920 research outputs found
Common geophysical characteristics of Campi Flegrei, Rabaul and Usu: Three volcanic events
Volcanogenic deformations during periods of
unrest are related to volcanic seismicity in various
ways. Magmas or geothermal fluids intrude
beneath volcanoes and cause deformations at the
surface gradually or rapidly. Mechanical energies
of the intrusions are converted to deformation
energy, earthquakes, and also explosions under
certain circumstances. Partition among the
three kinds of energies provides information of
the internal processes and yields a clue to their
origin. From the above standpoint, deformations
accompanying seismicity at Campi Flegrei,
Rabaul and Usu are discussed with the aid of
published data. To quantitatively correlate the
deformations and the seismicity, we discuss the
time-derivatives of uplift and release of seismic
energy, which are energetically interrelated. The
correlation between them is moderate at Campi
Flegrei, somewhat higher at Rabaul and high
at Usu, but the data sets are not always equal
in quality. The deformation volumes are also
different among the three volcanoes. In order
to standardize the volumes, seismic energies
released by unit volume of each deformation are
compared. The specific seismic energy is found
to increase from Campi Flegrei through Rabaul
to Usu. Such different behavior in seismodeformations
among the three volcanoes is
interpreted as differences in the mechanism of
volcanic activity, and in physical properties of the
mediums involved
Thirty years of precise gravity measurements at Mt. Vesuvius: an approach to detect underground mass movements
Since 1982, high precision gravity measurements have been routinely carried
out on Mt. Vesuvius. The gravity network consists of selected sites
most of them coinciding with, or very close to, leveling benchmarks to remove
the effect of the elevation changes from gravity variations. The reference
station is located in Napoli, outside the volcanic area. Since 1986,
absolute gravity measurements have been periodically made on a station
on Mt. Vesuvius, close to a permanent gravity station established in 1987,
and at the reference in Napoli. The results of the gravity measurements
since 1982 are presented and discussed. Moderate gravity changes on
short-time were generally observed. On long-term significant gravity
changes occurred and the overall fields displayed well defined patterns.
Several periods of evolution may be recognized. Gravity changes revealed
by the relative surveys have been confirmed by repeated absolute measurements,
which also confirmed the long-term stability of the reference
site. The gravity changes over the recognized periods appear correlated
with the seismic crises and with changes of the tidal parameters obtained
by continuous measurements. The absence of significant ground deformation
implies masses redistribution, essentially density changes without
significant volume changes, such as fluids migration at the depth of
the seismic foci, i.e. at a few kilometers. The fluid migration may occur
through pre-existing geological structures, as also suggested by hydrological
studies, and/or through new fractures generated by seismic activity.
This interpretation is supported by the analyses of the spatial gravity
changes overlapping the most significant and recent seismic crises
MODELING OF GAS COMPOSITION AND GRAVITY SIGNALS AT THE PHLEGREAN FIELDS CALDERA
Hydrothermal systems are known to play an
important role in the evolution of active calderas:
these volcanic systems periodically undergo dramatic
unrest crises, commonly involving ground
deformation, seismic activity and important changes
in several geophysical and geochemical parameters
monitored at the surface. These unrest crises may, or
may not, culminate with a renewal of the eruptive
activity, but in any case they bear important
consequences in densely populated regions. Early
warning and a prompt evaluation of the state of
evolution of the volcanic system are therefore
essential to ensure proper mitigation measures. A
proper interpretation of monitoring data, however, is
only achieved within the framework of a robust
conceptual model of the system. Recent research
work carried out at the Phlegrean Fields shows that
the recent evolution of the caldera is consistent with
the presence of a pulsating magmatic source,
periodically discharging CO2-enriched fluids into a
shallow hydrothermal system. Such pulsating
degassing affects the amount of heat and fluids
entering the hydrothermal system, the distribution of
fluid phases throughout the system, and their
composition. As a consequence, degassing controls
not only the composition of fluids discharged at the
surface, but also ground displacement and gravity
residuals. In this work, the TOUGH2 code has been
applied to study how different degassing scenarios
could affect the composition of discharged fluids and
the gravity signals recorded at the surface
Toward Absolute Gravity Networks to Monitor the Neapolitan Volcanoes
As well known, measurements of the time-space changes of the gravity field are a powerful approach to detect the masses change/redistribution in the underground, such as those at volcanoes.
The most common approach to detect gravity precursory signals is the use of repeated relative measurements at benchmarks on networks. This is very good, but has some and significant limits: 1) the networks must be linked to a reference station stable over long time and if possible external to the active area. This is quite simply for land volcanoes, but is generally difficult for volcanoes on islands, particularly when far from the terra firma; 2) to reach high precision, measurements require special operative procedure implying long time surveys; 3) relative gravity changes can be affected over the long-time by changes of instrumental sensitivity and loss of vacuum in the air-tight sealing system, therefore the instruments must be subject to continuous check.
