereference model can be constrained by assigning them aspecific density value and weights.

Setting the closeness of the constructed model to thereference model may also be defined for any specific cell.

Density gradients in specific parts of the model wereallowed to increase/decrease by three sets of directionalweights that defined the kind of smoothness for thedensity transition among the cells of the model. Low/high values will favour a more/less abrupt densitytransition in the final model.

The 3D mesh model was extended beyond the dataarea to prevent boundary effects. The mesh size wasgradually decreased toward the central part to allowmore detailed modelling beneath the Somma–Vesuviuscomplex, where more measured data are available.

Constraints were introduced within the referencemodel in subvolumes were density was not allowed tosignificantly change with respect to the reference values.

Because of the high detail needed in the inner part ofthe model and the consequent large number of finiteelements to handle, a great effort is expected to constrainthe model and to set up the weighting functions for eachcell. This was circumvented by building a set of reliabletools, including a versatile graphic interface. Use ofthese tools allowed: a) a fast conversion of theinformation represented by the partial bathymetry ofthe carbonate top (a 2D gridded data set) to constraintsfor sectors of the reference model; b) easy managing oflimited volumes of the model, as identified by thecoordinates of their boundaries; c) the individuation ofvolumes including known or presumed source boun-

daries and the assignment to them of specific directionalweights to warrant focussing, so avoiding an excessive-ly smoothed density model.

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