Hazard assessment at volcanic fields: the Campi Flegrei case history

Journal of Volcanology and Geothermal Research 112 (2001) 53±73 www.elsevier.com/locate/jvolgeores

Hazard assessment at volcanic ®elds: the Campi Flegrei case history Lucio Lirer a,*, Paola Petrosino a, Ines Alberico b b

a Dipartimento di Scienze della Terra, UniversitaÁ di Napoli Federico II, L. go S. Marcellino 10, 80138 Napoli, Italy C.I.R.AM, Centro Interdipartimentale di Ricerca Ambiente, UniversitaÁ di Napoli Federico II, Via Mezzocannone 16, 80134 Napoli, Italy

Received 1 June 2000; accepted 19 April 2001

Abstract Volcanological analysis of the 10 000 yr bp±1538 ad explosive activity at Campi Flegrei shows that the most common explosive eruptions are characterized by the emplacement of ¯ow or surge deposits, originating from the interaction between magma and shallow and/or sea water. The minimum volumes of pyroclastic products range between 0.04 and 0.7 km 3; the proximal areas covered by these products range from 3±4 to 40±50 km 2. The pyroclastic ¯ow and surge deposits occurring inside the caldera have been strongly controlled by pre-existent morphology; because of this, the area of present Napoli city was blanketed by approximately 5 m of pyroclastic deposits, during the last 5000 yr. Previous analysis suggests that the presence of even very low topographic obstacles may in¯uence pyroclastic density current run out such that future eruptive deposits would mainly be con®ned inside the caldera rim. We suggest that a future eruption at Campi Flegrei would not seriously involve the urbanized area of Napoli city located on the hills. On the contrary, the plains located on the eastern side of the caldera (Fuorigrotta, Bagnoli) would be the most damaged area. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Campi Flegrei; volcanic history; pyroclastic density current; volcanic hazard assessment

1. Introduction Volcanism in Campi Flegrei is characterized by: (1.) the presence of monogenic volcanic ash/tuff cones/rings, whose products are inter-layered or partially covered by the products of more recent activity (Fig. 1); (2.) the lack of evidence of a central plumbing system, though the caldera suggests the presence of a large central chamber; (3.) the occurrence of volcanic activity along

* Corresponding author. E-mail address: [email protected] (L. Lirer).

regional feeding fractures, which often allowed very small magmatic volumes to reach shallow depths; (4.) the evidence of only one eye-witnessed explosive event in historical times (1538 ad, eruption of Monte Nuovo); (5.) volcanic vents almost always located along the coast line, which has markedly withdrawn during the last 18 ka (Cinque et al., 1985); (6.) the presence of a ground water table at shallow depth (Oliveri del Castillo and Montagna, 1984; De Natale et al., 1991) The last two characteristics have in¯uenced the type of eruption, which has almost always been explosive with the exception of very rare effusive activity emplacing lava domes.

0377-0273/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0377-027 3(01)00234-7

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Fig. 1. Geomorphological sketch map of Campi Flegrei. The original topographic base is at the scale of 1:25 000.

The history of the Campi Flegrei activity is characterised by two highly explosive volcanic events with well constrained ejected volumes, dated at 37 ka bp (Campanian Gray Tuff, 80 km 3 of magma in dense rock equivalent (DRE)) and 12 ka bp (Neapolitan Yellow Tuff, 10 km 3 of magma in DRE), respectively (Pappalardo et al., 1999). These two events ejected pyroclastic volumes exceeding by 1±2 orders of magnitude those ejected before and after them at the Campi Flegrei volcanic ®eld, and they should be considered somewhat `anomalous' for the area. As a consequence of these two high magnitude explosive eruptions, both the volcanic ®eld physiography (Paleo Campi Flegrei Ð 37±12 ka bp and Campi Flegrei sensu stricto Ð ,12 ka bp±1538 ad) and the hydrogeological system underwent a signi®cant rearrangement (Scandone et al., 1991). The recent explosive activity (,12 ka bp±1538 ad, Campi Flegrei), which occurred after the emplacement of the Neapolitan Yellow Tuff, has the following characteristics: (1.) inside the caldera rims within the Fuorigrotta± Bagnoli depression, ¯ow and surge deposits quickly

decrease from a maximum thickness (120 m for the Astroni eruption, 70 m for the Monte Spina eruption and 40 m for the Averno eruption) toward very low values over 3±4 km; (2.) outside the caldera rims within the urbanised area of Naples stratigraphic sequences are mostly made up of fall products up to 10 m in thickness; (3.) in the distal areas towards the slopes of Vesuvio moderately thick fall deposits (10±40 cm) are interbedded with explosive and effusive Somma±Vesuvio products. 2. Recent up to historical explosive activity The six main explosive events studied here occurred in the 10 ka bp±1538 ad time span, displaying quite different activities, emplacement mechanisms, and erupted volumes. Table 1 shows their relative ages, areas covered by pyroclastic products and ejected volumes, both as pyroclastic material and as DRE. Eruption data, inferred from literature and from a partly published data bank of the senior author (Di

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Table 1 Age, area covered by products and ejected volume for Campi Flegrei eruptions used for volcanic hazard assessment Explosive eruption

Pomici Principali Monte Spina 1±2 Astroni Averno 1±2 Fossa Lupara Monte Nuovo

Age

10 000±8000 yr bp 4000 yr bp 3750 yr bp 3700 yr bp 3600 yr bp 1538 ad

Area covered (km 2)

