longterm forecast of eruption style and size at campi flegrei caldera italy

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Long-term forecast of eruption style and size at Campi Flegrei caldera (Italy)

Giovanni Orsi a,, Mauro Antonio Di Vito a, Jacopo Selva b, Warner Marzocchi c

a Istituto Nazionale di Geosica e Vulcanologia, Osservatorio Vesuviano, Via Diocleziano 328, 80124 Naples, Italyb Istituto Nazionale di Geosica e Vulcanologia, Sezione di Bologna, Via D. Creti 12, 40128 Bologna, Italyc Istituto Nazionale di Geosica e Vulcanologia, Sezione Roma 1, Via di Vigna Murata 605, 40123 Rome, Italy

a b s t r a c ta r t i c l e i n f o

Article history:

Received 12 May 2009

Received in revised form 20 July 2009Accepted 7 August 2009

Available online 12 September 2009

Editor: Y. Ricard

Keywords:

volcanic hazards assessment

eruption size

eruption style

Campi Flegrei caldera

The Campi Flegrei caldera is an active and restless volcano in the densely inhabited Neapolitan area of

southern Italy. Because of the very high value (lives, properties, infrastructures, etc.) exposed to potential

volcanic hazards, it is one of the areas at highest volcanic risk on Earth. In such a situation we have made an

attempt to contribute to assessment of its volcanic hazards by providing a quantitative probabilistic long-

term forecast of style and size of the next eruption.

We have evaluated the most relevant physical parameters of the 22 explosive eruptions of the Campi Flegrei

caldera over the past 5 ka. This time span has been taken as the reference period for volcanic hazards

assessment on the basis of the volcanic and deformation history of the caldera. The evaluated parameters

include dispersal, volume and density of the pyroclastic deposits, volume of erupted magma, total erupted

mass, and eruption magnitude. The obtained results permit a size classication of the explosive eruptions,

which are grouped into three sizes: small, medium, and large. On the basis of the reconstructed eruption

dynamics, we have considered a type event(s) representative of each size class and hypothesized the style of

the next event. An effusive eruption will likely generate a dome or very small lava ows, while an explosive

event of any size very probably will produce particles fallout and owage of pyroclastic density currents.

Using a Bayesian inference procedure, we have assigned a conditional probability of occurrence to each of

the eruption size classes. A small-size explosive eruption is the most likely event with a probability of about

60%; a large-size explosive eruption is the least likely event with a probability of about 4%; a medium-size

explosive eruption has a probability of occurrence of about 25%; an effusive eruption has about 11%probability of occurrence.

2009 Elsevier B.V. All rights reserved.

1. Introduction

Long-term volcanic hazards assessment and eruption forecasting,

especially for an active volcano within a densely inhabited area, are

among the most important tasks of modern volcanology (e.g.:Blong,

1984; Crandell et al., 1984; Tilling, 1989a,b, 2005; Sigurdsson et al.,

2000; Marti and Ernst, 2005). A comprehensive long-term hazard

estimation usually requires the denition of the portion of the entire

history of a given volcano to be taken as reference and the estimation

of vent location, type and size of the next eruption, and eruption

phenomena and their sequence during the event. In particular, the

probabilistic estimation of vent location, and type and size of the next

eruption is important as it permits a hazards zoning of the territory

and construction of volcanic hazards maps. Furthermore, for volcanic

elds and calderas, it takes into account the largest part of uncertainty

in volcanic hazards evaluation.

Long-term forecast of size and style of the next eruption has to be

based on detailed stratigraphical, sedimentological and volcanological

investigations of the eruptive products of a given volcano during the

portion of its history taken as reference. The results of these analyses

enable denition of eruption dynamics, particles transport and depo-

sition mechanisms, dispersal and volumeof the erupted products, and

nally volume, mass and discharge rate of the magma feeding each

past event. These parameters provide the basic for a quantitative

evaluation of the likely size classes of a future eruption, and of the

probabilities of occurrence of each of them.

The Campi Flegrei caldera (CFc) in the Neapolitan area is an ex-

cellent example of an active and restless volcano (Orsi et al., 1996).

Due to the very dense urbanization, it is one of the areas at highest

volcanic risk on Earth. The only volcanic hazard assessment based on

the entire set of available geological data for this structure was

recently attempted byOrsi et al. (2004)and later integrated byCosta

et al. (2009) and Selva et al. (submitted for publication-b).Orsi et al.

(2004)took the past 5 ka as the reference period for volcanic hazards

assessment based on the volcanic and deformation history of the

caldera. They subdivided the explosive eruptions that occurred in this

time span into three size classes, according to the area covered by the

Earth and Planetary Science Letters 287 (2009) 265276

Corresponding author. Tel.: +39 081 6108343; fax: +39 081 6108344.

E-mail address:[email protected](G. Orsi).

