Abstract
Following the significant ground uplift (\~1.8 m) of the 1982-1984
bradyseismic crisis, the recent history of Campi Flegrei volcanic
complex (Italy) has been dominated by a subsidence phase. Recent
geodetic data demonstrate that the subsidence has terminated, and
that positive ground deformation renewed in November 2004, at a low
but accelerating rate leading to about 4 cm of uplift by the end
of October 2006. As in previous episodes, ground uplift has been
accompanied by swarms of micro-earthquakes (M <= 1.4) in three distinct
episodes: October 2005, October 2006 and December 2006. Hypocenters
of these earthquakes are mainly located beneath the Solfatara Volcano
at depths ranging between 0.5 and 4 km. Inversion of S-wave spectra
indicates source radius and stress drop on the order of 30-60 m and
104-9 x 105 Pa, respectively. Fault plane solutions indicate predominantly
normal mechanisms. Accompanying the October 2006 swarm, we detected
intense long-period (LP) activity for about 1 week. These signals
consist of weak, monochromatic oscillations whose spectra exhibit
a main peak at frequency \~0.8 Hz. This peak is common to all the
stations of the network, and not present in the noise spectra, suggesting
that it is a source effect. About 75\% of the detected LPs cluster
into three groups of mutually similar events. Adjustment of waveforms
using cross-correlation allows for precise alignment and stacking,
which enhances signal onsets and permits accurate absolute arrival
picks, and thus better absolute as well as relative locations. Locations
associated with the three different clusters are very similar, and
appear to delineate the SE rim of the Solfatara Volcano at a depth
of about 500 m. The most likely source process for the LP events
involves the resonance of a fluid-filled, buried cavity. Quality
factors of the resonator cluster in a narrow interval around 4, which
is consistent with the vibration of a buried cavity filled with a
water-vapour mixture at poor gas-volume fractions. We propose a conceptual
model to interpret the temporal and spatial patterns of the observed
seismicity.
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