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15
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
DOI: 10.4408/IJEGE.2015-02.O-02
M
arta
Della Seta
(*)
, C
arlo
eSpoSito
(*)
, G
ian
M
arCo
MarMoni
(*)
, S
alvatore
Martino
(*)
,
a
ntonella
paciello
(**)
, C
riStina
perinelli
(*)
& G
ianluCa
Sottili
(***)
(*)
Sapienza Università di Roma - Dipartimento di Scienze della Terra e Centro di Ricerca CERI - Piazzale Aldo Moro, 5 - 00185 Roma, Italy
(**)
ENEA - C.R. Casaccia, Via Anguillarese, 301 - 00123 Roma, Italy
(***)
CNR - Istituto di Geologia Ambientale e Geoingegneria (IGAG) - Monterotondo, Roma, Italy
GeoloGical constraints for a conceptual evolutionary model
of the slope deformations affectinG mt. nuovo at ischia (italy)
extended abstract
i vulcani attivi sono strutture ad elevata dinamica morfoevolutiva che, come tali, possono facilmente dare luogo a condizioni di instabi-
lità gravitativa. tali instabilità possono manifestarsi a diverse scale, da poche centinaia di migliaia di metri cubi fino al collasso di interi
settori degli edifici vulcanici (flank instabilities s.s.). l’isola di ischia non fa eccezione a tali scenari; nel corso dell’olocene, è stata, infatti,
interessata da numerosi eventi di frana, variabili da colate di detrito e scorrimenti superficiali a collassi generalizzati di versanti rocciosi.
Questi ultimi sono, in genere, associabili alle dinamiche vulcano-tettoniche connesse al fenomeno di risorgenza calderica che ha interessato
la struttura del Monte epomeo.
il presente studio si concentra sulla deformazione gravitativa che interessa l’area del Monte nuovo ubicata lungo il versante occidentale
del Monte epomeo, originatasi a seguito di un catastrofico evento vulcano-tettonico verificatosi durante le ultime fasi di sollevamento .
in corrispondenza di questo rilievo affiorano depositi di flusso piroclastico alcali-trachitici messisi in posto 55 ka (tufo Verde del Monte
epomeo Auctt.) e lave trachitiche e fonolitiche costituenti parte del basamento dell’isola datate 133 ka.
attraverso un approccio multidisciplinare, questo studio propone un modello geologico-tecnico di dettaglio del versante del Monte
nuovo, portando ad un nuovo modello interpretativo della deformazione gravitativa che lo interessa. attraverso un rilevamento geomor-
fologico, sono state individuate scarpate di faglia, terrazzi morfologici, trincee e contropendenze, che forniscono dei vincoli cinematici
oggettivi al processo di deformazione gravitativa di versante. Un rilievo geomeccanico estensivo è stato finalizzato a derivare proprietà ge-
omeccaniche di ammasso e valutare la loro influenza sul processo in corso. al fine di vincolare ulteriormente il modello geologico-tecnico
del versante, sono state realizzate specifiche campagne geofisiche, consistenti in misure di rumore ambientale distribuite nell’intera area del
Monte nuovo ed interpretate secondo il metodo di n
akaMura
(1989), ovvero l’approccio dei rapporti spettrali tra componenti orizzontali
e verticali (HVSr). i risultati ottenuti hanno messo in luce la complessità della deformazione gravitativa in atto, portando ad identificare
la zona di taglio che guida l’instabilità gravitativa e consentendo l’attribuzione al fenomeno franoso di un meccanismo di scorrimento
biplanare composito. in particolare, le misure di rumore sismico effettuate hanno apportato un utile contributo alla definizione del modello
geologico, evidenziando un picco di risonanza a 0.8 Hz non direzionato e relazionabile alla risonanza di un ammasso a ridotta rigidezza
(riferibile alla porzione interessata dall’instabilità gravitativa), con uno spessore pari a circa 250 m, sovrapposto ad un substrato rigido.
il modello proposto ha, inoltre, messo in luce la stretta relazione esistente tra le emissioni idrotermali e l’assetto stratigrafico e strutturale
del versante di Monte nuovo, evidenziando una concentrazione di emissioni fumaroliche in corrispondenza delle brecce basali, dove la
circolazione di fluidi ha causato un’intensa alterazione, riconoscibile per la presenza di mineralizzazioni autigeniche secondarie, che rap-
presentano geotermometri in riferimento all’evento di alterazione idrotermale.
ai fini del presente studio, è stata, inoltre, prodotta una carta geologica e sono state ricostruite quattro sezioni geologiche, delle
quali due normali e due longitudinali alla direzione di deformazione (vedasi tavola allegata fuori testo). tali sezioni mettono in luce
l’esistenza di tre distinte zone di taglio, la più profonda delle quali, in accordo con le evidenze morfometriche e con le misure geofi-
siche, è localizzata ad una profondità di circa 250 m dal piano campagna. le evidenze geometriche hanno permesso di associare alla
deformazione di versante un meccanismo di scorrimento traslazionale di tipo biplanare che inviluppa un volume di circa 190 milioni di
m
3
. tale superficie è controllata da sistemi di giunti ad alto angolo, limitatamente alla zona di scarpata. Di contro, la superficie a basso
angolo, lungo la quale si esplica uno spostamento a componente prevalentemente orizzontale pari a circa 60 m, non appare controllata
da elementi strutturali. in definitiva, il modello evolutivo qui ipotizzato, che giustifica la deformazione gravitativa di versante in atto
presso Monte nuovo, vede un ruolo preponderante delle pressioni interne al sistema idrotermale ad esso soggiacente nello sviluppo e
propagazione verso monte della zona di taglio. le evidenze ottenute rafforzano le analogie geometriche e volumetriche esistenti tra la
deformazione in atto e grandi frane di versante verificatesi nel medesimo settore, lasciando aperta la possibilità di una analoga evolu-
zione verso condizioni di collasso generalizzato.
