Abstract
We analyse the seismic catalogue of the local earthquakes which occurred
at Somma-Vesuvius volcano in the past three decades (1972-2000).
The seismicity in this period can be described as composed of a background
level, characterised by a low and rather uniform rate of energy release
and by sporadic periods of increased seismic activity. Such relatively
intense seismicity periods are characterised by energy rates and
magnitudes progressively increasing in the critical periods. The
analysis of the b value in the whole period evidences a well-defined
pattern, with values of b progressively decreasing, from about 1.8
at the beginning of the considered period, to about 1.0 at present.
This steady variation indicates an increasing dynamics in the volcanic
system. Within this general trend it is possible to identify a substructure
in the time sequence of the seismic events, formed by the alternating
episodes of quiescence and activity. The analysis of the source moment
tensor of the largest earthquakes shows that the processes at the
seismic source are generally not consistent with simple double-couples,
but that they are compatible with isotropic components, mostly indicating
volumetric expansion. These components are shown to be statistically
significant for most of the analysed events. Such focal mechanisms
can be interpreted as the effect of explosion phenomena, possibly
related to volatile exsolution from the crystallising magma. The
availability of a reduced amount of high quality data necessary for
the inversion of the source moment tensor, the still limited period
of systematic observation of Vesuvius micro-earthquakes and, above
all, the absence of eruptive events during such interval of time,
cannot obviously permit the outlining of any formal premonitory signal.
Nevertheless, the analysis reported in this paper indicates a progressively
evolving dynamics, characterised by a generally increasing trend
in the seismic activity in the volcanic system and by a significant
volumetric component of recent major events, thus posing serious
concern for a future evolution towards eruptive activity.
Users
Please
log in to take part in the discussion (add own reviews or comments).