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hydrothermaL system infLuencing sLope-scaLe deformations at mt.
nuovo (ischia, southern itaLy): preLiminary resuLts from
2d-muLtiphysics numericaL modeLLing G.M. Marmoni1, A. Calabriso2,
S. Martino1, D. Borello2, M. Della Seta1, C. Esposito1, M.
Fiorucci1, P. Venturini21 Dip. Scienze della Terra, “Sapienza”
Università, Roma, Italy2 Dip. Ingegneria Meccanica ed Aerospaziale,
“Sapienza” Università, Roma, Italy
Introduction. Gravitational slope instabilities can be strictly
related to volcanic and hydrothermal systems as widely documented
in several case studies (Reid et al., 2001, 2004; Lopez et al.,
1993). Such relations are due to interactions between slope system
and inner forces (Bozzano et al., 2013), produced by a renewal of
the volcanic activity, magma or dikes emplacement, volcanic-related
seismicity, hydrothermal pressurization (Reid et al., 2004). This
close relation is strengthened by alterations effect produced by
aggressive hydrothermal fluids which can compromise the mechanical
properties of rocks and soils (John et al., 2008; Frolova et al.,
2014; del Potro et al., 2009) often jeopardizing mechanical
properties (Heap et al., 2012) and hence flank stability.
In order to evaluate the role of hydrothermal systems in
conditioning slope-scale deformations, a 2D multiphysics numerical
model was implemented for the Mt. Nuovo case study (Ischia Island,
Southern Italy), by coupling hydrothermal and mechanical
solutions.
Geological framework of Ischia Island. Ischia Island is one of
the most adapt volcanic systems for studying relations between
geothermal systems and development of slope-scale instabilities,
because of the presence of a well developed and stable geothermal
system and the occurrence during the Holocene of massive rock slope
failures (Della Seta et al., 2011).
These occurrences is closely related to volcano-tectonic
dynamics, mainly driven by magma emplacement and volcanic activity
renewal. In a cyclical recurrence pattern (de Vita et al., 2013),
slope instabilities would represent the surficial response to the
general gravitational disequilibrium induced by deep deformation
and caused by magmatic intrusion or pressurization.
The main volcano-tectonic event recorded in the geological
succession of Ischia Island is represented by an asymmetric
resurgence able to produce an uplift up to 900 m over a period of
30 ka, as testified by the present elevation above sea level of
marine sediments (Tibaldi and Vezzoli, 1998).
According to Rittmann (1930) such a resurgence could have been
generated by the emplacement of a shallow laccolith, or by an
increase of volume and pressure in the magma chamber (Tibaldi and
Vezzoli, 1998), which furthermore produced a vapor dominated
hydrothermal system. The Ischia hydrothermal system is
characterized by high heat flow (200-400 mW/m2) and geothermal
gradients ranging between 180 and 220 °C/km (Cataldi et al., 1991;
AGIP, 1987). The principal surface evidences of the presence of a
vigorous hydrothermal circulation are several thermal springs and
fumaroles with temperature up to 100°C, among which most vigorous
localized in the western sector of the island, in the Donna Rachele
fumarolic field. The geothermal system of Ischia is also
characterized by important pressure fluctuations, as testified by
several wells eruptions, occurred after the wells closure.
The underground fluid circulation mainly occurs within tuffs and
lavas through a dense cracks network in a multilayered aquifer,
controlled by the presence of fractured porous media separated by
low-permeability layers and impermeable horizons (Carlino et al.,
2014). According to Di Napoli et al. (2011), the hydrothermal
system in the Mt. Nuovo sector is fed by rainwater with significant
seawater inputs, as clearly shown by chemical and isotopic markers.
The tectonic discontinuities represented by the network of
sub-vertical NW-SE oriented faults which drove the resurgence,
strongly controlled the hydrothermal circulation, conditioning
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the lateral extension of aquifers and geothermal reservoir,
locally representing preferential pathways for upwards migrations
of fluids.
As documented in the literature (McKenna et al., 2004; Bachler
et al., 2003), structural alignment may control the geometry of
convective cells, fluid velocity, and thermal anomalies
distributions in geothermal system.
The volcano-tectonic evolution of Ischia Island also affected
the gravitational response of the edge of the resurgent block.
Slope instability-related deposits including rock falls, slides,
toppling as well as lahars and large scale debris flows and debris
avalanche at Ischia has been described by many authors inland and
above continental shelf all over the island (Chiocci et al., 2006;
De Alteriis et al., 2009). Some of the volcanoclastic deposits are
intercalated to primary volcanic, outlining a close relationship
among slope instability and volcano-tectonic activity (de Vita et
al., 2006).
