Abstract
The Campi Flegrei caldera is a complex structure in which resurgence
occurs through a simple shearing mechanism. The most uplifted block
inside the caldera is the La Starza block. The caldera has shown
signs of unrest since 1969 with two bradyseismic events which generated
a net vertical ground displacement of 3.5 m around the town of Pozzuoli.
The first event occurred between 1969 and 1972, when the ground was
uplifted of 1.74 m, while the second took place between 1982 and
1984 and produced a vertical displacement of 1.79 m. These events
have been followed by subsidence with small uplifts of few centimeters.
Uplifting events, despite the amount of ground displacement, are
always accompanied by seismicity which is absent during subsidence.
We have performed an analysis of the ground deformation and seismic
data collected since 1970. The results obtained on the two independent
sets of data are very much concordant. The ground deformation is
not quasi bell-shaped as previously suggested, instead its geometry
is strongly constrained by structural discontinuities. Seismicity
is confined in a rectangular area, NE-SW elongated and centered on
the La Starza block. The margins of this area are marked by earthquakes
whose hypocenters describe vertical planes which reach depths of
about 5 km. The hypocenters of the earthquakes located in the La
Starza block are shallower than 3 km. Focal mechanisms of the earthquakes
whose hypocenters describe a NNW-SSE fault plane in the Pozzuoli
bay show reverse fault solutions, while those of the earthquakes
located on the opposite side of the La Starza block show normal fault
solutions. All these data well constraints the hypothesis that resurgence
occurs through a simple shearing mechanism (Orsi et al., 1996). We
have also performed a finite elements modeling of the ground deformation
data. This modeling shows that the detected deformation can be accounted
for by an overpressure of 75 MPa in a source with a radius of 1 km,
located at 5 km depth in a medium characterized by the detected structural
discontinuities.
Users
Please
log in to take part in the discussion (add own reviews or comments).