600 32.5 41.0 13:5 ‡ 23:0 6.0 2.8

Volume ejected (km 3) Magma

DRE

0.38 0.60 0.70 0.21 0.15 0.04

0.21 0.38 0.40 0.13 0.09 0.02

Girolamo et al., 1984; Rosi and Santacroce, 1984; Di Vito, 1985; Lirer et al., 1987a,b; Rosi and Sbrana, 1987; Di Filippo et al., 1991; Mastrolorenzo, 1994; de Vita et al., 1999), have been reorganised. The distribution of pyroclastic deposits, the stratigraphic sequences of products mainly cropping out in proximal areas, their different facies and the chemical composition of juvenile components classi®ed according to Armienti et al. (1983) are reported here. Appendix A gives the UTM co-ordinates of the single site locations where the investigated stratigraphic sections crop out.

were deposited. Each eruptive phase lasted a few hours and displayed maximum calculated model column heights ranging between 12 (a) and 22 km (d). On the basis of the total lithic content, the calculated triggering depth varies between 0.5 and 2 km. The pumice fragments are trachytic and do not show any substantial chemical variation: DI values are 72.82 and 70.59 and normative Ne values are 6.11 and 3.71 for a and d, respectively (Di Girolamo et al., 1984). From the mineral assemblage, a magmatic temperature of 9008C and pre-eruptive pressure of 1000 bar (Lirer et al., 1987a) have been inferred.

2.1. Pomici Principali eruption

2.2. The Monte Spina eruption

The Pomici Principali eruption (Plate 1) represents the eruptive event opening the fourth cycle (Di Girolamo et al., 1984) of activity at Campi Flegrei, lasting from 10 ka bp to 1538 ad (Monte Nuovo eruption). The deposits are made up of seven subplinian±plinian pumice fall beds (named a, b 0 , b 00 , b 000 grouped as 3b, b, g and d), separated by thin ash layers (Lirer et al., 1987a). The Pomici Principali products are mainly dispersed towards the East and in distal outcrops, are generally interbedded with the Somma±Vesuvio products. The maximum thickness of the formation is 300 cm (in the Naples urban area), whereas the minimum is 10±15 cm (the Sperone site, Avellino), 40 km from the vent. The total calculated discharged volume is 0.14 km 3 DRE. The whole eruptive event was composed of seven phases displaying progressively growing magma discharge rate, as deduced from the increase in juvenile and lithic fragments grain size and in lithic fragments content. These phases alternated with quiescent intervals, during which the ®ner clasts

The Monte Spina eruption (Plate 2), dated by radiocarbon techniques between 4000 ^ 50 (Di Girolamo et al., 1984) and 4400 (Rosi and Santacroce, 1984) yr bp, occurred on the southern side of the horse-shoe shaped morphology bordering to the Agnano Plain, an area experiencing volcanic activity in the 6000±3700 bp time-span. Before the Monte Spina eruption took place, the Campi Flegrei central area had undergone a 40 m uplift of the central La Starza marine terrace to a subaerial environment (Cinque et al., 1985). The eruption consisted of two phases, here named Monte Spina 1 and 2, emplacing different deposits at increasing distances from the vent, with the following characteristics: Monte Spina 1 Ð inside the caldera: the initial breakthrough of the conduit explosively ejected fragments of older solid rocks and spatter of molten magma. The emplaced deposit is a coarse, pink breccia, cropping out in the restricted Monte Spina hill area. Monte Spina 1 Ð outside the caldera: a plinian

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Plate 1. Volcanological features of the Pomici Principali eruption.

phase with calculated column heights between 10 and 20 km (de Vita et al., 1999) distributed 0.12±0.15 km 3 of material that extend 40 km towards the NE and consist of a thinly waved, pinkish deposit made

up of an alternation of ash and pumice-fall layers (maximum thickness 10 m). Monte Spina 2: gravitational collapse of the eruption column emplaced topographically controlled

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Plate 2. Volcanological features of the Monte Spina eruption.

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coarse pyroclastic ¯ow deposits, reaching 30 m in thickness in the Vallone del Corvo area, becoming laterally ®ner grained and considerably thinner. The pyroclastic ¯ows cover an area of 32.5 km 2 inside the caldera. The purely magmatic character of the main part of the Monte Spina eruption can be inferred from the high degree of vesiculation of juvenile clasts (60±82%) and from density values (500±1000 kg/ m 3), measured for pumice fragments in the Vallone del Corvo pyroclastic ¯ow sequence. The chemical composition of the products varies from trachytic to alkali-trachytic. The Monte Spina 1 fall products have DI and Ne values of 73.4 and 2.3, respectively (Di Girolamo et al., 1984). For basal and topmost pumice fragments in the Vallone del Corvo pyroclastic ¯ow sequence, DI values are 86.6 and 83.2, and normative Ne values are 7.35 and 7.05 (Munno, personal communication). 2.3. The Astroni eruption The Astroni volcanic centre (Plate 3) is an ash-ringshaped volcano located in the southeastern part of Campi Flegrei caldera, belonging to the last cycle of activity at Campi Flegrei (Di Girolamo et al., 1984). The thickness of the products 3 km from the vent is 3 m, and thins to nothing 8±10 km from the vent. The eruptive sequence consists of several different phases (Di Filippo et al., 1991). Stage 1 is represented by the ascent of viscous magma through a ®ssure and the emplacement of the Caprara lava dome, K/Ar dated at 3750 yr bp (Cassignol and Gillot, 1982); stage 2 represents the main eruptive phase, during which very ef®cient magma±water interaction took place. The deposits, 14C dated at 3720 yr bp (Alessio et al., 1971, 1973), consist of surge deposits widely spread on a 41 km 2 area that made up the ash ring and, ®nally, a coarse pumice pyroclastic ¯ow. Stage 3 consists of strombolian activity occurring inside the ash ring, emplacing a scoria rampart (Toppo dell'Imperatrice) followed by a scoriaceous lava ¯ow (Rotondella). During the three phases about 0.7 km 3 of material, in DRE, was emplaced. The whole eruptive sequence was interpreted to have occurred in an environment, where a very shallow water table could exist. The ascent of viscous lava through a ®ssure (stage 1) was followed by the

