Abstract
Arrival times and waveforms of first and secondary arrivals (reflected
and converted waves) produced by active sources (on land and offshore
explosions) and/or natural sources (microearthquakes), can provide
significant seismic information about the smooth variations of the
velocity and attenuation of seismic waves and about the morphology
of the most important reflectors. The project 'Integrated seismic
methods applied to the investigation of the active volcano structure:
an application to the Campi Flegrei caldera' aimed both at developing
and using advanced techniques in order to model active and passive
seismic data recorded in the Campi Flegrei region, and at the com-
parison of the obtained velocity and attenuation measurements with
labora- tory data. In particular, spatial variation of the elastic
(P and S velocity values) and anelastic (P and S quality factors)
properties, jointly with recent informa- tion about the caldera thermal
state, can provide parameters necessary to a preliminary thermo-mechanical
modeling of the active physical processes. Structural complexity
of the upper crust in volcanic regions is due to mechanisms of magma
feeding and volcanic structure building. Moreover, variations in
rocks rheology and elastic parameters of the propagation medium can
be produced by an anomalous thermal field. Thus, a reliable model
representing the distribution of elastic/anelastic properties in
the propagation medium is needed both to understand and to reproduce
the mechanisms which drive the magma rising up and to monitor accurately
and continuously the seismic activity occurring during eruptive quiescence
periods and crisis episodes. Complexities of seismograms recorded
in volcanic regions are usually produced by strong heterogeneities
of the propagation medium, sometimes jointly to seismic source complexity
(e.g. volcanic tremor, low frequency earthquakes), and by the small
depth and magnitude of the seismic events. Since '80s, the availability
of digital, reliable and dense seismic networks enable to record
high quality data in volcanic regions, allows to interpret the seismograms
complexities. Arrival times, amplitudes, polarisations and waveforms
of the most important seismic phases can be analysed to discriminate
source and propagation effects. From a methodological point of view,
since the early '80s, travel time tomography of P and S first arrivals
from local earthquakes has provided new information on the active
volcanoes structure. In the last '80s-early '90s, the joint analysis
of active (explosive sources located in boreholes) and passive (local
microearthquakes) seismic data has been introduced in order to better
resolve the strongly non-linear inverse problem of the joint determination
of the propagation medium (seismic waves velocity in a 3D-grid nodes)
and hypocentral parameters (earthquake location and origin time).
The project has concerned the development and implementation of innovative
techniques of analysis and interpretation of active/passive seismic
data recorded in the Campi Flegrei. Thanks to bradiseismic crisis
occurred in the early '80s, the scientific background and the general
geological/geophysical knowledge are improved. The main goals of
the project was the definition of a 3D detailed crustal model of
the volcanic region and the identification and detection of the possible
deep reservoir of magma feeding. The project was based on the analysis
and interpretation of different seismic data set. The first one was
a data set of several hundreds of microearthquakes recorded during
the last bradiseismic crisis occurred on 1984. The second was the
DSS (Deep Seismic Soundings) seismic refraction data acquired during
the surveys conducted on 1980 and 1985 in the region. The latest
was a data set acquired in the framework of the MareVes97 offshore
seismic survey, an experiment devoted to the definition of the structure
of the Somma-Vesuvio volcanic complex. Moreover, during the project
development has been planned and carried out a new experiment (called
SERAPIS, SEismic Reflection/Refraction Acquisition Project for Imaging
complex volcanic Structures) based on off-shore seismic energization
and data acquisition on the sea-bottom. The experiment was performed
in September, 2001 during which the vessel NADIR of IFREMER (equipped
with 12, 16-liters airgun) produced more than 5000 airgun shots recorded
at a sea-bottom seismograph array of 62 OBS and 50 station installed
on-land. The data set acquired during the SERAPIS experiment integrated
the above mentioned data set and has been successfully used to infer
3D images of the volcanic structures of Campi Flegrei and Neapolitan
bay. The importance of studying the structure of Campi Flegrei is
related to the high volcanic risk associated even to a minor eruption
because CF is one of the main unrest calderas in the world where
several hundred thousands people live within its borders. In the
past the structure of the caldera has been mainly investigated by
a few km deep drillings, earthquake seismic tomography, gravity and
magnetic surveys and sporadic observations of teleseismic and wide
angle seismic data. The whole geophysical information indicate the
following features: (a) the evidence of sharp temperature gradients
at shallow depths (450 degrees measured at 3 km depth) (b) the presence
of a few km thick, inner basin characterized by low Vp, high Vp/Vs
and high Qp; (c) the shape of this basin is consistent with the gravity
low anomaly and appear to be the site where most of deformation is
concentrated during the recent ground uplift episodes; d) the possible
occurrence of a magmatic reservoir at about 4-5 km depth from teleseismic
observations and extrapolation of thermal data. Relevant open questions
still remained to be answered, mainly concerning the depth and lateral
extension of the shallow magmatic reservoir and the possible existence
of intra-crustal magmatic sill as it has been found in the nearby
Mt. Vesuvius. The main objective of the project was the detailed
reconstruction of the 3D structure of the Campi Flegrei caldera through
the inversion of passive and active data sets and the understanding
of the relationship between the elastic/anelastic properties and
the physical, mechanical and thermal properties of rocks forming
the caldera edifice. In the enclosed CD-Rom you will find appendixes
A and B.
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