different nature, not correlated with the global dynamics of
the area (point instability, surface ground settling, etc.).
From the comparison of data recorded during the
measurements of January and September 1997 (only
along the coast line) with those ones of February 1998, it
is possible to point out that in a temporal interval of 13
months, the monthly mean velocity of subsidence is a
constant value of about - 4.3 ± 0.4 mm/month.
With regard to tiltmetry, the scene obtained by data
processing confirms the subsidence phase interesting the
phlegrean area.
It is quite promising the comparison between the results
obtained by INSAR data processing and spirit leveling
data. Really, such a comparison has been carried out
between data set referring to periods lightly different
(height variations between March 1995 and March 1996
spirit leveling measurements with an August 1995/August
1996 interferogram), depending on the possibility to
choose, for the interferometric data, a more favourable
baseline, of only 11 m of length.
A linear extrapolation of the height values of the
benchmarks at the period of August 1995 and August
1996, will allow a more significant comparison between
the results of the two different techniques.
For this particular case, the first step has been an
appropriate visualisation of spirit leveling data, which
should permit to discriminate the areas with higher vertical
ground displacements from those ones with lower or no
displacements. Such a result is shown in fig. 12, where the
height differences between the above mentioned spirit
leveling measurements are reported. As expected, it is
possible to note how the area of maximum vertical
displacement corresponds to the center of Pozzuoli, with
an entity of the ground vertical displacements radially
decreasing towards the borders of the city. Such a trend
shows a good agreement with the results obtained from
the interferogram of fig 5.
1*36* 1*40“ 1*44' 1" 48'
Long., E of Rome, Monte Mario
40 s 54'
40* 50'
-60 -50 -40 -30 -20 -10 0 10
A h (mm) between 3/96 and 3/95 spirit leveling measurements at Phlegrean Fields
Fig. 12 Height variations from spirit leveling measurements to be compared with the interferogram of fig.5
4.2 The island of Ischia
As in the case of the Phlegrean Fields, also for the island
of Ischia the spirit leveling is, among the surveying
techniques used by the Osservatorio Vesuviano, that one
which is used since more time (more than 20 years). Such
measurements have thus allowed to evaluate the
temporal trend of the deformation field acting in the island,
which results extremely complex. In fig. 13 it is possible to
point out the most interesting results (reference period:
May 1990/May 1997), where a subsidence interesting the
NW sector of the island is shown. The maximum value of
ground subsidence, pointed out in seven years for the
benchmark n. 100, is - 9.4 cm (Forio d'lschia). In this
case, the extrapolated subsidence value is more than 1
cm/year.
This is probably due to a gravitational movement of the
detrital layer covering the volcano-tectonic structure of Mt.
Epomeo green tuff. The results of spirit leveling
measurements have thus suggested, as mentioned, the