0012-821X/$ see front matter 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.epsl.2009.08.013

Contents lists available at ScienceDirect

Earth and Planetary Science Letters

j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l
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pyroclastic deposits that were assumed to be representative of the

eruption magnitude.

The main goal of this paper is to improve the current volcanic

hazards assessment for CFc by providing a quantitative probabilistic

long-term forecast of style and size of the next eruption. To reach this

goal, we have improved the magnitude estimation of each of the

explosiveeruptions of the reference period through a detailed analysis

of their pyroclastic sequences, focusing on the evaluation of variable

physical parameters. The new magnitude estimations and their evolu-tion through time have been used as the primary data for a long-term

probabilistic forecastingof thesizeof thenext eruption at CFc, through

a methodology based on a Bayesian inference that accounts for both

prior models and past activity of the volcano during the reference

period (Marzocchi et al., 2004, 2008, submitted for publication).

2. The Campi Flegrei caldera

The CFc, together with SommaVesuvius strato-volcano and Ischia

volcano island, is one of the three active volcanoes of the Neapolitan

area (Orsi et al., 2003) (Fig. 1). The caldera system formed from two

major collapses related to the Campanian Ignimbrite (39 ka) and the

Neapolitan Yellow Tuff (NYT; 15 ka) eruptions (Orsi et al., 1992, 1996,

2004, and references therein).

In the past 15 ka, volcanism has been very intense within the NYT

caldera, which has been affected by an ongoing resurgence (Orsi et al.,

1996; Di Vito et al., 1999). The resurgence occurs through a simple-

shearing mechanism (Orsi et al., 1991) and has generated a maximum

upliftof about90 m of the LaStarzablock,composedof a marineterrace.

In recent decades, the NYT caldera has been affected by two major

bradyseismic events, in 196972 and 198284, that generated 170 and

180 cm of maximum ground deformation, respectively (Barberi et al.,

1984, 1989; Orsi et al., 1999a). These events were followed by an

irregular but net subsidence interrupted by several minor episodes

in 198889 (7 cm), 1994 (


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by a deep reservoir with the top at about 8 km depth, and individual

eruptions were fed by small magma batches located at less than 8 km

depth. They have also shown that at present there is no evidence of

magma batches larger than 1 km3 at less than 4 km depth.

Location of the eruption vents is a good tracer of the structures

active through time. During the rst and second epochs, vents were

mostlylocated along thestructural boundary of theNYT caldera, while

during the third epoch the majority was in the northeastern sector of

the NYT caldera

oor, that is along normal faults cutting through thenortheastern portion of the resurgent block (Orsi et al., 1996, 1999a).

Duringthe secondperiod of quiescence (8.24.8 ka),priorto theonset

of the third epoch, the stress regime within the NYT caldera changed

(Orsi et al., 1996; Di Vito et al., 1999). This new regime still persists, as

shown by the dynamics of the recent unrest episodes, that have been

interpreted as short-term, transient events within the long-term

deformation related to resurgence (Orsi et al., 1999a,b).

Orsi et al. (2004)performed the rst volcanic hazards assessment

at the CFc that took into consideration all the geological, volcanolog-

ical, petrological and geophysical data available in literature. Their

work has been later integrated into more quantitative and probabi-

listic analyses of some aspects of volcanic hazards assessment such as

tephra fallout (Costa et al., 2009) andlocationof a futureeruptionvent

(Selva et al., submittedfor publication-b). Based upon thevolcanicand

deformation history of the past 15 ka, and the ongoing dynamics,

which persists since the last change in stress regime occurred prior to

onset of the third epoch (4.83.8 ka), Orsi et al. (2004) concluded that

the past 5 ka should be considered as the portion of the entire history

of the CFc system to be taken as reference for volcanic hazards

assessment and eruption size and style forecasting. They suggested

that time and location of the next eruption will be dictated by the

stress regime, although they could not rule out building up of over-

pressureunrelated to tectonic processesas a possible triggering factor.

According to the characteristics of the past volcanic events, the authors

concluded that a future explosive eruption will very likely include

magmatic and phreatomagmatic explosions with generation of sus-

tained columns and particles fallout, and pyroclastic density currents

(pdcs). Consideringthe shorttimeintervals at whicheruptionsfollowed

each other during the past eruptive epochs, Orsi et al. (2004) alsosuggested that a future eruption in the AgnanoSan Vito area, which is

within the area at highest probability of vent opening, could be therst

of a series of events closely spaced in time.

3. Physical parameters of the explosive eruptions of the past 5 ka

To date the only systematic attempt to evaluate the size of the past

explosive eruptions of the CFc was made by Orsi et al. (2004). The

authors, lackinga satisfactoryevaluationof thevolume of thepyroclastic

deposits emplaced over the past 15 ka, especially of those laid down by

pdcs, assumed the area covered by pyroclastic deposits as representa-

tive of the magnitude of the eruptions from which they were generated.