injection of water in the conduit zone, causing extensive magma fragmentation. The deposits of this phase (stage 2) are alternating dry and wet surge deposits, possibly caused by variation in the magma/ water ratio, up to a ®nal purely magmatic pyroclastic ¯ow. Stage 3 represents explosive strombolian activity at the end of the eruption. The surge deposits contain typical internal structures (Wohletz and Sheridan, 1979); in the sand wave facies dune, antidune, and chute and pool structures are predominant in comparison with massive and planar structures. The lava and scoria fragments from the Astroni eruption products are trachytic in composition; DI ranges from 76.06 to 75.34, and normative nepheline from 4.42 to 2.04 (Di Girolamo et al., 1984), for basal pumice fragments and top scoriaceous lava ¯ow, respectively. 2.4. The Averno eruption The Averno eruption (Plate 4) occurred in 3700 ^ 50 yr bp (Alessio et al., 1971) in the western sector of Campi Flegrei. The basal part of the deposits consists of alternating pumice (0:5 , Mdw , 22.4) and ash layers …3:6 , Mdw , 3:7†: In its upper part, the Averno sequence consists of ®ner, sandy layers with accretionary lapilli and rare intervals of matrixsupported coarser clasts interbedded with pumice layers (Mastrolorenzo, 1994). The eruption style has been interpreted as an alternation between magmatic and phreatomagmatic phases. Five phases (Lirer et al., 1987c) have been recognised. Stage 1 was characterised by several sustained column pulses with maximum calculated column heights never exceeding 11 km, depositing the basal lapilli and ash layers (Averno 1); stages 2 and 4 were characterised by episodes of magma±water interaction, column collapse, and deposition of wet surges; stages 3 and 5 showed a decrease in magma±water interaction and the occurrence of new sustained eruptive columns of purely magmatic phases. The deposits of stages from 2 to 5 are here labelled as Averno 2. The chemical composition of pumice fragments is trachytic and shows only slight chemical variation from the base to the top of the stage 1 sequence: DI values range between 85.28 and 82.65 and normative Ne between 0.20 and 0.69. The products of the other

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Plate 3. Volcanological features of the Astroni eruption.

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Plate 4. Volcanological features of the Averno eruption.

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Plate 5. Volcanological features of the Fossa Lupara eruption.

stages are chemically very similar to those of the basal part of the sequence. 2.5. The Fossa Lupara eruption The Fossa Lupara volcano (Plate 5) is located in the eastern sector of Campi Flegrei caldera, north of Astroni crater, making up the original Fossa Lupara

volcano, before anthropic activity completely obliterated it. The only report regarding the Fossa Lupara activity is by De Lorenzo and Simotomai (1914), which also presents the topographic features of the three craters before anthropic changes. From the description of these authors, we infer that the Fossa Lupara activity could have been strombolian, as testi®ed by the presence of decimetre-sized scoria

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fragments mixed with bombs in pyroclastic fall deposits. In the innermost crater these authors also describe the presence of a possibly ®ssure-fed trachytic lava ¯ow, 20 m thick, linked to a 52 m long and 30 m deep fracture (La Senga fracture) on the eastern ¯ank of the internal crater. Mean diameters of 780, 460, and 235 m and mean heights of 110, 130, and 110 m are reported for the outermost, internal and innermost crater, respectively. At present the only exposed sequence of Fossa Lupara volcano products can be found in the Grotta Papera quarry, where they are exposed on the slope of the outermost crater. The Fossa Lupara deposits are 10 m thick and are divided by a 3700 ^ 50 yr bp (Alessio et al., 1971) palaeosol from the underlying products of the Astroni ash ring. The Fossa Lupara sequence shows structural and stratigraphic features that make it possible to identify different phases during the whole eruption. Stage 1 involved the opening of explosive activity through an impulsive column phase, emplaced a narrowly spread purplish ash deposit with basal columnar jointing that can be ascribed to an hot pyroclastic ¯ow depositional mechanism. Stage 2 was a strombolian phase with the emplacement of several meters of fall beds mainly made up of coarse scoria fragments (up to 10±20 cm diameter) with a few lithic fragments and lava bombs. Stage 3 involved the emission of a possibly ®ssure fed trachytic lava ¯ow, no longer exposed in the ®eld, but very evident at the beginning of the last century, as reported in De Lorenzo and Simotomai (1914). The chemical composition of the products varies from alkali-trachytic to trachytic along with the whole sequence of the Grotta Papera quarry Fossa Lupara products. DI values range between 79.74 and 76.98 and normative Ne between 0.10 and 3.23 from basal ashes to the top of the lava ¯ow (Rosi and Sbrana, 1987). 2.6. Monte Nuovo eruption The Monte Nuovo eruption (Plate 6) is the only Campi Flegrei eruptive event that occurred in historical times, testi®ed to and documented by several accounts. This eruption was preceded by a long period of ground uplift, located over a wide area, becoming more and more intense and concentrated in the area adjacent to the site of the vent during the two days