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M. della seta, C. esposito, G.M. marmoni, S. martino, A. paciello, C. perinelli & G. sottili
16
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
abstract
ischia island was the scenario of several Holocene slope in-
stability events occurred at different scales, from shallow mass
movements, triggered by meteo-climatic forcing, up to massive
rock slope failures such as large debris avalanches these last ones
related to the volcano-tectonic dynamics of a resurgent caldera.
the present study focuses on the gravitational deformation that in-
volves Mt. nuovo, located in the western portion of Mt. epomeo
resurgent block. a high-resolution engineering-geological model
was reconstructed according to a multi-modelling approach sup-
ported by field geo-structural evidences and constrained by pas-
sive seismic investigations. it revealed a complex morpho-struc-
tural setting and led to the identification of a multiple compound
mechanism, involving a rock mass volume of about 190 million
of cubic meters.
the obtained geological model shows a partial structural
control of the pre-existing tectonic pattern on slope deformation
mechanisms, highlighting geometric and volumetric similarities
between the Mt. nuovo ongoing deformation and an already oc-
curred rock avalanche. the defined conceptual evolutionary mod-
el allows to hypothesize the role of inner pressures constraining
the shear zone initiation and propagation and making reliable a
future scenario of generalized collapse.
Starting from these new field and laboratory data, numerical
models will be reconstructed in order to depict the evolution of
the gravitational slope deformation, evaluate its sensitivity and
constrain future evolutionary instability scenarios.
K
eywordS
: Ischia Island, hydrothermal system, slope instability, volcanic
hazard
introduction
active volcanoes can be affected by gravity-induced slope
deformations up to generalised failure related to their hetero-
geneous structures and rapid growth. the development of dis-
equilibrium conditions can evolve through instabilities at dif-
ferent scales, with volumes of a few 10
5
m
3
(e.g., rolling bocks)
to some 10
7
m
3
during the collapse of entire sectors of the vol-
cano edifices. Several factors have been described as triggers
for volcano flank collapses (B
ozzano
et alii, 2013; M
C
G
uire
,
2006), including cryptodome emplacements, such as Mount
Saint Helens (v
oiGht
, 1981), dyke intrusions as described in the
canary islands (S
ieBert
, 1984), seismicity, such as at Bandai
San (S
ekiya
& k
ikuChi
, 1889) and hydrothermal pressurization
(r
eid
, 2004). these events mostly affect polygenic volcanic
edifices where voluminous, weak, and hydrothermally altered
rock portions can facilitate collapse events. the hazards as-
sociated with lateral collapses are very relevant in the case of
volcanic islands due to their potential to generate tsunamis.
ischia island, an emerged portion of the phlegraean Volcanic
District, is characterized by widespread and recurrent instability
processes (d
e
a
lteriS
et alii, 2009; d
ella
S
eta
et alii, 2011)
due to volcano-tectonic activity and associated to topographic
perturbations. the strict interconnection between active volcan-
ism, evolution of the magmatic and hydrothermal systems and
widespread slope instabilities at ischia island (e.g., d
e
v
ita
et
alii, 2006) developed over the last thousands years through cy-
clical phases of volcanic unrest with associated modifications
in the stress-strain conditions acting on the slopes. Specifically,
stress changes induced by shallow magma reservoir and/or hy-
drothermal groundwater dynamics, associated with topographic
effects and structural and lithological factors exerted an active
and passive control on slope instabilities, as recorded by several
landslide deposits overlaid by, and interbedded to, volcanic suc-
cessions and/or submarine deposits.
Here we investigate the slope deformations active in the
western sector of the Mt. epomeo area which represents one
of the most significant gravitational processes active at ischia.
Based on new geological, geomorphological, geomechanical
and geophysical data, we propose an high-resolution engineer-
ing-geological model of the slope to provide constraints on the
ongoing gravitational slope deformations and to depict possible
future scenarios associated with volcanic unrest episodes.
GeoloGical settinG
the densely populated ischia island is an emerged portion
(about 46 km
2
) of the active phlegraean Volcanic District. the
phlegrean structural setting is related to the plio-pleistocene ex-
tensional phase, active along the tyrrhenian margin of the apen-
nine chain, which led to the formation of the campanian plain
graben (o
rSi
et alii, 2003; d
e
v
ita
et alii, 2006; d
e
v
ita
et alii,
2013). the phlegrean volcanism developed through multiple
large caldera forming events alternating to less intense explosive
activity from scattered, mostly monogenetic eruptive centers. on
ischia island (Fig. 1), the oldest volcanic activity was dominant-
ly effusive with the emplacement of a trachytic and phonolitic
lava (tl) plateau (dated at ~150 ka) cropping out in the south-
eastern sector (v
ezzoli
, 1988). later on, between 150 and 74
ka, small lava domes formed the Mt. Vezzi, Mt. Barano and Mt
Vico reliefs, along with small eruptive edifices in the coastal area
of Sant’angelo, punta chiarito, capo negro, punta imperatore
(C
hieSa
et alii, 1987; d
e
v
ita
et alii, 2013; Fig. 1).
the main Mt. epomeo Green tuff (MeGt) caldera forming
event opened a new phase of activity after about 20,000 years
of quiescence (o
rSi
et alii, 1991; t
iBaldi
& v
ezzoli
, 1998). the
MeGt unit, alkali-trachytic in composition, is characterized by
a greenish colour due to zeolitization in a seawater-rich environ-
ment (a
ltaner
et alii, 2013), possibly related to an undersea em-
placement environment, i.e., the submerged caldera floor.