The most important events, occurred since 3 ka, are
characterized by volumes of hundreds Mm3, representing the major
catastrophic mass movements documented in the island since the
Holocene.
Ongoing deformations still involve the edge of the resurgent
block in Mt. Nuovo area over a volume of about 160-190 Mm3 (Della
Seta et al., 2011, 2015). This gravitational slope deformation was
reported as triggered by a catastrophic volcano-tectonic event that
took place around 460÷470 BC in the external portion of the edge of
resurgent block (Della Seta et al., 2011 and references therein),
where historical occurrence of large debris avalanche has been
documented.
The spatial distribution as well as the morphological and
geo-structural similarities between Mt. Nuovo slope deformations
and already detached debris avalanche (Della Seta et al., 2015)
strengthen the analogy between the evolution of the occurred
landslide and the gravitational ongoing-deformation, leading to
assume such landslides as the local evolution to collapse of a wide
deforming sector.
Based on a high-resolution engineering-geological model a
preliminary conceptual model of the slope-scale deformations
involving the Mt. Nuovo sector was already proposed (Della Seta et
al., 2015). This conceptual model highlights possible relations
between hydrothermal system and ongoing slope deformation. In fact,
increased pore pressures can induce transient stress field changes,
so increasing the instabilities of the slope instabilities and
driving fastly toward a generalized failure.
Multiphysics 2D numerical modelling of coupled
hydrothermal-slope system. In order to investigate possible
thermo-mechanical interactions between the Ischia hydrothermal
system and the deep gravity-driven slope deformation a numerical
modelling was designed. Starting from the conceptual model of the
geothermal reservoir proposed by Carlino et al. (2014), a 2D
thermo-fluid dynamic simulation was performed by the use of FEM
COMSOL® code. A multilayer aquifer was defined, fixing a reservoir
located at depth ranging between -150 and -800 m a.s.l.. A
numerical analysis was performed, assuming a stationary, conductive
and convective geothermal heat flux coupled with fluid flow in
porous medium and fixing thermal boundary conditions. The 2D
thermo-fluid model was validated by matching experimental thermal
data obtained from deep wells (AGIP, 1987), i.e. verifying the
coherence of the numerical thermal output respect with the path of
measured temperature. The model of the deep hydrothermal system was
combined with the engineering-geological model of the Mt. Nuovo
slope, applying the output of the deep model as thermal
conditioning of the slope-system thermal model. The free-convection
in porous medium was solved by introducing a Bousinnesq buoyancy
term to the Brinkman’s momentum equation, linking resulting fluid
velocity to the heat transfer equation, thus accounting for the
lifting force due to thermal expansion (Hossain and Wilson 2002).
In order to assess the role of structural elements in the
development and characteristics of thermal convection, fault zones
were implemented, by introducing linear permeable elements. The
modelling results highlight a well developed steady-state
convection in the shallow reservoir of Ischia western sector,
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showing the clear role of tectonic elements in the ascent of
fluid and confirming the hypothesis of strict relation to both the
stratigraphic and structural setting (Della Seta et al., 2015). The
main structural control of Mt. Nuovo gravitational slope
deformations can be attributed to mayor fault lines. Preferential
paths for fluid rising may have played a dual role in controlling
the pore-pressure distribution and inducing the localization of
hydrothermal alteration in the rock mass. The thermo-fluid dynamic
model fits the temperature log-profile measured in deep wells (Fig.
1) and outputs the thermal- baric field within the Mt. Nuovo
slope.
Perspectives. The preliminary results obtained so far encourage
further numerical modelling, in order to evaluate the mechanical
response of the slope system with respect to transient hydrothermal
conditions and to estimate long-term effect of thermal anomalies on
rock mass creep process (Chigira et al., 1992).
The here proposed numerical modelling will be enriched by
specific laboratory physical and mechanical data derived by
thermo-mechanical investigation on Mt. Epomeo Green Tuff.
The obtained results constitute a starting point for studies
focused on the dynamics and mechanics of the gravity-induced slope
evolution affecting the Mt. Nuovo area, providing the thermo-baric
range which controls the evolution of the gravitational
deformations.
Thermo-mechanical modelling of the Mt. Nuovo slope will be
implemented to quantify the landslide hazard and infer possible
paroxysmal scenarios toward general collapse.Aknowledgements. This
research was funded by “Sapienza” University of Rome in the frame
of the project “Influence of geothermal systems and related thermal
regime variations on the onset and development of large slope
instabilities in the island of Ischia” (2015 - prot. C26A15FH3L -
P.I. Dr. Carlo Esposito).
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