Measurements on network have the advantage to permit to define the position and the geometry of the masses change/redistribution, even if they suffer from the lack of information about the rate and/or quick changes, since variations are assumed linearly changing over the time between two consecutive surveys, usually spaced out some months or years. This is a fundamental information in the prediction of the volcanic activity changes and/or of volcanic eruptions and leads to conclude that measurements on networks cannot be excluded from a monitoring program, but moving toward absolute methodologies is advisable.
Nowadays, and since some years, this is possible due to the commercially availability of a portable field absolute gravimeter (Micro-g_LaCoste A10), which we acquired at the end of 2014.
The main advantages of the absolute measurements on networks are: i) they are independent from any reference and the field operation are faster and easier, permitting much frequent measurements and reducing the lack of information between two consecutive surveys; ii) they are directly linked to standards of time and length therefore fairly independent from instrumental references and drift, avoiding loss of long-term information; iii) the measured value can be used without loop reductions, post processing and benchmark links. This can be translated into large advantages, such as saving of human resources, survey’s time and costs.
Before to start field surveys, we carried out several measurements to test the performances of the instrument and mainly to verify the repeatability of the measured value. The test was performed at the Old Building of the Osservatorio Vesuviano, on Mount Vesuvio, that is a very low noised and good logistic site; it is one of the absolute stations installed in 80’s in the Neapolitan area which value has been measured several time till 2010.
In June 2015, starting from Campi Flegrei, we set the first absolute gravity networks on the Neapolitan volcanoes, formed by stations coinciding or close to the benchmarks of the already existing relative networks.
Here we present and discuss the data collected during the test and the field surveys; we also will describe the new absolute networks
MISURE GRAVIMETRICHE ALL’ISOLA DI PANTELLERIA
Nel mese di giugno 2009 è stata condotta una campagna gravimetrica di precisione all’isola di Pantelleria, dove le misure vengono effettuate con una periodicità biennale data la debole attività dell’isola. Le misure sono state rilevate sull’intera rete che è costituita da 24 stazioni (figura 1), molte delle quali ubicate su vertici della locale linea di livellazione e tutte collegate a due stazioni assolute istituite nel 1993 rispettivamente a Pantelleria e a Khamm
Misure gravimetriche a L’Aquila. Confronto con i valori rilevati nel 1995
Nel febbraio 2010 si è dato inizio alla
realizzazione di una rete gravimetrica a
L’Aquila con l’istituzione di una
stazione assoluta. La stazione è stata
realizzata nel Palazzo Comunale della
città di L’Aquila (Palazzo Margherita
– foto a), nel centro della città , in un
locale al piano terra (foto b). Sono
state effettuata anche misure
gravimetriche relative al fine di
misurare il locale gradiente verticale
della gravitĂ (foto c) e riportare il
valore assoluto ad una stazione
satellite esterna all’edificio (foto d).
Nel corso della campagna è stato anche
effettuato il collegamento gravimetrico
tra la nuova stazione e quella istituita
nel 1995 nei laboratori sotterranei dei
Laboratori Nazionali del Gran Sasso
(LNGS) (foto e) dove è stato anche
misurato il gradiente verticale. Tale
punto fu allora istituito quale nodo
fondamentale della costituenda “Rete
Gravimetrica Nazionale di Ordine
Zero”
AGGIORNAMENTO SULL’ATTIVITA’ DI SORVEGLIANZA SVOLTA NEL 2009 AI CAMPI FLEGREI MISURE GRAVIMETRICHE
Dal 16 febbraio al 18 marzo 2009 si è svolta una campagna gravimetrica ai Campi Flegrei. Le misure sono state rilevate sull’intera rete che è attualmente costituita da 28 vertici (figura 1), tutti posizionati in corrispondenza, o in prossimità , di capisaldi di livellazione e collegati alla stazione assoluta di Napoli assunta quale riferimento
RENDICONTO SULL’ATTIVITA’ DI SORVEGLIANZA SVOLTA NEL 2008 (1 Gennaio – 31 Dicembre 2008)MISURE GRAVIMETRICHE AI CAMPI FLEGREI
Nel 2008 sono state effettuate due campagne gravimetriche ai Campi Flegrei; la prima nel mese di febbraio, la seconda dal 20 ottobre al 11 novembre. In entrambre le campagne le misure sono state rilevate sull’intera rete che, nel corso della secondo rilevamento, è stata amplianta con l’istituzione di due nuovi vertici posizionati lungo la linea costiera, e precisamente tra le stazioni di Bagnoli e La Pietra
RENDICONTO SULL’ATTIVITA’ DI SORVEGLIANZA SVOLTA NEL 2008 (1 Gennaio – 31 Dicembre 2008) MISURE GRAVIMETRICHE AL VESUVIO
Nel corso del 2008 sono state eseguite due campagne gravimetriche nell’area vesuviana; la prima nel mese di aprile, la seconda a cavallo dei mesi di novembre e dicembre. In entrambe le campagne, i dati sono stati rilevati su 31 dei 32 vertici costituenti la rete in quanto la stazione “Baracche Forestali”, ubicata lungo la linea che corre all’interno della Forestale, è andata distrutta nel 2007 a causa di lavori di ripristino nell’area
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