Based on depositional and componentry features, the MeGt
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GeoloGical constraints for a conceptual evolutionary model of the slope deformations
affectinG mt. nuovo at ischia (italy)
17
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
has been divided in: a) an intra-caldera member made up of two
pyroclastic flow deposits (Upper and lower) separated by a vol-
canoclastic deposit; b) an extra-caldera sequence with pumice
ash-fall deposits and widespread lag-breccia deposits in a rela-
tively distal setting (B
rown
et alii, 2008). the MeGt intra-cal-
dera succession displays a variable thickness with a maximum of
70 m for the lower pyroclastic flow unit (lMeGt) and 200 m for
the upper pyroclastic flow unit (UMeGt). Both units are opened
Fig. 1 -
a) Geological sketch map of Ischia Island; b) structural sketch map of the Neapolitan volcanic district; c) location of the study area (modified after
D
ella
S
eta
et alii, 2011)
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M. della seta, C. esposito, G.M. marmoni, S. martino, A. paciello, C. perinelli & G. sottili
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Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
by a basal lithic breccia (Fig. 2) with a thickness varying from 20
to 50 meters. eluvial-colluvial deposits and debris deposit close
the stratigraphic succession hiding the tuff and lava outcrops, as
clearly shown in the Mt. nuovo area, where these deposits are
largely distributed (Fig. 1).
the MeGt is a zeolitized trachytic ignimbrite consisting in
an altered ash matrix that contains altered pumice and sanidine
along with plagioclase, biotite, horneblende, augitic clinopy-
roxene phenocrysts and lithic ejecta in minor amount; mafic
xenocrysts such as olivine rarely occur. the phenocrysts are
generally unaltered. no differences in MeGt components have
been found between lMeGt and UMeGt. a detailed miner-
alogical and geochemical study on the MeGt and an associ-
ated polymictic breccia (a
ltaner
et alii, 2013) showed that the
phenocryst assemblage of MeGt consists mainly of K-feldspar
crystals of both pyrogenetic (primarly) and authigenetic origin,
chabazite and/or phillipsite zeolites and clay minerals associ-
ated with minor amounts of analcime and calcite. the clay min-
erals comprise illite/smectite randomly interstratified, Fe-rich
illite and minor smectite. plagioclase, biotite, horneblende and
clinopyroxene phenocrysts are minor component.
after the major MeGt caldera-forming event, a series of ex-
plosive eruptions (until ~33 ka ago) took place from vents located
in the Se- and nW sectors of the island. later on, effusive and
small volume explosive eruptions took place from scattered vents
until the last eruption occurred in aD 1302.
a greenish-yellow tufite, associated with the marine re-
working of MeGt (named “tufite del Mt. epomeo”; v
ezzoli
,
1988), records a marine sedimentation phase in the caldera
depression. this deposit is unconformably topped by alter-
nate fossiliferous white siltstones, whitish volcanic ashy lay-
ers and yellow sandstone (named “colle Jetto Fm.”; v
ezzoli
,
Fig. 2 - Outcrops of (a) MEGT, (b) MEGT lithic breccias, (c) Donna Rachele fumarolic fields, (d) Rione Bocca Trachytic Lavas (TL)
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GeoloGical constraints for a conceptual evolutionary model of the slope deformations
affectinG mt. nuovo at ischia (italy)
19
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
1988). the resurgence of the caldera produced an asymmetric
uplift of the central sector of the island that generated the Mt.
epomeo relief. this resurgence could have been generated by
the emplacement of a shallow laccolith (r
ittMann
, 1930), or
by magma chamber dynamics (t
iBaldi
& v
ezzoli
, 1998; o
rSi
et alii, 1991). a
CoCella
et alii (1999) and M
olin
et alii (2003),
based on analogue models of caldera collapse and resurgence,
propose an asymmetric “trapdoor” mechanism, controlled by
a network of nW-Se oriented sub-vertical faults. the uplift,
between ~700 m and 900 m over the last 30 kyr, corresponding
to an average uplift rate of about 3 cm/yr, has been estimated
through the present-day elevation of the outcropping marine
sediments (t
iBaldi
& v
ezzoli
, 1998). the caldera resurgence
played a key role in the geological and geomorphological evo-
lution of ischia island. in fact, this event strongly controlled the
volcano-tectonic activity, the local seismicity and the gravita-
tional processes that involve the edge of the resurgent block.
Superimposed to the above mentioned crustal deformation dy-
namics, a persistent hydrothermal system, with high heat flow
(200-400 mW/m
2
; C
ataldi
et alii, 1991) and several thermal
springs and gas vents (i.e. fumaroles with temperature up to
100°c), is driven by a a relatively shallow magmatic body. the
most stable and active fumaroles are localized in the rione
Bocca area (Donna rachele; see plate 1). in this area, the shal-
low circulation of thermal fluids pervades semilithoid pyroclas-
tic rocks (tuffs) and lavas through a dense cracks network; the
geometry of thermal fluid circulation consists of multilayered
aquifers confined by low-permeability layers with vertical dis-
placements induced by faulting (C
arlino
et alii, 2014). in the
Mt. nuovo sector the hydrothermal system is dominantly fed by
rainwater with significant seawater inputs, as clearly evidenced
by chemical and isotopic markers (d
i
n
apoli
et alii, 2011).