Based on this assumption and considering the area covered by pyro-clastic deposits thicker than 10 cm, they subdivided the explosive

events of the past 15 ka, into three classes: low-, medium-, and high-

magnitude eruptions. The deposits of the low-magnitude eruptions

covered areas


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eld data andtheir linearinterpolation on semi-logarithmic plots, and

uses the ArcView GIS software for data analysis.

APD, the area covered by the entire pyroclastic sequence, was

calculated as the area covered by a sequence not less than 10 cm thick

on eld data, and then extrapolated to that covered by not less than

1 cm of deposit (Dell'Erba, 2003).VPDwas calculated as the volume of

the entire pyroclastic body, including both pyroclastic-fallout and pdc

deposits thicker than 1 cm (Pyle, 2000; Fierstein and Nathenson,

1992, 1993; Dell'Erba, 2003; Costa et al., 2009). Considering thatthe magmas feeding the explosive eruptions of the past 5 ka have

been dominantly trachytic in composition, the VDREwas evaluated by

assuming a magmatic density of 2500 kg/m3 (Romano et al., 2003).DPDwas evaluated on the basis of the density of each lithotype of the

pyroclastic sequence, resultingfrom eld and laboratory analyses, and

its percentage in volume of the entire deposit. Density values for

pyroclastic-fallout deposits with variable sedimentological character-

istics were already presented by Orsi et al. (2004). The eruption

magnitude, according toPyle (2000), was calculated by the formula

M= log10TEM; kg 7 1

The obtained results (Table 1) show a signicant variability for all

the calculated parameters. Excluding the AMS eruption, by far the

largest event of the past 5 ka, APD1cmvaries from 24 to 893 km2,VPDfrom 0.02 to 0.33 km3, VDREfrom 0.01 to 0.19 km

3, TEM from 0.4 to

4.7 kg1011, andMfrom 3.5 to 4.7.

The frequency distribution of physical parameters such asAPD, VPD,

VDRE, and TEM follows a power law, with the lower values for each of

them being the most frequent (Fig. 2). The distribution of theAPD1cmvalues, shows two breaks, which allow the entire set of data to be

subdividedin threeclasses. The small-area class includes 15 eruptions,

the medium-area class includes 6 eruptions, and the wide-area class

only includes one eruption. The same subdivision into three classes,

although not as obviously as for APD1 cm, can also be made using the

VPD, VDRE, and TEM frequency distributions. A gap in the magnitude

values around 4.3, which separatesevents of the small-area fromthose

of the medium-area class, can be taken as the boundary between the

low-magnitude and the medium-magnitude classes of eruptions. Alsoin this case, AMS is the only event in the high-magnitude class.

The variation through time of the measured physical parameters

(Fig. 3) sheds light on the past behavior of the volcanic system. Orsi

et al. (2004) presented and discussed the variation of theAPD10cm over

the past 15 ka. They found a striking similarity in the variation of this

parameter during the rst (159.5 ka) and third (4.83.8 ka) epoch of

activity, and grouped the explosive eruptionsin three size classes. Our

results conrm and extend those ofOrsi et al. (2004). During the third

epoch of activity, all the measured parameters roughly increased

untill the large AMS eruption and then decreased.

The Volcanic Explosivity Index (VEI), developed by Newhall and

Self (1982) by integrating a variety of parameters (Walker, 1973,

1980), is widely used to express the size of an eruption. The use of the

VEI index implies the assumption that magnitude, depending on thetotal erupted mass of magma, and intensity, related to the rate of

magma release and column height, are correlated. However this

correlation commonlyexists for largeeruptions, but not necessarily for

medium- and small-size events (Pyle, 2000). Large eruptions generate

high columns and produce large volumeof ejecta, while medium- and

small-size eruptions (especially phreatomagmatic events) can gener-

ate low columns and many pyroclastic currents. In the latter case, a

discrepancy arisesfrom VEIvalues calculated on thebasisof volumeof

ejecta (magnitude) or column height (intensity). For these reasons,

and because the volcanic eruptions of the past 5 ka have been domi-

nated by phreatomagmatic fragmentation processes, it does not seem

appropriate to apply the VEI index to categorize the past eruptions of

the CFc. Instead, magnitude is the most diagnostic parameter for such

a purpose. In fact, magnitude depends upon the volume of erupted

magma andpyroclastic deposits,i.e.,on theerupted mass, a parameter

even more useful to evaluate the impact of an eruption on the envi-

ronment. Considering the magnitude as well as the parameters upon

which it depends (Fig. 4) and their frequency distribution (Fig. 2), the

explosive eruptions of the past 5 ka can be subdivided according to a

threefold classication, which includes a small-, a large-, and a

medium-size class. Each class is characterized by a range of variability

of the measured parameters (Tables 1 and 2;Fig. 4). The large-size

class includes only theA

MS eruption; the medium-size class includes6 eruptions, namely Agnano 3, Paleo Astroni 2, Astroni 3, 4, 5, and 6;

and the remaining 15 eruptions fall within the small-size class.