immediately preceding the eruption. On 29 September 1538, a week-long eruption began in the western sector of Campi Flegrei forming a new volcanic edi®ce, Monte Nuovo. Several accounts were written in the months following the event (delli Falconi, 1538; Marchesino, 1538; da Toledo, 1539; del Nero, 1538; Simone Porzio, 1551); some were later translated into English by Hamilton (1776) and Lobley (1889). From these contemporary accounts, it has been possible to reconstruct the different phases of the eruption and to correlate them with the different units cropping out in the ®eld (Di Vito et al., 1987; Lirer and Rolandi, 1987b). Stage 1 was a phreato-magmatic phase, during which a large amount of external water (meteoric or sea water) came into contact with magma, and emplaced a 70-m-thick deposit, making up most of the volcano. Two depositional units can be recognised, one displaying a maximum thickness of 7 m, the other a maximum thickness of 70 m. The bulk of the deposits consists of an essentially disordered arrangement of lithic fragments of lava and yellow tuff up to about 10 cm in size and rounded pumice fragments set in a coarse ash matrix. The chaothic texture of the deposits and the emplacement in¯uenced by pre-existing topography make it possible to ascribe it to a pyroclastic ¯ow mechanism. In stage 2, the exhaustion of water supply and of volatile content caused two days of very scarce explosive activity. The eruption resumed on 3rd October with a strombolian eruptive phase, intermixed with minor water±magma interaction episodes. During this phase, deposits consisting of an alternation of ash beds and scoria layers up to 2±3 m thick were emplaced. Finally, on 6th October, a violent magmatic phase, emplacing a strongly directional scoria ¯ow, signalled the end of the eruption. During this phase, 24 people, attempting to climb the vent, were caught by surprise and killed. The chemical composition of the erupted products is trachytic. DI values range between 88.39 and 87.2 and normative Ne between 9.15 and 2.16 from base to top. 3. Type and distribution of products The plinian Pomici Principali eruption and Monte

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Plate 6. Volcanological features of the Monte Nuovo eruption.

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for Monte Spina 1, Averno 1 and Fossa Lupara eruptions; magmatic with pyroclastic ¯ow deposits for Monte Spina 2 and Astroni in its last explosive phase; and hydromagmatic with base surge deposits for Averno 2 and Astroni. The circular distributions indicate the absence of control by topography on the distribution of products whereas the irregular distribution outside the caldera suggests a strong topographic control on the emplacement of products, even though paleotopography was modest (100±150 m). 4. The role of topography

Fig. 2. Reconstruction of Campi Flegrei paleomorphologies: (a) before the Averno eruption (.3700 yr bp) and (b) before the Monte Nuovo eruption (3700 yr bp±1538 ad time span).

Spina 1 explosive events gave rise to elliptically distributed subplinian±plinian fall deposits over an area of 600 km 2, along a E±NE oriented variable dispersal axis, ranging in thickness from about 230± 100 cm in the city of Naples to 50±20 cm, respectively, in the distal area (about 30 km from the source). Inside the caldera, in contrast, the distributions of the products in the proximal area (see Plates 2±6) shows almost circular to quite complex shapes. This is a consequence of the different types of eruptive dynamics and emplacement mechanisms: magmatic with plinian±subplinian±strombolian fall deposits

The control of topography is particularly evident on the distributions of the explosive products of Mt Spina 2 and Astroni (see Plates 2 and 3), partially controlled by the Posillipo and Camaldoli hills (see Fig. 1), which reduced their accumulation in today's urban area located topographically above the caldera. Another example of the control exerted by the topography is represented by the Averno eruption products. A comparison between the present topography and the thickness and distribution of the Averno 2 pyroclastic surge, deduced from Di Vito et al., 1988; Lirer et al., 1990 was performed through the 3D representation using the GIS software ArcView 3.1. As a ®rst step, the Digital Elevation Model (DEM) of the present Campi Flegrei topography was mapped onto a N±S grid of resolution 10 £ 10; derived from a 1:25 000 topographic map. The products of the Monte Nuovo and Averno 2 eruptions were than `stripped' from the DEM and the tentative determination of the paleo-morphology prior to these explosive events (Fig. 2a) was obtained. The co-analysis of paleomorphologies and thicknesses of the pyroclastic deposits allow the reconstruction of the pre Monte Nuovo eruption topography (Fig. 2b) and shows that the spread of the Averno 2 pyroclastic surge deposits were controlled by the pre-existing Archiaverno ashring morphology, that acted as a barrier to the basal and denser portion of the eruptive cloud. The distribution of the Averno 2 deposits has been used to quantify the effects of topography on the distribution of pyroclastic surge deposits. Fig. 3 reports the pre-Averno topographic pro®le (dotted lines) in three different directions from the Averno

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vent compared with the thickness of the pyroclastic surge deposit, as deduced from the isopach map of Fig. 3a. The shaded area beneath the curves gives information on the area covered by pyroclastic surge deposits at a distance .500 m from the source, where the ongoing pyroclastic clouds encountered the Archiaverno crater rim representing the nearest natural obstacle towards the SE. The pyroclastic cloud encountered no obstacle toward the SW (Fig. 3b), in the seashore direction, and deposit thickness goes to zero at about a 3000 m distance from the vent. The thickness of the pyroclastic deposit decreases much more rapidly where natural obstacles are present, such as the Archiaverno crater rim and the Monte Ruscello plain (toward N Ð Fig. 3c) or the Archiaverno crater rim (towards the W±SW± Fig. 3d). In particular, the thickness±distance curve changes its slope as the pyroclastic cloud reaches the Archiaverno barrier and we can infer a different rheological behaviour for the more dilute portion of the pyroclastic ¯ow that surmounted the barriers. This inference is also supported by ®eld textural evidence in the Monteruscello area (2 km away from the vent), where the deposits are very thin and ®ne-grained. Finally, as regards the distribution of Monte Nuovo pyroclastic ¯ow deposits (see Plate 6), the control was exerted by Mt Gauro and Mt delle Ginestre. As a consequence, the Roman Temple of Apollo was protected against destruction and the connection between the `Inland Sea of Averno' and the sea was obliterated, causing both the Averno lake and the coastal Lucrino lake to be formed (see Plate 6). 5. Volcanic hazard assessment 5.1. General remarks Volcanic hazard, H, is the probability that a destructive volcanic event will occur in a given area within a given period of time. It must be evaluated based on the past history of a volcano and is quite Fig. 3. (a) Isopach map of the Averno 2 eruption pyroclastic surge deposit (dotted lines, thickness in meters). (b±d) Pre-Averno topographic pro®les (dotted lines) in three different directions from the Averno vent compared with the thickness of pyroclastic surge deposit. Pro®le traces in (a).