GeomorpholoGical features
the resurgence event strongly conditioned the geomorpho-
logical evolution of the island, particularly the gravitational
processes that involve the edge of the resurgent block. Histori-
cal chronicles documented since the 8
th
century Bc and recent
geological and geomorphological studies allowed many authors
to reconstruct the history of major slope instability events at
ischia island (v
ezzoli
, 1988; d
el
p
rete
& M
ele
, 2006; t
iBaldi
& v
ezzoli
, 2004; d
e
v
ita
et alii, 2006; d
ella
S
eta
et alii, 2011
and references therein). these events, characterized by dif-
ferent mechanisms, age and triggers are dated from the early
Holocene. the oldest recognized events (up to 1000 years Bp)
are mainly represented by large lahars (> 0.2 km
2
), while debris
avalanches were triggered exclusively by volcano-tectonic fac-
tors (earthquakes and/or eruptions). nonetheless, it is likely that
events triggered by meteo-climatic conditions occurred in this
period but were not preserved and/or documented.
Between 1000 and 100 years aD both volcano-tectonical-
ly and meteo-climatically triggered events have been docu-
mented, but the first ones are represented only by small lahars
(<0.2 km
2
), slumps and debris/rock-slides. Finally, in the last
100 years slope instability at ischia was exclusively triggered
by meteo-climatic factors, causing mainly shallow landslides,
rock falls and slumps , as in the well-documented case of Mt.
Vezzi in 2006, on which specific studies have been already pub-
lished (iJeGe 02/2007).
a detailed stratigraphic study by d
e
v
ita
et alii (2006) evi-
denced a complex succession of intercalated primary volcanics
and slope instability-related volcanoclastic deposits. this study
outlined the clustering in space and time of mass movements
before, during and after the main periods of volcanism, high-
lighting, in the last period of activity, between ca. 5.5 and 1.9 ka,
a strong interplay among slope instability and volcano-tectonic
activity. in particular, the presence of paleosols and tephras in
the stratigraphic succession allowed to define four main phases.
this cyclicity was interpreted as the evidence that resurgence
has occurred through the alternation of periods of uplift and pe-
riods of volcano-tectonic quiescence. During quiescence, new
magma intrusion triggered resurgence accompanied by seismic-
ity and slope instability, with or without renewal of volcanic
eruptions. the most gravitationally unstable slopes of the island
are the north-, northwest- and southwest facing steep flanks of
Mt. epomeo and in the Serrara Fontana basin, on the southern
flank of the block, as testified by the widespread volcanoclastic
deposits they are covered by.
the lowlands between casamicciola and Forio are, in fact,
mostly covered by secondary volcanoclastic deposits emplaced
due to slope instability events. Single scars and bodies of slope
failure have been identified and mapped in this sectors of the
island by d
ella
S
eta
et alii (2011), after geomorphological,
stratigraphic and textural analyses. the related events, mostly
occurring since 3 ka, include large debris avalanches (basal con-
tact of the main event outcrops in pietre rosse), large lahars and
minor mass movements such as rock falls, slumps, debris and
rock slides, and small lahars (Fig. 3).
Massive rock slope failures affected the strongly fractured
and hydrothermally altered rock masses of the most uplifted
sectors of the Mt. epomeo, where often characterized by the
intersection of fracture systems and associated fumaroles.
the massive flow of fragmented rocks followed the initial
collapse from a rock slope through expanding and shattering
mechanisms responsible for spreading and run-out of debris
avalanches.
the topography of debris avalanche deposits shows typi-
cally natural levees, marginal and distal cliffs, and linear ridges
close to the margins of the deposit and a common hummocky
topography (S
ieBert
, 1984; G
liCken
, 1998) due to the hetero-
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M. della seta, C. esposito, G.M. marmoni, S. martino, A. paciello, C. perinelli & G. sottili
20
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
geneity of the deposits, mainly made up of a massive, chaotic,
poorly sorted, vesicular matrix, which incorporates megaclasts
and large blocks up to hundreds of cubic meters
mainly com-
posed of MeGt. the topographic overprint suggests that some
lahar events occurred, mostly on top of debris avalanche bod-
ies, producing debris flow depositional bodies, some of which
also entered the sea. Bathymetric surveys have highlighted also
the existence of extended deposits to the north, northwest and
south of the island, the latter interpreted as the products of a sin-
gle catastrophic event that occurred in historical times from the
southern slopes of Mt. epomeo (Serrara Fontana), named the
ischia Debris avalanche (iDa; C
hioCCi
& d
e
a
lteriiS
, 2006; d
e
a
lteriiS
et alii, 2010), whose estimated volume ranges between
tens to hundrends Mm
3
. in addition to these large landslides,
which represent the major catastrophic mass movements oc-
curred in the island from the Holocene, a still active deep seated
gravitational slope deformation (DSGSD) was reported as trig-
gered by a catastrophic volcano-tectonic event that took place
around 460÷470 Bc.
the slope-scale gravitational deformation affects the
northwestern flank of Mt. epomeo in correspondence of Mt.
nuovo (d
ella
S
eta
et alii, 2011). this deformation involves a
steep slope (almost 60°) in the northwestern corner of the Mt.
