Abstract
We have analyzed 42 microearthquakes (1<ML<3) that occurred at Campi
Flegrei volcanic area during an intense swarm-like activity (more
than 500 events in half a day) in the last period of strong ground
uplift (1982-1984). Focal mechanisms of these earthquakes have been
computed by a probability method, which uses P wave polarity and
S wave polarization data. Composite focal mechanisms for different
groups of earthquakes have also been computed by the same technique.
Results show a well-defined fault plane, in agreement with the elongation
of the epicentral pattern. The orientation and dip of this fault
plane, together with other observations about hypocenter locations
and focal mechanisms of the whole seismicity accompanying the ground
uplift episode, suggest the presence of an elliptical fracture system
at the caldera center. This fracture system, dipping toward the center
of the caldera, should represent the limit of the most recent caldera
collapse, which is also indicated by a negative Bouguer gravity anomaly.
Moreover, an important differential feature among various groups
of earthquakes has been shown, namely, a rotation up to about 30
deg of the strike of the compressive stress axis, between the easternmost
and the westernmost events. The resulting pattern indicates that
the axis is always oriented towards the town of Pozzuoli, which roughly
represents the very center of the caldera, and also the center of
symmetry of the ground deformation. These observations strongly suggest
that earthquakes are generated by the variation of the stress field
associated with the deformation, along prefractured zones, because
of their greater weakeness. This study provides, then, the first
direct evidence that both seismicity and ground deformations have
a common origin.
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