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