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dif®cult to quantify, because even when the historical record of eruptions of a volcano is known in detail, it is not possible to work out statistical models that can ®t the overall activity (Wickman, 1966; Carta et al., 1981). The volcanological analysis of the activity at Campi Flegrei younger than 10 000 yr bp makes it possible to stress the following considerations that must be taken into account for the scenario of a possible eruption and the degree of hazard in this area.

² The proximal areas covered by these products range between 3±4 and 40±50 km 2. ² The stratigraphic sequences, cropping out in the city of Naples, show evidence that over the last 5000 yr, mostly pyroclastic fall products and only very rare pyroclastic ¯ow and surge deposits have accumulated, reaching a total thickness of about 5 m.

² Over the last 5000 years, the explosive activity was concentrated in a time interval of 500 yr (between 4000 and 3500 yr bp). The eruption of Monte Nuovo occurred 3500 yr after the last explosive event. So far a cyclic periodicity for activity at Campi Flegrei volcanic ®eld cannot be clearly deduced. ² The activity occurred on the western and eastern sides of the caldera, but on the latter side it was slightly more intense in magnitude and frequency. ² There is no clear evidence of any pattern of spatial migration of activity inside the caldera. ² At present, inside the caldera, there is evidence of fumarolic activity along the structural alignment (Lirer et al., 1987c) extending through Agnano Plain±Pisciarelli±Solfatara±Mofete. ² The most recurrent dynamics of explosive eruptions produced the emplacement of ¯ow-surge deposits, due mostly to the interaction between magma and shallow and/or sea water. Though to a lesser extent, subplinian±strombolian explosive dynamics are also possible consequences of the beginning or the end of the magma/water interaction. In this way the hydromagmatic eruption of the Astroni ash ring followed by both the Toppo dell'Imperatrice scoria rampart inside the crater and by the subsequent scoria deposits of Fossa Lupara spatter cone could represent a good example. ² In the recent activity the maximum volumes of ejected pyroclastic products range between 0.04 and 0.7 km 3 (Table 1).

On the basis of the presented data, choosing one of the eruptions, which occurred in the last 5000 yr as a type scenario could be speculative, since past eruptive events displayed a wide variety of activities, deposits and emplacement directions. Fig. 4a reports pyroclastic ¯ow and surge deposits distributions for all the eruptions taken into account here. Superimposition of these areas allows us to deduce the global spatial extent of pyroclastic ¯ow and surge deposits emplaced during the last 5000 yr at Campi Flegrei, reported in Fig. 4b. This map is a simple representation of the hazard of invasion by pyroclastic ¯ow, when only the past history of the Campi Flegrei volcanic ®eld is taken into account. However, many other factors can in¯uence the hazard related to a future eruptive event, such as the site of vent opening and the topographic control on the distribution of products. As to the site of vent opening, the pattern of volcanic activity over the last 5000 yr, recorded in the western and eastern caldera areas, suggests a higher probability of new magmatic uprise in the eastern area than in the western one (Scandone and D'Andrea, 1994; Alberico et al., 2001). The future eruption considered here in order to make hazard predictions has a vent located inside the caldera along the rims of the maximum uplift area of the last bradyseismic crisis, and involves magmatic volumes # 0:5 km 3. No plinian fallout phase is expected, since plinian episodes represent low frequency events for the area. Subplinian and strombolian activity could represent a minor phase

6. Possible future eruptions

Fig. 4. (a) Pyroclastic ¯ow and surge deposits emplacement area for the eruptions of the last 5000 yr at Campi Flegrei. (b) Volcanic hazard areas for hypothesised accumulation of pyroclastic density current deposits.

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of a possible future explosive event, but the main part of the erupted products would be made up of pyroclastic ¯ow and surge deposits, emplaced by a radially propagating 150±200 m high pyroclastic cloud. The present topography has been reconstructed to hypothesise the eventual distribution of pyroclastic ¯ow and surge deposits in a possible future eruption. The pro®les from the centre of the Agnano plain and the inferred location of a future vent are shown in Fig. 5. Using the 3D mapping ArcView 3.1 computer software, the total area of the Agnano plain has been calculated at 5.5 km 2. Considering an average elevation of 100 m, a volume of about 0.2 km 3 has been deduced. Note that, in this section, the term pyroclastic density current has been used as a comprehensive term (Valentine, 1998) for pyroclastic surge and ¯ow deposits, referring to the general depositional mechanism, and not to speci®c genesis or ®eld features. For a future eruptive event, taking into account pyroclastic volumes comparable to those ejected during the Averno 2 phase (0.21 km 3; Lirer et al., 1990), the possibly erupted products would ®ll up the Agnano plain. Topographic barriers would be more effective towards the East (average elevation around 150 m), whereas towards the S (Fig. 5c), about 400 m from the hypothetical vent, topographic obstacles are only ,60 m high and could be overridden by the more dilute part of the pyroclastic density current. If ejected volumes far exceeding those of the Averno eruption (i.e. a Monte Spina 2 type eruption) are taken into account, then the topography around the Agnano plain could be easily surmounted by density currents; however, external hills (Posillipo and Camaldoli hills) would certainly prevent the spreading of pyroclastic density currents towards the Napoli urbanised area located on the hills. The role of topographic barriers played by external hills would be enhanced by their distance from the vent; low volume pyroclastic density current deposits moving over a plain surface are, in fact, expected to lose the main part of associated energy within a short distance from the vent (Allen, 1982).

