Page 414 - Caldera Volcanism Analysis, Modelling and Response

A New Uplift Episode at Campi Flegrei Caldera (Southern Italy) 389

contains magma or fluids of magmatic origin and is located at 3 km depth (see also
De Natale et al., 2006; Troise et al., 2007). Such overpressures subsequently
involved the shallower aquifers (for instance, after fracturing of the rock volume
between the magmatic fluids and the aquifer). The subsidence is therefore
interpreted as water deflation toward the external rocks from a prolate ellipsoidal
aquifer, located at depths between 1.5 and 2.5 km. The presence of continuous GPS
measurements in the area since 2000 makes, for the first time, a detailed comparison
of horizontal to vertical displacement ratios between two uplift periods possible.
The observed DH max /DV max ratio for the 2000 and the 2004–2006 uplifts is slightly
less than 0.35 and seems indicative of an oblate spheroidal source. However, since
the maximum elevation point is likely to be located in the Gulf of Pozzuoli
(see Beauducel et al., 2004), where no measurements exist, this ratio is best
regarded as an upper limit. These observations allow the formulation of a model for
the occurrence of small uplift episodes, and more generally for the fast ground
deformation episodes at Campi Flegrei. In fact, in the light of the previously
mentioned models, the rather small ratio between horizontal and vertical
displacements observed during the small uplift episodes is indicative of over-
pressures in the deeper source of fluids of magmatic origin, evidenced by Battaglia
et al. (2006), with only limited or no involvement of the shallower aquifers.
A large uplift episode could then occur when the initial pulse of overpressure,
causing significant fracturing of the upper rocks, migrates into the shallower
aquifers. Such interpretation is consistent with the observations of Chiodini et al.
(2003) that similar peaks of CO 2 follow both large and small uplift episodes, with a
time lag of several months, thus indicating that uplift episodes are associated with a
significant input of CO 2 from below, almost independently from the size of uplift.
This is in agreement with a model in which all the uplift episodes start with
an overpressure in a deep CO 2 rich source that is filled with magma or fluids of
magmatic origin and is injected into shallower layers by the overpressure. In the last
few months, the present uplift episode has been also accompanied by a consistent
increase in CO 2 flux and other magmatic indicators (Chiodini, personal
communication), and by a peak in microseismicity in October (maximum
magnitude around 1) which consisted of swarms of low-frequency (around 1 Hz)
events, with peak rates of several events per hour (Del Pezzo, personal
communication). The increase in CO 2 flux is consistent with an increased supply
of magmatic fluids from below, whereas the occurrence of low-frequency seismicity
seems to be new at Campi Flegrei, where this kind of earthquakes has never been
noted before, at least not with similar high rates of occurrence. This low-frequency
seismicity can reflect magmatic gas or water overpressure in fractures. If the
evolution of this phenomenon follows the previous mini-uplift episodes, we should
expect the phenomenon to stop within 1–2 months. On the contrary, if the
DH max /DV max ratio increases (indicating an involvement of the shallower levels),
we should expect a new large unrest, similar to what occurred in 1969–1972 and
1982–1984.
The model of activity has important implications on how hazardous an eruption
could be. In fact, if the deeper source of overpressure evidenced in this paper
directly represents the magma chamber, or the increase of magmatic fluid pressure is