Considering the huge vertical ground displacements
recorded during the recent bradyseismic crises of
1969/72 and 1982/84, such measurements are yearly
repeated along the whole altimetric network and half-
yearly along part of this one.
The last spirit leveling measurements along the whole
network (February 1998) have been performed using
autolevels equipped with plain-parallel lamina
micrometers, one electronic autolevel and couples of
rods with invar ribbon.
The measurements have been submitted to the least
squares adjustment with the method of indirect
observations, with a final value of the standard
deviation per unit of weight go = 0.913 mm.
In fig.2 are shown the height variations (reference
period: penultimate measurements of January
1997/February 1998) of the benchmarks belonging to
the most significant line of the phlegrean network: the
dotted lines represent the limits of the confidence
interval at 95% for the series of data being compared.
Fig.2 Vertical displacements (1/97-2/98) along the coast
line in the Phlegrean Fields
2.2 Tiltmetry
The crustal deformation interesting the phlegrean area
is continuously measured in some particular places by
means of a monitoring system of the ground tilt
variation.
The physical entities thus measured are 8u z /8x and
8u z /8y where x, y and z represent the three coordinate
axes and u z represents the vertical component of the
displacement.
The acquisition of their variation in time is carried out by
means of particular transducers called biaxial electronic
tiltmeters whose specifications allow a range of ± 800
pradiants with a resolution of 0.1 pradiants.
The phlegrean tiltmetric network (fig. 1) consists at
present of four stations three of which (DMA, DMB and
DMC) are located into an underground gallery in Via
Campana (Pozzuoli) and aligned along the NS
direction; they are about 2 km far from the area of the
maximum vertical displacement (benchmark n.25, see
fig.2). The fourth station (BAI) is located into a cave of
Baia castle and is about 4 km far from the other ones.
Data recorded by every sensor, included the thermic
one, are sent by cable at the acquisition unit which
performs the digital conversion with a frequency of 2
cycles/hour (but in any case programmable), the
acquisition of the feeding voltage and their
management while waiting for the teletransmission by
telephonic line at the central unit located at the
surveillance center of the Osservatorio Vesuviano.
Data postprocessing allows to obtain the temporal
evolution of the tilt vector. In fig.3 are shown the
inclinations in pradiants (prad, NS and EW
components) and the temperature in degrees
centigrade recorded at the four stations; in abscissas is
reported the time, in days.
The total tilt recorded in the first 1998 half-year is so
divided:
DMA: 55 prad in SSW direction, associated with a
temperature increase of 4.5°C;
DMB: 30 prad in ESE direction, associated with a
temperature increase of 3.3°C;
DMC: 21.5 prad in NE direction, associated with a
temperature increase of 2.7°C;
BAI: 6 prad in NNE direction, associated with a
temperature increase of 6.5°C.
Fig.3 Phlegrean Fields tilt data recorded in the first
1998 half-year (NS and EW components,
besides the temperature)
The above mentioned tilt data have been successively
reprocessed, also on the base of the knowledge of
every station response to thermic and baric variations
(site response), estimating for the first 1998 half-year a
ground tilt corrected of about 6 pradiants in the SE
quadrant. With regard to BAI station, contrarily to data
recorded in the last years, a low value for ground tilt has
been pointed out, certainly related to tidal weight
variations and to site effects.
2.3 GPS
During the first 1997 half-year has been installed in the
Phlegrean Fields area a GPS network formed by 30 3D
vertices (fig. 1), four of which to be used as permanent
stations.
Six vertices of the network have been installed in the
crateric area of the Solfatara volcano, also in the frame
of a multimethodological project aimed to the monitoring
of many geophysical and geochemical parameters in
the area. The disposition of the vertices of the whole
network, planned for the monitoring of eventual relative
displacements of the principal structures inside the
phlegrean area, defines three main NS lines and two
main EW lines.
Starting from autumn 1997 a first measurement of the
whole phlegrean network has been carried out; for the