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7. Dynamic overpressures and damage Finally, we consider the speci®c features and destructive power associated to possibly erupted Campi Flegrei pyroclastic density currents. The main part of the pyroclastic cloud, con®ned inside the Agnano plain, is expected (Lipman, 1976; Valentine et al., 1992) to be a simple cooling unit, constituting the denser part of the deposit. The portion of the density current over-riding the topographic obstacles would be the more diluted fraction of the whole eruptive cloud, possibly constituting compound pyroclastic ¯ow and cooling units, spread in different paths and controlled by pre-existing topography. For all the pyroclastic density current deposits, however, the absence of topographic slope (McEwen and Malin, 1980) and the occurrence of cooling due to frequent magma/water interaction episodes have to be considered as factors constraining the speed and consequent spatial distribution of these deposits. Maximum travel distance for Averno 2 pyroclastic surge deposits were 3 km, whereas maximum travel distances of 15 km have been hypothesised by De Vita et al. (1999) for Monte Spina lower concentration ¯ows overtopping the morphological boundary of the Campi Flegrei caldera, that are 2 cm thick about 10 km away from the vent in the Aversa area. Physical parameters associated with Averno 2 pyroclastic surge deposits allowed Lirer et al., 1990 to de®ne a pyroclastic column fountain (jet region) of 300 m just before the collapse. This phenomenology could be well represented by the model two numerical approach of Valentine and Wohletz (1989). Using the 300 m collapse height and the maximum thickness of the deposit of 40 m in the source area, it is possible to deduce a particle concentration of 1 £ 1021 and, as a consequence, using the dynamic pressure±velocity diagram of Fig. 6 (Valentine, 1998), for a theoretical initial velocity of 30 m/s, a dynamic overpressure exceeding 14 kPa can be deduced. The thickness of the deposit rapidly decreases, and we hypothesize that the most concentrated part of the pyroclastic density current stops at a few tens of meters from the source,

Fig. 5. (a) Present topography of Campi Flegrei area, with the location (black circle) of a possible future vent. (b) Topographic pro®le before and after the Averno 2 pyroclastic surge deposit emplacement. (c±f) Present topographic pro®les in different directions from the possible future vent.

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L. Lirer et al. / Journal of Volcanology and Geothermal Research 112 (2001) 53±73

8. Conclusions on hazard assessment On the basis of the data presented and of the past history of this volcanic area, producing a map of hazard related to a hypothetical future event would be rather complex and perhaps speculative. However, the pattern of volcanic activity over the last 5000 yr, recorded in the western and eastern caldera areas, suggests a greater volcanic hazard for the Pozzuoli and Quarto towns, as well as for the areas of Soccavo, Pianura, Fuorigrotta and Bagnoli, than for the towns of Bacoli and Monte di Procida, since the probability of new magmatic uprise might be higher in the eastern area than in the western one (see Fig. 4b). The Camaldoli and Posillipo hills could protect the part of Naples city outside the caldera from the emplacement of pyroclastic density current deposits. Fig. 6. Velocity±dynamic overpressure diagram (modi®ed from Valentine, 1998) for the Averno 2 surge deposits.

as is testi®ed in the ®eld by the presence of a basal sandy to coarse grained ¯ow layer restricted to the narrow source area. The values of main grain-size parameters (mean diameter Ð Mdw and sorting Ð s ) for this layer con®rm it is coarser …1:59 , Mdw , 0:16; 1:77 , s , 2:21† than the whole part of the deposit, made up by ash layers …4:76 , Mdw , 3:54; 1:69 , s , 2:17† representing the more diluted and widely distributed portion of the pyroclastic density current. Applying the same calculation as before, the particle concentration of the pyroclastic density current about 500 m away from the vent becomes 1 £ 10 22 or even smaller, implying, for a theoretical velocity of 0±50 m/s, a range of dynamic overpressures between 7 and 14 kPa. In this range of overpressure, the damage would be complete for forests and green areas; masonry and precast concrete houses would suffer little or no structural damage, but glass windows, false ceilings and interior partitions might be heavily damaged. Most heavy infrastructure elements, such as roads and railways, are not appreciably damaged by pyroclastic density currents in this overpressure range even if some structural damage could come from earthquakes possibly associated with the eruptive events. For the restricted area nearer to the source, the emplacement of a more concentrated pyroclastic density current could imply a signi®cantly higher destructive power.

9. Risk implications Risk is de®ned as `the expected loss to a given element or a set of elements resulting from the occurrence of a natural phenomenon of a given magnitude' (UNDRO, 1982). It may be quanti®ed using the following relationship (UNESCO, 1972; Fournier d'Albe, 1979): RˆE£V £H where E is the element at risk, and includes the population, property, economic activity, public service, and so on, that are under the threat of disaster. Vulnerability, V, is a measure of the fraction of the value, which is likely to be lost as a result of a given event. H is the volcanic hazard. We must take into account that, in the whole area, starting from the probable source located inside the caldera, risk is controlled by high volcanic hazard and much more by high exposed value degree, Campi Flegrei being a heavily inhabited place (Fig. 7). Therefore in this area a value of acceptable risk for the community should be de®ned. The co-analysis of volcanic hazard and present exposed value in Campi Flegrei area makes it possible to stress that resident people are exposed to a high enough risk that there is a strong need for a detailed knowledge of mechanisms of past volcanic activity and an accurate surveillance, to rapidly decide, in any speci®c case, how to handle the evacuation plans.

L. Lirer et al. / Journal of Volcanology and Geothermal Research 112 (2001) 53±73

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Fig. 7. Relationships between volcanic hazard and exposed value inside the Phlaegrean caldera.