epomeo resurgent block (Fig. 4) and has a planimetric extent
of about 1.6 km
2
. the main diagnostic features of such a defor-
mational process consist in counter-slope terraces and opened
deep trenches with a direction almost parallel to the slope face
(Fig. 4). this block seems presently to be affected by an esti-
mated westward displacement rate of maximum 15 mm/year, on
the basis of precision levelling, GpS surveys, and DinSar data
(M
anzo
et alii, 2006).
the morphological evidence of slope deformation and the
historical occurrence of large avalanches are diagnostic ele-
ments to define the evolutionary style of this area. the deforma-
tion affecting the western part of Mt. epomeo resurgent block,
and in particular the Mt. nuovo gravity-induced slope instabil-
ity, is the response to a stress field initially connected with the
recurrent inflation and deflation phases, these last ones related
Fig. 3 - Distribution of the major volcano-tectonically triggered slope instability landforms. b). Topographic profile across the Mt. Epomeo resurgent
block (modified after D
ella
S
eta
et alii, 2011)
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GeoloGical constraints for a conceptual evolutionary model of the slope deformations
affectinG mt. nuovo at ischia (italy)
21
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
to the pressurization/depressurization or intrusion/migration
of magmas at shallow depth. in addition destabilizing actions,
possibly related to the pressurization of the local hydrothermal
system, cannot be neglected.
according to z
aniBoni
et alii (2013), the possible evolution
of the active Mt. nuovo slope deformations toward a general-
ized collapse, that can be assimilated to the already occurred rock
avalanche in the south-western adjacent sector, could produce a
tsunami wave able to reach high speeds (30 m/s) and height (more
than 5 m) and therefore configuring significant risk conditions
for the coasts of both the island and the campanian region. in
order to derive constraints to the geometry and mechanisms of the
gravitational slope deformation, as well as to evaluate the possi-
ble evolution of this process, an high-resolution geological model
has been reconstructed.
enGineerinG-GeoloGical modellinG
Methodology
in order to identify the shear zones that presently drive the
gravitational slope deformations of Mt. nuovo and to infer mech-
anisms and volumes associated with this gravity-induced process,
a high-resolution engineering-geological model was reconstruct-
ed through a multidisciplinary approach including geological, ge-
omorphological, geomechanical and geophysical investigations.
new field observations, combined with geological data from
literature, allowed to identify the geological framework in which
the Mt. nuovo slope deformation is taking place.
the starting point of this modelling was a geological and geo-
mechanical survey focused on the identification and mapping of the
two MeGt flow units along with their basal lithic breccias, which
allowed to reconstruct the stratigraphic setting and to assess the role
Fig. 4 - a) Panoramic view of the Mt. Epomeo relief from the Forio Plain; the Mt. Nuovo area, involved in the ongoing gravitational deformation is clearly
visible on the left side of the picture. b) NW-SE view of the deformed block and of the apical zone of one of the documented rock avalanches. c)
View of the Falanga plateau and Mt. Nuovo (downslope) from the top of Mt. Epomeo
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Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
of tectonic elements on the gravity-induced slope deformation.
By means of geomorphological surveys and terrain analyses
(in GiS environment), fault scarps, terraces, saddles, trenches
and morphological counter-slopes were identified, thus providing
more details and constraints to the geometry of the slope defor-
mation. at the same time, a geomechanical survey was carried
out to derive the properties of the outcropping jointed rock mass
and to evaluate their influence on the gravitational process. an
inventory of the fumaroles located within the rione Bocca and
the Forio plain areas was also realized (see the annexed plate 1),
based on both available data (d
ella
S
eta
et alii, 2011; C
hiodini
et alii, 2004) and field observations. Several fumaroles were in-
ventoried, among which the most active and persistent are located
in the Donna rachele area. Moreover, a lot of smaller gas vents
were recognized along the Mt. nuovo and Falanga slopes, partly
characterized by an intermittent behavior and partly extinct.
Seismic geophysical investigations were carried out to char-
acterize the local seismic response of the Mt. nuovo slope as well
as to derive other constrains to the geological model. at this aim,
ambient seismic noise measurements were performed during two
distinct field surveys over an area of approximately 1 km
2
, from
Mt. epomeo to Mt. nuovo, along a section that crosses the main
surveyed structural and geomorphological features from Mt.
epomeo to Mt. nuovo (see plate 1). each measurement station
was equipped with a 3-component seismometer and acquired for
at least 1 hour. a le-3D/5s seismometer by lennartz electronic
GmbH coupled with a reFteK 130-01 data-logger, set to a 250
Hz sampling frequency, was used for the first survey; the other
measurements were carried out using a 1.4 Hz Sl06 acquisition
unit by Sara electronic instruments, set to a 200 Hz sampling
frequency. the seismic noise records were processed by Geopsy
software (www.geopsy.org) developed in the frame of the SeSa-
Me project (B
ard
& SeSaMe t
eaM
, 2004). the time histories,
were de-trended sampled with a 40 s moving time window, 5% co-
sine tapered, converted to the frequency domain and smoothed by
a Konno-ohmachi function (k
onno
-o
hMaChi
, 1998, b=40) to get
average spectra of the three components and average HVSr (Hori-
zontal to Vertical Spectral ratio) according to n
akaMura
(1989);
the distribution of HVSr values on the horizontal plane (HVSr
rotate) was moreover taken into account.