According to thecharacteristics of theexplosive eruptionsof thepast

5 ka (de Vita et al., 1999; Di Vito et al., 1999, submitted for publication;

Isaia et al.,2004;Orsi et al.,2004; Costa et al.,2009; Tonarini et al., 2009),

we have singled out a representative type event(s) for each size class

(Table2). Syntheticdescriptionsof themaincharacteristicsof these type

eruptions are reported inAppendix A. The large-size class is character-

ized by AMS, the only high-magnitude eruption of the past 5 ka.

Eruptions of the medium-size class are represented by the Astroni 6

event. For the small-size class eruptions, Monte Nuovo and Averno 2

are suggested as two type events. Monte Nuovo, a tuff-cone forming

eruption, was dominated by phreatomagmatic explosions, while

Averno 2, a tuff ring-forming event, alternated magmatic phases, with

generation of sustained columns, to phreatomagmatic explosions.

4. Probability estimation of the size of the next eruption at

Campi Flegrei

The previouslydescribed data on the explosiveeruptions and those

available in literature on the effusive events, were used to estimate

the probability of each of the size-class eruptions at CFc for the next

eruption. For such a purpose, the eruptions have been grouped in 4

distinct size classes: size class 1 includes the effusive eruptions, while

size classes 2 through 4 include the dened small, medium and large

explosiveeruptions. By means of Bayesian inference, we have estimated

the conditional probability of an eruption of each of the 4 possible size

classes, given that an eruption occurs. Note that, following the consid-

erations reported above, the conditional probability of an eruption ofeach size class is assumed independent from its vent location.

In Bayesian inference, each probability is considered as a random

variable,characterized by its probability density function (e.g., Gelman

et al., 1995; Marzocchi et al., 2004, 2008, submitted for publication).

This process permits to take into account any kind of available infor-

mation (i.e., a priori and theoretical beliefs,models, expert opinions,as

wellas geological, historical and monitoring data). It alsoallows to deal

with aleatory and epistemic uncertainties in a structured, quantitative

and explicit fashion. Details on Bayesian inference and on the specic

choices adopted are reported inAppendix B. Here we only recall that

the prior/theoretical beliefs, models, expert opinions, etc., are sum-

marized in aprior distribution, whichis then combined withpast data.

The obtained combination is theposterior distribution and represents

the nal estimate of the procedure. The prior distribution is setthrough parameters such as theaverage probabilities to each class and

the equivalent number of data (see Eqs. (3) and (4) inAppendix B).

In our application to CFc, we have assigned an average probability

of 0.05 to the effusive eruptions size class (1) and of 0.95 to the

explosive eruption size classes (2 through 4). This choice is quite

arbitrary, butit roughly reectsthe effusiveversus explosiveeruptions

ratio over the past 15 ka (Di Vito et al., 1999; Orsi et al., 2004). Within

the explosive eruption size classes (2 through 4), we have set prior

probabilitiesaccording to a powerlaw estimatedby considering all the

historically known volcanic eruptions (Simkin and Siebert, 1994), in

analogy with Marzocchi et al. (2004) for Mt. Vesuvius. Therefore,

these choices lead to best guesses of theprior distributionof 0.05, 0.79,

0.13, and 0.03, respectively. The equivalent number of data has been

set to 1, i.e., the variance (epistemic uncertainty) is the maximum

268 G. Orsi et al. / Earth and Planetary Science Letters 287 (2009) 265276


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Fig. 2.Frequency distribution of parameters calculated for the 22 explosive eruptions of Campi Flegrei caldera over the past 5 ka. (Data of Table 1).

269G. Orsi et al. / Earth and Planetary Science Letters 287 (2009) 265276


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value allowed (seeMarzocchi et al., 2004, submitted for publication).

As regards past data, outof the25 eruptionsthatoccurredoverthe past

5 ka, 3 are effusive, while 15 are small-size, 7 medium-size, and 1

large-size explosive events.

The results (posterior probability distribution) obtained for the

four size classes are reported inTable 3. The probability distribution

for each size class is described through parameters such as best guess

probabilities (average) and condence intervals (10th, 50th and 90thpercentiles).

It is noteworthy that such probability distributions, despite being

an important result by themselves, are a basic component for both

achieving a comprehensive probabilistic volcanic hazards assessment

(Marzocchi et al., submitted for publication; Selva et al., submitted for

publication-a), and establishing quantitative decision-making proce-

dures (Marzocchi and Woo, 2007, 2009; Woo, 2008).