The ®rst possible step to be taken by local governments would be a signi®cant improvement of facilities needed for a quick evacuation of the area; ®rst of all from the areas inside the caldera as a consequence of signi®cant premonitory signs. Among these, bradyseismic crises, representing a probable sign of a new volcanic activity such as occurred before the 1538 ad eruption, have to be taken into particular account. In the longer term, however, it is desirable that the whole Campi Flegrei area should change its character of being a heavily inhabited place, since the evacuation of families from their houses could be quite dif®cult to manage. On the contrary, exploitation of the natural features (i.e. Parco degli Astroni, Solfatara) should be encouraged, so developing the tourist resources that the area offers. Also, it would be better

if business settling in the Campi Flegrei area were represented by of®ces and service societies, instead of factories and production facilities that could be heavily damaged in case of an eruption with a serious economic loss. In this regard, the dismantling of steel factories at Bagnoli, with the reconversion towards non-residential buildings, can be considered a ®rst step that must be followed by many others aiming to achieve the same results. Acknowledgements The authors wish to acknowledge L. Wilson and an anonymous reviewer whose suggestions greatly improved the manuscript. A grateful thank you goes to R. Scandone, who read an early draft.

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L. Lirer et al. / Journal of Volcanology and Geothermal Research 112 (2001) 53±73

Appendix A Site

City/town

UTM co-ordinates

Monte Spina Vallone del Corvo Celle La Pietra Marigliano Quarry Colli Aminei Pomigliano d'Arco Quarry Sperone Quarry Grotta Papera Quarry Astroni Crater Averno Crater La Schiana Monte Ruscello Monte Nuovo Crater

Napoli Napoli Napoli Napoli Marigliano (NA) Napoli Pomigliano d'Arco (NA) Sperone (AV) Napoli Pozzuoli (NA) Pozzuoli (NA) Pozzuoli (NA) Pozzuoli (NA) Pozzuoli (NA)

429.560 430.865 428.159 429.053 452.857 435.500 448.395 466.433 427.342 428.362 422.223 422.437 422.188 423.262

References Alberico, I., Petrosino, P., Scandone, R., Lirer, L., 2001. The evaluation of volcanic risk in Campi Flegrei (Italy). Abstract XI EUG Symposium Strasbourg 9±12 April, 2001. Alessio, M., Bella, F., Improta, F., Belluomini, G., Cortesi, C., Turi, B., 1971. University of Rome Carbon-14 dates IX. Radiocarbon 13, 341±395. Alessio, M., Bella, F., Improta, F., Belluomini, G., Cortesi, C., Turi, B., 1973. University of Rome Carbon-14 dates X. Radiocarbon 15, 165±178. Allen, J.R.L., 1982. Developments in Sedimentology. Sedimentary Structures. Their Character and Physical Basis, vols. 30 A and B. Elsevier, Amsterdam, 386 pp. Armienti, P., Barberi, F., Bizouard, H., Clocchiatti, R., Innocenti, F., Metrich, N., Rosi, M., Sbrana, A., 1983. The Phlegraean Fields: Magma evolution within a shallow chamber. J. Volcanol. Geotherm. Res. 17, 289±311. Carta, S., Figari, R., Sartoris, G., Sassi, E., Scandone, R., 1981. A statistical model of Vesuvius and its volcanological implication. Bull. Volcanol. 44 (2), 129±151. Cassignol, C., Gillot, P.Y., 1982. Range and effectiveness of unspiked potassium±argon dating: experimental groundwork and application. In: Odin, G.S. (Ed.), Numerical Dating in Stratigraphy. Wiley, New York, pp. 160±179. Cinque, A., Rolandi, G., Zamparelli, V., 1985. L'estensione dei depositi marini olocenici nei Campi Flegrei in relazione alla vulcano-tettonica. Boll. Soc. Geol. It. 104, 327±348. De Lorenzo, G., Simotomai, H., 1914. I crateri di Fossa Lupara nei Campi Flegrei. Atti Acc. Sc. Fis. e Mat. Ser. 2 16 (5), 1±50. De Natale, G., Pingue, F., Allard, P., Zollo, A., 1991. Geophysical and geochemical modelling of the 1982Ð1984 unrest phenomena at Campi Flegrei caldera (Southern Italy). J. Volcanol. Geotherm. Res. 48 (1/2), 199±222.

4.520.235 4.500.609 4.519.423 4.519.267 4.532.426 4.524.500 4.529.191 4.532.437 4.523.430 4.521.992 4.522.097 4.524.281 4.522.500 4.520.782

Di Filippo, G., Lirer, L., Maraf®, S., Capuano, M., 1991. L'eruzione di Astroni nell'attivitaÁ recente dei Campi Flegrei. Boll. Soc. Geol. It. 110, 309±331. Di Girolamo, P., Ghiara, M.R., Lirer, L., Munno, R., Rolandi, G., Stanzione, D., 1984. Vulcanologia e Petrologia dei Campi Flegrei. Boll. Soc. Geol. It. 103 (2), 349±413. Di Vito, M., Lirer, L., Mastrolorenzo, G., Rolandi, G., Scandone, R., 1985. Volcanological map of Campi Flegrei (1:50,000). Di Vito, M., Lirer, L., Mastrolorenzo, G., Rolandi, G., 1987. The 1538 Monte Nuovo eruption (Campi Flegrei, Italy). Bull. Volcanol. 49, 608±615. Di Vito, M., Gattullo, V., Lirer, L., Mastrolorenzo, G., 1988. L'eruzione di Averno (3700 yr bp) bp nei Campi Flegrei. Atti dello Stato di Avanzamento della Convenzione Univ Regione Campania sul Bradisimo Flegreo, Naples. delli Falconi, M.A., 1538. Del'incendio di Puzzuolo. Marco Antonio delli Falconi all'illustrissima Signora Marchessa della Padela nel MDXXXVIII. Published in Hamilton, 1976, p. 70. Fournier d'Albe, E.M., 1979. Objectives of volcanic monitoring and prediction. J. Geol. Soc. London, 321±326. Hamilton, W., 1776. Campi Flegrei observations on the volcanoes of the two Sicilies, P de Fabris ed. Naples. Lirer, L., Mastrolorenzo, G., Rolandi, G., 1987a. Un evento pliniano nell'attivitaÁ recente dei Campi Flegrei. Boll. Soc. Geol. It. 106, 461±473. Lirer, L., Rolandi, G., Di Vito, M., Mastrolorenzo, G., 1987b. L'eruzione del Monte Nuovo (1538) nei Campi Flegrei. Boll. Soc. Geol. It. 106, 447±460. Lirer, L., Luongo, G., Scandone, R., 1987c. On the volcanological evolution of Campi Flegrei. Eos 68, 226±324. Lirer, L., Di Vito, M., Giacomelli, L., Scandone, R., Vinci, A., 1990. Contributo delle analisi granulometriche alla ricostruzione della dinamica dell'eruzione di Averno (Campi Flegrei). Boll. Soc. Geol. It. 109, 583±597.

