Page 429 - Caldera Volcanism Analysis, Modelling and Response

404 Micol Todesco

of hot fluids. This mechanism is particularly suited to explain ground deformation
during non-eruptive unrest, when ground uplift is followed by a subsidence phase
that cannot be ascribed to magma withdrawal. If uplift is generated by an increase
in pore pressure, subsidence may occur as fluids propagate and eventually discharge
at the surface and gradually dissipate the initial overpressure. In the case of
Phlegrean Fields caldera, a long scientific tradition has suggested the involvement
of hydrothermal fluids in the two recent episodes of non-eruptive unrest, each of
which was accompanied by remarkable ground uplift (bradyseism) (Olivieri del
Castillo and Quagliariello, 1969; Casertano et al., 1976; Bonafede, 1991; Gaeta
et al., 1998; Orsi et al., 1999; De Natale et al., 1991, 2001; Castagnolo et al., 2001).
Recent findings substantiate the concept and emphasise the role of hydrothermal
system during recent unrest events (Chiodini et al., 2003; Todesco et al., 2004;
Todesco and Berrino, 2005; Battaglia et al., 2006).

5. An Example of Assessing the Role of Hydrothermal
Processes During Unrest: Solfatara (Phlegrean
Fields Caldera, Italy)

The Phlegrean Fields caldera (Figure 1) represents an optimal site for
modelling hydrothermal fluid circulation. Volcanic surveillance was started here in
the early 1980s and long data series are now available to describe the caldera’s recent
evolution. The last major unrest took place between 1982 and 1984 and involved
seismic activity, ground deformation (with ground uplift up to 1.8 m), positive

Figure 1 The Phlegrean Fields caldera, and the Solfatara crater, where fumaroles and di¡use
degassing are concentrated. Locations of two gravity stations (Solfatara and Serapeo) are also
shown (modi¢ed afterTodesco and Berrino, 2005).