5. Discussion and nal remarks

Oneof thebasicassumptionsof this work is that the characteristics

of all the eruptions of the CFc over the past 5 ka and the physical

parameters measured for all the 22 explosive events are indicative of

the past behavior of the system and form a solid scientic basis for a

long-term forecast of style and size of a future eruption. We have

estimated the most relevant physical parameters of the explosive

eruptions, such as area covered by pyroclastic deposits thicker than 1

and 10 cm, volume and density of the pyroclastic deposits, volume of

erupted magma, total erupted mass, and eruption magnitude. The

obtained results have enabled us to perform a size classication of the

past eruptions and a probabilistic forecast of style and size of the next

event. According to size, the 22 explosive eruptions of the past 5 kacan be grouped in a threefold classication, which includes a small-, a

medium-, and a large-size class (Fig. 4). On the basis of the charac-

teristics of these eruptions, we have singled out a type event(s) for

each size class. AMS is the only large-size eruption and therefore the

type event for this class,whilewe have identied Astroni 6 as the type

event for the medium-size class. For the small-size class, we have

identied the Monte Nuovo and Averno 2 eruptions as type events,

because, although of similar size they have had different dynamics

and impact on the environment.

This understanding of the dynamics of the past eruptions permits

some general hypotheses on the style of the next event. An effusive

eruption will be very likely dominated by extrusion of viscous lavas,

which will generate a dome or very small lava ows. An explosive

eruption of any size very probably will be characterized by alternation

Fig. 3.Variation through time of some parameters calculated for the 22 explosive eruptions of Campi Flegrei caldera over the past 5 ka. Inset gures expand the time scale to better

show details.



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of magmatic and phreatomagmatic phases, which will produce

particles fallout and owage of pdcs. At present it is impossible to

hypothesize a time sequence of the various eruption dynamics and

related phenomena during the course of a future event. Finally, a

sequence of eruptions following each other in the same vent area at

short (years or tens of years) time intervals cannot be ruled out.

A probability of future occurrence has been assigned to each of the

eruption size classes by means of a Bayesian inference procedure. Toeach class has been attributed an a priori distribution based on a

power law distribution typical for the global catalogue. This prior

distribution has been then updated through the likelihood containing

all the information of the eruptions that occurred over the past 5 ka.

The obtained results (Fig. 5) show that the most likely event to occur

is an explosive eruption of the small-size class with a conditional

probability of about 60%, the least likely event to occur is one of the

large-size class with a conditional probability of about 4%, while an

explosive eruption of the medium-size class has a conditional

probability of occurrence of about 25%. The conditional probabilityof occurrence of an effusive eruption is about 11%.

Fig. 4.Area1cmversus selected parameters calculated for the 22 explosive eruptions of Campi Flegrei caldera over the past 5 ka.

Table 2

Physical parameters variation and type eruption(s) for each of the dened size classes

for explosive events.

Size A1cm(km2)

VTephra(km3)

VDRE(km3)

TEM

(kg1011)

Magnitude Type

esruption(s)

Large >500 >0.40 > 0.3 > 5 >5 Agnano

Monte Spina

Medium 5001000 0.150.40 0.10.3 25 4.35 Astroni 6

Small 0500 00.15 00.1 02


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The results presented in this paper increase the current knowledge

on the past behavior of the CFc system and supply both the scientic

community and the civil authorities with an important element in their

effortsto assess volcanic hazardsand to develop volcanic risk mitigation

measures. Furthermore, the low probability value of the large-size class

seems to suggest that risk mitigation measures based on the medium-

size class event could be the most reasonable. On the other hand, the

probabilities of all size classes should concur to dene a comprehensive

evaluation of volcanic hazards and risk (Marzocchi et al., submitted

for publication; Selva et al., submitted for publication-a).

Acknowledgements

The research has been carried out in the framework of the INGV-

DPC joint projects on the Italian active volcanoes. F. Dell'Erba is

thanked for the kind help in eld work and data processing. Two

anonymous reviewers are thanked for improving a rst version of the

manuscript.

Appendix A

A.1. AgnanoMonte Spina eruption

The AgnanoMonte Spina is thehighest-magnitudeeruptionof the

CFc over the past 5 ka (de Vita et al., 1999 and references therein)(Fig. 6). The measured physical parameters of this eruption are

reported inTables 1 and 2.de Vita et al. (1999)presented a detailed

reconstruction of the entire erupted sequence, dated the eruption

at 4,10050 years bp, and well constrained shape and size of the

volcano-tectonic collapse that accompanied the event. The erupted

sequence includes beds generated by contrasting fragmentation and

transportation dynamics. Magmatic Plinian/sub-Plinian fallout depos-

its commonly alternate with base-surge beds of phreatomagmatic

origin. Moreover, during some eruption phases, the contrasting erup-

tive dynamics were almost contemporaneous (Dellino et al., 2001,

2004). Variation of lithological features and occurrence of a well-

dened erosional unconformity allowed the authors to subdivide

the whole sequence into six Members named A through F, each

representing the deposit of one eruption phase. Members were fur-

ther subdivided into sub-Members, based upon sedimentological

characteristics.