Under specific conditions, i.e. flat topography and horizon-
tal subsurface layers, a peak of significant level (>2 according
to Bard & SeSaMe team, 2004) in the HVSr curve points out
the resonance frequency (f
0
) of a softer soil overlaying a bed-
rock. at the present, nevertheless, this technique is applied also in
more complex geo-morphological conditions and can contribute
to obtain useful information about subsurface structures. in our
case-study, the evidence of a 1D resonance frequency was used
to to indirectly assess the depth of the deformed rock mass. Due
to these basic assumptions, the presence/absence of seismic reso-
nance was used to derive other constrains to the geological model
and to confirm the reconstructed one. this control was performed
relating the f
0
value to the thickness of a “soft” layer, to indirectly
assess the depth of the landslide rock mass. a Vs value of 900
m/s was assumed for the MeGt (S
trollo
et alii, 2015), based
on measures available for similar tuffs in the phlegraean area
(i.e neapolitan Yellow tuff) (n
unziata
et alii, 1999). it is worth
noting that Vs values for such a weak-rock range between 500 e
1000 m/s depending on physical conditions (degree of hardening,
texture, fractures density) as well as on depth.
the litotechnical features of the MeGt were well treated by
several authors which analyzed geotechnical parameters and me-
chanical behavior respect to weathering and alteration (p
ola
et
alii, 2014). a decay of mechanical properties was clearly high-
lighted in tests performed on zeolite-rich materials as the MeGt,
that showed a marked influence of high temperatures on the
compressive and tensile strength (h
eap
et alii, 2009). Such a be-
haviour can be regarded as significant in the hyper-thermal area
of ischia, as rock mass rheology can be strongly controlled by
thermo-baric conditions.
Results
the detailed geological survey allowed to highlight the stratig-
raphy and the depositional features of the units outcropping in the
Mt. nuovo study area. the oldest unit is represented by the tl of
rione Bocca (133 ka; v
ezzoli
, 1988), which is intensely jointed
and pervasively altered by hydrothermal fluid circulation (Fig. 2).
over this unit the massive, ashy deposits of the MeGt crop
out extensively. our high-resolution geological map (see plate 1)
distinguishes the two main flow units (lower MeGt and Upper
MeGt) of which it is composed (B
rown
et alii, 2008). it was also
possible to identify and map, for both lower and Upper MeGt,
the basal levels of lithic breccia that indicate the initial phase of
the Mt. epomeo eruption activity. these levels are represented by
clast-supported deposits composed by tl blocks and heterometric
lapilli. lava blocks from both lMeGt and UMeGt basal breccia
are porphyritic (porphyric index, pi between 5 and 25 vol %) with
a phenocryst assemblage dominated by sanidine and subordinate
plagioclase ± clinopyroxene ± biotite ± oxides. Microlites of alka-
li-feldspar, occasionally joined to plagioclase and clinopyroxene,
are included in the rock groundmass ranging from holocrystalline
to hyalopilitic. no clear difference has been found between the
lMeGt and UMeGt lava blocks of breccia, except that, in the
lower MeGt, mafic minerals are always present, whereas in the
Upper MeGt, if present, clinopyroxene and/or biotite occur as mi-
crophenocrysts (Fig. 5). notably, in correspondence of both the ba-
sal breccia levels an intense hydrothermal flow activity is clustered,
as highlighted by several fumaroles. Depending on the distance
from the fumarole vents, the inner circulation of fluids causes in-
cipient to intense alteration of the breccia levels that, in lava blocks,
background image
GeoloGical constraints for a conceptual evolutionary model of the slope deformations
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23
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
Fig. 5 - Optical photomicrographs (plane polarized light) of samples of Mt. Epomeo Green Tuff. Fresh a) to moderately altered b) porphyric trachytes
from LMEGT breccia. Altered samples from this breccia level show a partial to complete replacement of groundmass phases by analcime and clay
minerals. Weakly c) to highly altered e) porphyric trachytes of UMEGT breccia. In the samples collected at the fumaroles (e) also phenocrysts are
affected by hydrothermal alteration showing a pervasive substitution of them with clay minerals. (f) Mt. Epomeo Green Tuff sample in which fresh
sanidine, plagioclase, biotite and oxide phenocrysts are enclosed in the altered ash matrix. San: sanidine; Biot: biotite; Plg: plagioclase; Ox: oxide
background image
M. della seta, C. esposito, G.M. marmoni, S. martino, A. paciello, C. perinelli & G. sottili
24
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
is recognizable by the presence of secondary authigenic minerali-
zations in the groundmass giving an orange to orange-red colour to
the rock. the authigenic mineral assemblage analcime + montmo-
rillonite + kaolinite + hematite recognized by X-ray diffraction
analyses on altered samples suggests temperatures of hydrothermal
alteration in agreement with the temperature of fumaroles (up to
100°c; a
ltaner
et alii, 2013 and references therein). the phen-
ocrysts assemblage recognized in the MeGt samples is the same
to that observed by a
ltaner
et alii (2013) (sanidine, plagioclase,
biotite, clinopyroxene and horneblende). the secondary authigenic
mineralizations detected by X-ray diffraction analyses on a sample
of MeGt (at pizzone site - ne of Mt. nuovo, see Fig. 1) are simi-
lar to those reported by a
ltaner
et alii (2013) in Green tuff clasts
of polymictic breccia samples (K-feldspar + analcime + clay min-
erals ± phillipsite ± calcite); in our sample the phillipsite zeolite and
calcite are absent. the different types of alteration minerals identi-
fied indicate that MeGt experienced several alteration events; in
particular, the mineral association of phillipsite ± chabazite, inter-
stratified illite/smectite, Fe-rich illite and low amount of analcime
point to an alteration event at low-temperature (<70°c) whereas
the association of authigenetic K-feldspar, analcime Fe-rich illite
and interstratified illite/smectite suggests a hydrothermal alteration
episode at temperature higher than 70°c, possibly up to 100-150°c
(a
ltaner
et alii, 2013).
the results of geomechanical survey focused on reliable out-
crops of MeGt highlighted that the welded cineritic matrix is
interested by pervasive high angle conjugated systems and low-
angle sets of discontinuities, dipping from 50 to 70° and 25-30°
(J2, F; see Fig. 6), respectively. the main joint sets are a nS +/-
10° trending system dipping westward around 50° and a ne-SW
oriented set dipping 20-30° to Se (J1; see Fig. 6). the high-angle
planes are related to both major regional tectonic systems and
local systems responsible for the resurgence of the Mt. epomeo,
while the low-angle ones correspond to gravitational shear planes.
