Numerical simulations of magma chamber dynamics at Campi Flegrei, and associated seismicity, deformation and gravity changes
Creators: name: Vassalli, Melissa <1977>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2008
Subjects: GEO/11 Geofisica applicata

description: Understanding the complex relationships between quantities measured
by volcanic monitoring network and shallow magma processes is a crucial
headway for the comprehension of volcanic processes and a more realistic
evaluation of the associated hazard. This question is very relevant at Campi
Flegrei, a volcanic quiescent caldera immediately north-west of Napoli (Italy).
The system activity shows a high fumarole release and periodic ground slow
movement (bradyseism) with high seismicity. This activity, with the high
people density and the presence of military and industrial buildings, makes
Campi Flegrei one of the areas with higher volcanic hazard in the world.
In such a context my thesis has been focused on magma dynamics due
to the refilling of shallow magma chambers, and on the geophysical signals
detectable by seismic, deformative and gravimetric monitoring networks that
are associated with this phenomenologies. Indeed, the refilling of magma
chambers is a process frequently occurring just before a volcanic eruption;
therefore, the faculty of identifying this dynamics by means of recorded signal
analysis is important to evaluate the short term volcanic hazard.
The space-time evolution of dynamics due to injection of new magma
in the magma chamber has been studied performing numerical simulations
with, and implementing additional features in, the code GALES (Longo et al.,
2006), recently developed and still on the upgrade at the Istituto Nazionale di
Geofisica e Vulcanologia in Pisa (Italy). GALES is a finite element code based
on a physico-mathematical two dimensional, transient model able to treat
fluids as multiphase homogeneous mixtures, compressible to incompressible.
The fundamental equations of mass, momentum and energy balance
are discretised both in time and space using the Galerkin Least-Squares and
discontinuity-capturing stabilisation technique. The physical properties of
the mixture are computed as a function of local conditions of magma composition,
pressure and temperature.The model features enable to study a broad
range of phenomenologies characterizing pre and sin-eruptive magma dynamics
in a wide domain from the volcanic crater to deep magma feeding
zones.
The study of displacement field associated with the simulated fluid dynamics
has been carried out with a numerical code developed by the Geophysical
group at the University College Dublin (O’Brien and Bean, 2004b),
with whom we started a very profitable collaboration. In this code, the seismic
wave propagation in heterogeneous media with free surface (e.g. the
Earth’s surface) is simulated using a discrete elastic lattice where particle interactions
are controlled by the Hooke’s law. This method allows to consider
medium heterogeneities and complex topography.
The initial and boundary conditions for the simulations have been defined
within a coordinate project (INGV-DPC 2004-06 V3_2 “Research on active
volcanoes, precursors, scenarios, hazard and risk - Campi Flegrei”), to which
this thesis contributes, and many researchers experienced on Campi Flegrei
in volcanological, seismic, petrological, geochemical fields, etc. collaborate.
Numerical simulations of magma and rock dynamis have been coupled as
described in the thesis.
The first part of the thesis consists of a parametric study aimed at understanding
the eect of the presence in magma of carbon dioxide in magma in
the convection dynamics. Indeed, the presence of this volatile was relevant
in many Campi Flegrei eruptions, including some eruptions commonly considered
as reference for a future activity of this volcano. A set of simulations
considering an elliptical magma chamber, compositionally uniform, refilled
from below by a magma with volatile content equal or dierent from that
of the resident magma has been performed. To do this, a multicomponent
non-ideal magma saturation model (Papale et al., 2006) that considers the
simultaneous presence of CO2 and H2O, has been implemented in GALES.
Results show that the presence of CO2 in the incoming magma increases its
buoyancy force promoting convection ad mixing. The simulated dynamics
produce pressure transients with frequency and amplitude in the sensitivity
range of modern geophysical monitoring networks such as the one installed
at Campi Flegrei .
In the second part, simulations more related with the Campi Flegrei volcanic
system have been performed. The simulated system has been defined
on the basis of conditions consistent with the bulk of knowledge of Campi
Flegrei and in particular of the Agnano-Monte Spina eruption (4100 B.P.),
commonly considered as reference for a future high intensity eruption in this
area. The magmatic system has been modelled as a long dyke refilling a small shallow magma chamber; magmas with trachytic and phonolitic composition
and variable volatile content of H2O and CO2 have been considered. The
simulations have been carried out changing the condition of magma injection,
the system configuration (magma chamber geometry, dyke size) and the resident
and refilling magma composition and volatile content, in order to study
the influence of these factors on the simulated dynamics. Simulation results
allow to follow each step of the gas-rich magma ascent in the denser magma,
highlighting the details of magma convection and mixing. In particular, the
presence of more CO2 in the deep magma results in more ecient and faster
dynamics. Through this simulations the variation of the gravimetric field has
been determined.
Afterward, the space-time distribution of stress resulting from numerical
simulations have been used as boundary conditions for the simulations of
the displacement field imposed by the magmatic dynamics on rocks. The
properties of the simulated domain (rock density, P and S wave velocities)
have been based on data from literature on active and passive tomographic
experiments, obtained through a collaboration with A. Zollo at the Dept. of
Physics of the Federici II Univeristy in Napoli. The elasto-dynamics simulations
allow to determine the variations of the space-time distribution of
deformation and the seismic signal associated with the studied magmatic dynamics.
In particular, results show that these dynamics induce deformations
similar to those measured at Campi Flegrei and seismic signals with energies
concentrated on the typical frequency bands observed in volcanic areas.
The present work shows that an approach based on the solution of equations
describing the physics of processes within a magmatic fluid and the
surrounding rock system is able to recognise and describe the relationships
between geophysical signals detectable on the surface and deep magma dynamics.
Therefore, the results suggest that the combined study of geophysical
data and informations from numerical simulations can allow in a near future
a more ecient evaluation of the short term volcanic hazard.