The eruption began with phreatomagmatic explosions that pro-

duced a highly expanded ash cloud and deposition of an ash bed,

followed by a magmatic explosion that generated a low (about 5 km)

eruption column from which was deposited the upper fallout bed of

sub-Member A1. Collapse of this column and phreatomagmatic explo-

sions generated the sequence of ash-to-lapilli size surge- and fallout-

beds of sub-Member A2.

Formationof a pulsating eruption columnmarked the beginningof

the second phase of the eruption. This column reached a maximum

height of about 23 km, was dispersed towards the east, and deposited

the fallout layer B1 up to a distance of 45 km. Its destabilization

generated the basal pyroclastic surges of sub-Member B2, while its

Fig. 5.Relative proportions of conditional probability for all the events likely to occur

(best guess values) at Campi Flegrei caldera.

Fig. 6. Areal distribution of AgnanoMonte Spina deposits; the dashed line delimits the area of the pyroclastic density current dispersion, the solid lines encompass the 10 cm

isopachs of the main fallout deposits, the asterisk indicates the vent area. Modied afterde Vita et al. (1999) and Costa et al. (2009) .



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subsequent waning produced owage of high particle-concentration

pdcs and a lag breccia within the Agnano area. At this stage, a network

of fractures, foreshadowing a volcano-tectonic collapse, likely formed

and became the site of scattered vents. Opening of fractures allowed

ground water to access different parts of the magma reservoir.

Therefore, the ensuing explosions were triggered by magma/water

interaction processes of variable efciency and generated the pdc

deposits of the upper part of sub-Member B2.

The second phase was followed by a pause in the eruption duringwhich heavy rains caused diffuse erosion along steep slopes and then

thene ash suspended in the atmosphere fell on the ground to form

Member C, a bed with abundant mud lumps.

An increase in fracturing of the roof rocks of the magmatic feeding

reservoir, probably caused lowering of the lithostatic pressure with

consequent magma volatile exsolution and migration towards the

surface. Interaction of the rising magma with the overheated geother-

mal system caused its ashing and resumption of the eruption, with

phreatomagmatic explosions and deposition of the pyroclastic-surge

beds at the base of Member D (sub-Member D0). The subsequent

explosions were magmatic and generated a Plinian column that reached

a maximum height of 27 km, was dispersed towards the northeast,

and deposited a fallout sequence (sub-Member D1) up to a distance of

45 km. The main episode of volcano-tectonic collapse took place during

this phase. Related to this episode, alternation of phreatomagmatic ex-

plosions and/oreruption column collapse generatedpdcs that deposited

the sequence of pyroclastic deposits characterized by different facies,

according to distance from thevent and to substrate morphology of sub-

Member D2.

The latest stages of volcano-tectonic collapse were accompanied

by vent migration towards the northwestern part of the collapsed

area. Variation of the structural setting of the Agnano area allowed a

large amount of ground water and geothermal uids to access the

reservoir. Phreatomagmatic explosions produced an expanded cloud,

which deposited the sequence of base surge beds of sub-Member E1,

and were followed by a short-lived magmatic eruption column, which

generated sub-Member E2, and by phreatomagmatic explosions that

generated the sequence of pdc and minor fallout deposits of sub-

Member E3. The nal phase of the eruption was characterized by

phreatomagmatic explosions producing ash deposits (Member F).

Accumulation ofne particles suspended in the atmosphere, favored

also by rainfall, marked the end of the eruption.

A.2. Astroni volcano

The Astroni volcano (Tonarini et al., 2009, and references therein)formed between 4.1 and 3.8 ka, in the northwestern portionof the area

affected by the volcano-tectonic collapse related to the AMS eruption

(de Vita et al., 1999)(Fig. 7). It is the best example of a long-lasting

activity at CFc, as it was constructed through at least 7 eruptions of

variable magnitude, following each other at short time intervals within

the same vent area. The deposits of the 7 eruptions, named Unit 1

through 7 (Fig. 7), are separated by erosional unconformities or thin

paleosols containing evidence of Eneolithic settlements (Marzocchella,

1998), suggestingtimebreaks between eruptions. Theeruptiveunitsare

composed ofne- to coarse-ash pdc deposits and subordinate fallout

layers. These eruptions, whose vents, although all located within the

present crater, migrated through time, were characterized by phreato-

magmatic with minor magmatic explosions, only two events ended

with low-energy explosions and lava extrusion. Unit 6, whose vent was

located in the southeastern sector of the present crater, comprisesne-

to coarse-ash beds with intercalated coarse pumice lenses and beds. A

coarse pumice Plinian layer forms the base, while more pumice fallout

beds occur in theupper portion of the sequence. The Plinianlayer is the

only such layer of the entire Astronisequence, and wasdeposited by an

eastward-dispersed eruption column, which reached a maximum

height of about 20 km. The measured physical parameters of this

eruption are reported inTables 1 and 2.