Fig. 6 - Geological section of the gravitational deformation affecting the Mt. Epomeo western slope. Fumaroles as well as seismic noise measurement
locations are projected from the surface position to the cross-section. The dashed black line represent the shape of the already detached avalanche
of Pietre Rosse scar area, shifted 150 m eastward. The synthetic stereoplot of the main joint sets measured in the Mt. Nuovo area (left) and in the
scar area of the Pietre Rosse rock avalanche (right) are also shown
background image
GeoloGical constraints for a conceptual evolutionary model of the slope deformations
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25
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
resonance could be related to an approximately 250 m thick soft
layer constituted by the MeGt which is part of the landslide mass
and overlay the tl constituting the landslide bedrock (Fig. 6).
Such a seismic resonance is no more evident moving from
Mt. nuovo toward the Falanga plain where the MeGt is not yet
involved in the landslide process so justifying a more reduced
impedance contrast respect to the tl.
concerning the sector between the Falanga plain and the top
of Mt. epomeo, the measurement at station St_4 shows a 7 Hz
HVSr peak that is consistent with the presence of a shallow uncon-
solidated terrain, partially made of a rock slide mass and partially
of eluvial-colluvial deposits generated by weathering and hydro-
thermal alteration. Based on the obtained frequency and taking into
account the Vs values typical for such kind of deposits (500-600
m/s) it is possible to estimate a debris thickness of about 15 meters,
that is in agreement with the collected evidence. the other noise
measurements do not show significant HVSr peaks: this absence
of resonance can be ascribed to the presence of major faults or shear
zones that increase the rock mass jointing conditions of both the
landslide mass and its bedrock, reducing their impedance contrast.
Moreover, many noise measurements show HVSr values
higher than 2 within a wide frequency range as the FFt ampli-
tudes of the horizontal components are generally higher than
the vertical ones. according to d
el
G
audio
et alii (2007), such
an effect can be referred to a topographic amplification due to
peculiar landform geometries like steep slopes or sharp ridges,
that control the directional distribution of FFt. in addition to the
approximation related to the assumed 1D model, further uncer-
tainties derive from lack of stratigraphic or active seismic data,
useful for calibrating the seismo-stratigraphic model.
the here proposed engineering-geological model highlights
the correspondence between the gas vent location and the basal
breccias outcropping at the base of the two MeGt flow units.
Such a correspondence is evident in both the Donna rachele (ri-
one Bocca) fumarolic field and in the Mt. nuovo area and could
be related to the contrast of permeability between the MeGt
breccia levels and the massive lava, which represent the local
hydrothermal reservoir. Based on the here reported engineering-
geological section (Fig. 6) the fumarolic emergences are justified
as they rise from a complex net of faults and fractures, i.e. they
are related to the structural setting of the slope and the ongoing
gravitational deformation. the presence of extinct fumaroles up-
slope the main surface seems to confirm that both lithological and
structural elements control the emergence of ascent fluids from
the deep reservoir. in this framework, structural discontinuities
represent preferential escape paths for the ascent of fluids, be-
cause of the induced fracturing, while the gravitational surfaces
may have interrupted the lateral continuity of the reservoir, pre-
venting the ascent of fluids in the highest areas and leading to the
extinction of gas vents.
the most pervasive systems also control the widespread landslide
process affecting the Mt. nuovo slope as they are located at the
edge of the main escarpments, where discontinuities isolate large
unstable blocks, such as those identified in the Falanga plain.
the joint trends have a direct correspondence in the morpho-
logical trenches observed upslope from the deformed volume,
where they developed parallel to the slope suggesting a gravita-
tional origin.
the alignment of the aforementioned landforms with the
main tectonic elements suggests a main tectonic control: pre-
existing discontinuities, combined with high slope gradient, led
to a slow mass rock creep process that evolved in a shear-zone
driven deep landslide.
Based on the collected geological data it was possible to draw
four engineering-geological cross-sections of the Mt. epomeo
western slope.
these sections intercept three distinct gravitational shear
surfaces: the main one is about 250 m deep, while the two minor
ones cut the topographic surface in correspondence of the top of
Mt. nuovo and downslope, respectively.
the geometric evidence allow to classify the slope deformation
as a “type e” structurally-defined compound “constrained at toe”
slide (h
unGr
& e
vanS
, 2004) with main rupture surfaces following
an high angle joint set at scarp (F, J2 joint set, see Fig. 6) and a basal
shear zone cutting the rock mass. the resulting landslide mecha-
nism is a mainly translational one even if a rotational contribution
to the sliding can be deduced by retro-deforming the landslide mass
starting from its present setting. Geometry of secondary failure sur-
faces, counter-slope terraces and evidence of back-tilted markers
within the outcropping tuffs (i.e. elongated scoria) also indicate a
rotational component in the landslide mass movement.