L. Lirer et al. / Journal of Volcanology and Geothermal Research 112 (2001) 53±73 Lipman, P.W., 1976. Caldera collapse breccias in the western San Juan Mountains, Colorado. Geol. Soc. Am. Bull. 87, 1397± 1410. Lobley, J.L., 1889. Mount Vesuvius. A Descriptive, Historical, and Geological Account of the Volcano and Its Surroundings. Roper and Drowley, II Ludgate Hill, London. Marchesino, F., 1538. Copia de una lettera di Napoli che contiene li stupendi et grandi prodigi apparsi sopra Pozzuolo. From Parascandola, 1946. Mastrolorenzo, G., 1994. Averno tuff ring in Campi Flegrei (South Italy). Bull. Volcanol. 56, 561±572. McEwen, A.S., Malin, M.C., 1980. Dynamics of Mount St. Helens' 1980 pyroclastic ¯ows, rockslide avalanche, lahars and blast. J. Volcanol. Geotherm. Res. 37, 205±231. del Nero, F., 1538. A letter to Niccolo del Benino on the earthquake at Pozzuoli by which the Monte Nuovo was formed in 1538. Published in Lobley, 1889, 368 pp. and in German in the Neues Jahrbuch fur Geologie, of Leonhard Brown, 1846. Oliveri del Castillo, A., Montagna, S., 1984. Nuovi elementi sulla connessione tra termo¯uido±dinamica e geodinamica ai Campi Flegrei (Napoli): bradisismo 1982±1984. Proc. Annu. Congr. Geophys. Litosphere Roma, 46. Pappalardo, L., Civetta, L., D'Antonio, M., Deino, A., Di Vito, M., Orsi, G., Carandente, A., de Vita, S., Isaia, R., Piochi, M., 1999. Chemical and Sr-isotopical evolution of the Phlegraean magmatic system before the Campanian Ignimbrite and the Neapolitan Yellow Tuff eruptions. J. Volcanol. Geotherm. Res. 91 (2/4), 141±166. Parascandola, A., 1946. Il Monte Nuovo e il Lago Lucrino. Boll. Soc. Nat. 55, 173±175. Rosi, M., Santacroce, R., 1984. Volcanic hazard assessment in the Phlegraean Fields: a contribution based on stratigraphic and historical data. Bull. Volcanol. 47 (2), 359±370. Rosi, M., Sbrana, A., 1987. The Phlegrean Fields. C.N.R. Quad. Ric. Sci. 114, 1±175. Scandone, R., D'Andrea, M., 1994. Il rischio vulcanico. In: Di

73

Donna, V., Vallario, A. (Eds.), L'ambiente: Risorse e Rischi. Liguori, Napoli, pp. 130±150. Scandone, R., Bellucci, F., Lirer, L., Rolandi, G., 1991. The structure of the Campanian Plain and the activity of the Neapolitan volcanoes (Italy). J. Volcanol. Geotherm. Res. 48, 1±31. Simone Porzio, 1551. De con¯agratione Agri Puteolani Simoni Portii Neapolitani Epistola. Simoni Portii Opera, Florence. In: Lobley, 1889. da Toledo, P.G., 1539. Letter published in Hamilton, 1776, p. 45. UNDRO, 1982. Volcanic Emergency Management. United Nations, New York. United Nations Environment Program 1987 Environmental Data Report. Basil Blackwell, Oxford. UNESCO, 1972. Report of consultative meeting of experts on the statistical study of natural hazards and their consequences. Document SC/WS/500, 11 pp. Valentine, G.A., 1998. Damage to structures by pyroclastic ¯ow and surges, inferred from nuclear weapons effects. J. Volcanol. Geotherm. Res. 87, 117±140. Valentine, G.A., Wohletz, K.H., 1989. Environmental hazards of pyroclastic ¯ows determined by numerical models. Geology 17, 641±644. Valentine, G.A., Wohletz, K.H., Kieffer, S.W., 1992. Effects of topography on facies and compositional zonation in calderarelated ignimbrites. Geol. Soc. Am. Bull. 104, 154±165. de Vita, S., Orsi, G., Civetta, L., Caradente, A., D'Antonio, M., Deino, A., di Cesare, T., Di Vito, M.A., Fisher, R.V., Isaia, R., Marotta, E., Necco, A., Ort, M., Pappalardo, L., Piochi, M., Southon, J., 1999. The Agnano-Monte Spina eruption (4100 yr bp) in the restless Campi Flegrei caldera (Italy). J. Volcanol. Geotherm. Res. 91, 269±301. Walker, G.P.L., 1973. Explosive volcanic eruptions: a new classi®cation scheme. Geol. Rundsch. 62, 431±446. Wickman, F.E., 1966. Repose-Period Patterns of Volcanoes. V: General Discussion and a Tentative Stochastic Model. Arkiv fur Mineralogi och Geologi 4 (5), 353±366. Wohletz, K.H., Sheridan, M.F., 1979. A model of pyroclastic surge. Geol. Soc. Am. Spec. Pap. 180, 177±194.