On the basis of stratigraphical and sedimentological characteristics

of the entire Astroni sequence,Isaia et al. (2004)have reconstructed

the history of the volcano, which is here summarized. The rst erup-

tion was dominated by phreatomagmatic with subordinate magmatic

explosions and was followed by a pause. Volcanism resumed with the

Fig. 7.Areal distribution of the deposits of the 7 eruptions of Astroni volcano. The solid line 6 encompasses the entire sequence of the Astroni Unit 6. The dashed line 6 is the 10 cm

isopach of the coarse fallout deposit of Unit 6. Modied afterIsaia et al. (2004) and Costa et al. (2009) .



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second and lowest energy eruption of the volcano, which was mostly

phreatomagmatic and generated contemporaneous particles fallout

and pdcs owage. A subsequent short pause was interrupted by the

third and largest eruption of the volcano, which was characterized

by alternation of phreatomagmatic and magmatic explosions, with

dominant phreatomagmatic events only during its nal stages.

Another quiescence preceded the fourth eruption, which was very

similar in dynamics to the previous one, although more energetic. A

newpause in the activity took place until volcanism resumed with thefth eruption, which was explosive and ended with lava extrusion.

Another pause was interrupted by the eruption that produced

Unit 6. This eruption began with magmatic explosions generating a

sustained column, the only Plinian column in the entire life of the

volcano, and continued with prevailing phreatomagmatic and sub-

ordinate magmatic explosions. In the nal phase of the eruption,

magmatic explosions prevailed and formed low columns that

deposited fallout beds. This sixth eruption likely was followed by a

short pause, interrupted by the last eruption of the volcano, which

began with an explosivephaseand ended with low-energy explosions

and lava effusions within the crater area.

A.3. Averno 2 eruption

The Averno 2 eruption occurred 3,700 50 years bp (Alessio et al.,

1971) and was characterized by magmatic and phreatomagmatic

explosions that produced a sequence of pyroclastic-fall and pdc

deposits. The whole sequence has been subdivided into three members

named A through C, from base upsection(Costaet al.,2009; DiVitoet al.,

submitted for publication) (Fig. 8). Isopachs and isopleths maps of

fallout deposits, as well as dispersal of ballistic fragments and areal

distributionof facies of pyroclasticdensity current deposits, suggest that

the vent migrated from southwest to northeast, likely along a NESW

trending fault system. The measured physical parameters of this

eruption are reported inTables 1 and 2.

The eruption began with prevailing magmatic and subordinate

phreatomagmatic explosions, which generated the sequence of

Member A, distributed over an area of about 37 km2. The magmatic

explosions produced at least six eruption columns, which reached amaximum height of 10 km and deposited fallout beds, dispersed

towards the southwest. The phreatomagmatic explosions produced

pdcs that deposited thin ash layers around the vent area.

The second phase of the eruption was characterized by phreato-

magmatic with subordinate magmatic explosions. The latter mostly

occurred at the beginning and at the end of the eruption phase. The

activity of this phase generated pdcs with subordinate low eruption

columns, which deposited Member B, distributed preferentially north

of the volcano over an area of about 34 km2. This member includes a

sequence of cohesive, massive to cross-laminated, generally ne-ash

surge beds, containing abundant accretionary lapilli and minor lapilli-

sized pumice fragments. Fallout deposits are concentrated in the

lower and upper parts of the sequence.

The third and last phase of the Averno 2 eruption also generated

explosions driven mainlyby variably efcient water-magmainteraction

and subordinately by magmatic fragmentation. The former mechanismproduced pdcs, while the latter generated low eruption columns. The

deposits of this phase formed Member C, distributed over an area of

about 12 km2, preferentially north of the vent area. The pdcs deposited

a sequence of cross-laminated to plane-parallel ash-surge beds and

bedsets containing lapilli-sized pumice fragments. This sequence in-

cludes a few thin fallout layers deposited by the low eruption columns.

The surge beds are sometimes overlain by massive ne-ash, accretion-

ary lapilli-rich fallout beds, and underlain by lenses of traction carpet or

by erosional unconformities. On the northernank of thevolcano, large

ballistic blocks and bombs form impact sags on surge deposits.

A.4. Monte Nuovo eruption

The Monte Nuovo eruption, the most recent event of the CFc, has

been reconstructed through both geological, volcanological and petro-

logical investigations, and analyses of historical documents (Di Vito

et al., manuscriptin preparation)(Fig.9). Theeruption,which lastedone

week and was fed by three vents, was characterized by three phases

separated by pauses in the activity. The main vent (MV) was located in

the present crater of the Monte Nuovo tuff cone, whereas two minor

vents were along the southern (SV) and northeastern (NEV) slopes

of the cone. The entire sequence of deposits has been subdivided in 5

members named A through E, from base upsection. The measured

physical parameters of the eruption are reported inTables 1 and 2.