the bi-planar compound setting of the main sliding surface
involve a rock mass volume of about 190 Mm
3
with a total hori-
zontal displacement of about 65 m (Fig. 6). More in particular,
the flat portion of sliding surfaces seems to be not controlled by
structural elements (i.e. strata, joints or faults, see Fig. 6) since
they result in a counter slope setting (this is the case of the geo-
logical contacts) or in a high-angle down-slope dipping (this is
the case of main faults and joint sets). on the other hand, the
high-angle portion of the sliding surfaces corresponds to main
faults that displace Mt. nuovo from the top of the Falanga plain,
revealing a significant structural control.
the seismic noise measurements provided a useful contri-
bution to the reconstruction of such a geological setting as they
pointed out a HVSr resonance peak at 0.8 Hz at stations St_1,
St_3, St_8 (see Fig. 6 for their projection along the geological
cross-section). as the HVSrs does not show directivity, we could
relate this resonance to an impedance contrast between a soft-rock
layer and a bedrock, i.e. under almost 1D condition. taking into ac-
count the geological setting of the Mt. nuovo slope, the observed
background image
M. della seta, C. esposito, G.M. marmoni, S. martino, A. paciello, C. perinelli & G. sottili
26
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
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evidence strengthens the analogy between the evolution of the oc-
curred landslide and the gravitational ongoing-deformation, lead-
ing to assume a generalized collapse of the slope as a possible
ultimate scenario for the Mt. nuovo landslide evolution.
conclusions
this study proposes a high-resolution engineering-geological
model of the Mt. nuovo (nW of Mt. epomeo) at ischia island,
which is involved in a shear-zone-driven gravity-induced slope
deformation. the model was constrained by major morpho-struc-
tural field evidence of the landslide mechanism and by the results
of ambient noise measurements. the here experienced multidis-
ciplinary approach allowed to provide more constraints to the
understanding of the gravitational deformation affecting the Mt.
nuovo in terms of structural control, rock mass properties and
involved volumes.
Moreover, the here proposed geological model highlights that
the hydrothermal emissions are strictly related to both the strati-
graphic and structural setting of the area.
this model represents the first step to define the gravitational
conceptual evolutionary model of the
mt. nuovo onGoinG deformation
Based on the high-resolution engineering-geological model
of Mt. nuovo a preliminary conceptual model was derived to jus-
tify the ongoing gravity-induced slope deformations. this model
relates stress field within the hydrothermal system (due to high-
pressure fluids) with crack initiation at the foot of the slope (i.e.
where less confining pressures exist) that evolve in fractures con-
centrated along highly jointed rock mass zones (Fig. 7). these
fractures evolve in a proper shear zone propagating upslope until
they reach the pre-existing volcano-tectonic elements, i.e. those
related to the resurgence mechanism. this shear zone favored the
sliding deformation and allowed the opening of deep trenches
(Fig. 7). the projection on Mt. nuovo section (a-a’; Fig. 6) of
the scar surface of the already detached rock avalanche in the ad-
jacent south-western slope (d
ella
S
eta
et alii, 2011), outlines an
extraordinary similarity in terms of both geometry and shape of
the reconstructed sliding surfaces (Fig. 6). also the geostructural
setting of the rock avalanche scar area, reveals very similar fea-
tures respect to the one obtained at Mt. nuovo (see Fig. 6). this
Fig. 7 - Conceptual evolutionary model of the Mt. Nuovo slope deformation
background image
GeoloGical constraints for a conceptual evolutionary model of the slope deformations
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27
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
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both the landslide slope and the underlying hydrothermal system.
to this aim, a thermodynamic numerical model will be performed
by considering a multilayer hydrothermal system and a conduc-
tive-convective heat transfer. this model will be coupled with a
thermo-mechanical stress-strain model of Mt. nuovo slope, to
infer possible interactions between the thermo-baric field of the
hydrothermal system and the ongoing landslide process.
acKnoWledGements
this research was funded by “Sapienza” University of rome
in the frame of the project “landslide triggering in areas affected
by intense morphodynamics due to volcanism, seismicity and ero-
sional processes” (anno: 2012 - prot. c26a12Mt4F) p.i. prof. S.
Martino. the authors wish to thanks Matteo Fiorucci, Guglielmo
Grechi, chiara Sbarigia and luca Schilirò for their collaboration
to field activities; prof. Mario Gaeta and prof. Danilo palladino
for the suggestions and revisions provided to volcanological sur-
veys and minero-petrographic investigations.
evolution of the Mt. nuovo relief providing new constrains to the
ongoing deformations. the here proposed model also points out a
strict relation between geological setting and hydrothermal system,
which pervasively permeates the geological succession, and sug-
gests possible role of inner pressures in conditioning the evolution
of the ongoing deformations. the highlighted features confirm the
complexity of the landslide process by the identification of a mul-
tiple compound mechanism with a main rupture surfaces, about
200-250 m deep and involving a volume of about 190 Mm
3
.
Future studies will be devoted: i) to better constrain the local
seismostratography; ii) to sample and characterize the thermo-
mechanical behavior of the MeGt; iii) to quantify the thermo-
baric conditions of the hydrothermal system capable to lead the
ongoing landslide process toward a general collapse. Moreover,
since it is not a-priori negligible, the effect due to transient actions
related to earthquakes, teleseismic and/or bradiseismic events
will be modelled, as they can interact with the landslide-involved
mass (l
enti
et alii, 2015) and modify the stress-strain field of
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Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
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