Page 402 - Caldera Volcanism Analysis, Modelling and Response

A New Uplift Episode at Campi Flegrei Caldera (Southern Italy) 377

caldera formed by subsequent collapse episodes (Rosi and Sbrana, 1987; Orsi et al.,
1996). However, only the innermost caldera, with a radius of about 3 km, is clearly
visible by geophysical data including gravity Bouguer anomalies (Rosi and Sbrana,
1987; Capuano and Achauer, 2003) and seismic tomography (Judenherc and Zollo,
2004). This caldera was formed about 15,000 years ago by the ‘‘Neapolitan Yellow
Tuff’’ eruption. A larger caldera, with a radius of about 6 km, can be identified by
geological limits, but does not have a deep geophysical signature. Some literature
ascribes this larger caldera to an ignimbrite eruption that occurred about 37,000
years BP and produced the Campanian gray tuff deposits (Rosi and Sbrana, 1987;
Orsi et al., 1996); however, there is no global consensus on such a model (see, for
instance, Rolandi et al., 2003). The volcanic edifices in Campi Flegrei caldera are
mainly monogenetic tuff cones and tuff rings, produced by hydromagmatic
eruptions with alkali-trachitic magma (Rosi et al., 1983).
The history of crustal movements at Campi Flegrei has been reconstructed for
the last 2,000 years by the integration of different data: geological, archeological
and, for the last 40 years, precise geodetic measurements (Parascandola, 1947;
Dvorak and Mastrolorenzo, 1991; Morhange et al., 1999; Del Gaudio et al., 2005;
Gottsmann et al., 2006; Morhange et al., 2006; Pingue et al., 2006) (see Figure 2).
The secular trend of ground deformation is characterized by subsidence at a rate
of about 1.5 cm/year. Superimposed on such a long-term trend are at least two
episodes of large uplift which occurred around 600 and 1500 AD; each lasted 50–60
years, and the last eruption culminated in the only historical eruption of 1538
(Morhange et al., 1999, 2006; Bellucci et al., 2006)(Figure 2a).
Two unrest episodes (called bradisisma, from the Greek word for ‘‘slow
earthquake’’) occurred from 1969 to 1972 and from mid-1982 to December 1984
(Barberi et al., 1984)(Figure 2b). The first event produced a net uplift of about
1.7 m. The pattern of ground deformation was a nearly circular lens centered near
Pozzuoli. Only weak seismic activity was recorded during this phase. The second
event presented a similar uplift pattern, but compared to the previous event this
episode was accompanied by more seismicity, both in the number of earthquakes
and their magnitude (Dvorak and Gasparini, 1991; De Natale et al., 1995; Troise
et al., 1997). Shallow earthquake hypocenters (depth from 1 to 4 km) occurred from
mid-1983 to December 1984 and damaged buildings in Pozzuoli. A magnitude 4.2
earthquake with epicenter in Pozzuoli occurred on October 4, 1983. In 1984, the
town was almost completely evacuated and about 40,000 people were relocated.
The total uplift recorded between January 1982 and December 1984 amounted to
1.8 m. The most deformed area had a radius of about 3 km around Pozzuoli, at the
center of the caldera. Since the end of 1984, the ground has started to subside with
a peak subsidence rate of about 7–8 cm/year in 1985 and at a progressively smaller
rate until 2001–2002, when the subsidence ended and about 0.9 m of the total uplift
was recovered (De Natale et al., 2006). Superimposed on the subsidence phase were
some small and fast uplift episodes which occurred in 1989, 1994, and 2000, i.e.
with an apparent period of about 5–6 years (Gaeta et al., 2003; Lanari et al., 2004).
These smaller uplift episodes reached a maximum uplift of about 1–8 cm, lasted
4–10 months, and were accompanied by seismic swarms of micro-earthquakes
(MLo2) lasting from few days to 1–2 months.