The eruption began on September 29, 1538, at 7 p.m., and its rst

phase, which was the main phase of the entire event, lasted two days,

until the night of September 30. This phase generated almost contin-

uous phreatomagmatic with subordinate magmatic explosions, pro-

ducing pdcs and minor short-lived, low eruption columns, whichdeposited members A and B. Member A, composed of a sequence of

plane-parallel to undulatedne- to coarse-ash beds forms the largest

part of the Monte Nuovo tuff cone, and was erupted in about 12 hours

through the MV. Phreatomagmatic explosions at the SV produced

mainly pyroclastic density currents that deposited Member B. This

member, composed of wavy ne-ash deposits containing coarse

pumice and lithic fragments, is distributed only in the southern sector

of Monte Nuovo. Strombolian explosions at the SV and NEV deposited

Fig. 8. Areal distribution of the deposits of Averno 2 eruption. Curve a =cumulative

dispersal of the fallout deposits of Member A; curve b =cumulative pyroclastic density

current deposits of Member B; curvec=cumulative pyroclastic density current depos-

its of Member C. Modied afterCosta et al. (2009).

Fig. 9.Areal distribution of the deposits of Monte Nuovo eruption. Curve a =cumula-

tive dispersal of fallout deposits; curve b =cumulative pyroclastic density current

deposits. Modied afterOrsi et al. (2004).



11/12

the sequence of coarse scoria fallout deposits dispersed over narrow

areas around the eruption vents, which form Member C. This activity

marked the end of the rst phase and was followed by a pause that

lasted two days.

The eruption resumed on October 3 at 4 p.m. and lasted until the

next night. This second phase of the eruption was characterized by a

discontinuous sequence of low-energy phreatomagmatic and mag-

matic explosions at the MV, which deposited the sequence of Member

D. The phreatomagmatic explosions produced laminated to massiveash surge deposits, while the magmatic events produced coarse

pumice and scoria fallout deposits.

Followinga briefpause, thethird phaseof theeruption beganat 4 p.

m., October 6, and lasted few hours. This last phase was characterised

by low-energy magmatic explosions from the MV. This activity was

likelycharacterized by theexplosionof a small dome grownduring the

preceding pause, followed by minor, very low energy events at the

crater of the MV, which produced Member E. This member includes

fallout layers composed of dense to low-vesiculated, angular dark

clasts. During this phase, 24 people died while climbing the slopes of

the newly formed cone at the end of the second pause in the activity.

Appendix B

Bayesian inference is the process oftting a probability model to a

set of data andsummarizingthe results by a probability distribution of

the parameters of the model. To assess the conditional probability iof an eruption of size class i, given the set of data y and that an

eruption occurs, a probabilistic model to link the variables must be

used, i.e., the Bayes' Theorem:

i= ijy= ipriory ji=y 2

where the index i is relative to the size class of interest; [i|y] is the

posterior distribution, given the set of data y; [i]prior is the prior

distributioncontainingthe current knowledgebasedonly on theoretical

models, beliefs and/or expert opinion;[y|i] is the sampling distribution

(theso-called likelihood function), that is the probability distribution of

observing the datay given a specic probability of occurrence in the i-thevent (i); [y] is a normalizing factor accounting for the total probability

of observing datay.

The choice of the functional form of both prior distribution and

likelihood function is the core of the Bayesian inference, and requires

some physical and statistical assumptions. In the application pre-

sented here, we have modeled the prior distribution [i]prior with a

Dirichlet distribution with 4 possible outcomes, i.e., Di4(.) (see

Marzocchi et al., 2004, 2008, submitted for publication).This choice

simply assumes that size classes represent a set of mutually exclusive

and exhaustive events. The likelihood function [y|i] is modeled by a

multinomial distribution (seeMarzocchi et al., 2004, 2008, submitted

for publication), assuming that any datum in the y data set is inde-

pendent fromthe others. Withthese choices,the posteriordistribution

is still a Dirichlet distribution because the Dirichlet is the conjugatedistribution in the multinomial model (Gelman et al., 1995). This

choice is subjective and other distributions can be used, such as the

Gaussian distribution for the logistic transformation of the probability.

In practice, the differences associated with the use of reasonable

distributions are not typically signicant; for this reason, the Dirichlet

(or the Beta) distribution is the most used in practical applications

(see, e.g.,Gelman et al., 1995).

The posterior Dirichlet distribution can be expressed as

i= Di41;2;3;4 3

where

k =

k

+ 3+ yk k= 1;

; 4 4

In Eq. (4) the unknowns iand , andyiconcern the prior model

and the past data, respectively. The central value i represents the

best guess (aleatory uncertainty) of the prior distribution, the equiva-

lent number of data , being a measure of the dispersion around i,

represents our condence on the best guess (epistemic uncertainty).yi is the number of past eruptions in size class i. A more detailed

discussion about this formulation can be found in Marzocchi et al.

(2008, 2009)and in the references therein.

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longterm forecast of eruption style and size at campi flegrei caldera italy

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