Аннотация
We review the main results, with several new analyses, obtained in
recent times about the structure, present dynamics and hazard evaluation
at Somma-Vesuvius volcanic complex. We present a global review and
interpretation of structural features, both at local and regional
scale, constrained both by seismic and petrological data. The local
structure of Somma-Vesuvius is reviewed in three depth ranges, shallow,
intermediate and deep. The shallow velocity structure is inferred
by the joint inversion of shot and local earthquake arrival time
data. The main feature pointed out at shallow depth is a high velocity
anomaly at the crater axis, extending down to about 5 km of depth.
Such an anomaly, first observed at Vesuvius, seems to be common to
many other volcanoes. It can be interpreted in terms of the presence
of solidified residual magma in the shallow conduits, accumulated
in last eruptive cycles. The local seismicity is strongly clustered
around this anomaly, due to the focusing effect of the rigidity contrast.
The seismic occurrence appears as a result of the superposition of
a background level, mainly due to gravitational instability of the
Vesuvius cone, and of intense activity episodes, which possibly reflect
episodic internal activity. Two main zones of magma accumulation
in the upper crust are evidenced by the joint interpretation of seismic
and petrological data. The first one, located in the depth range
4-6 km, is mainly constrained by the crystallisation depth of phonolitic
magmas which fed Plinian and sub-Plinian eruptions; the second one,
around 11-15 km of depth, is mainly constrained by reflected-converted
seismic waves, and in agreement with crystallization depths inferred
for the moderate eruptions. The study of the deep structure, performed
by regional tomography with teleseisms, further points out magma
roots at higher depths (15-30 km). An additional result for the deep
structure, studied at regional scale and very important for geodynamic
interpretations of the Tyrrhenian volcanisms, has been the evidence
for a subducting slab under the Apennines, in an area where previous
models hypothesised a slab window. New original studies of crystal
growth (phenocrystals and microlites) on the eruptive products allow
to infer typical times of magma rising from such reservoirs, which
appear very low, on the order of minutes to tens of minutes. Static
deformation at this volcano, in the last 30 yr, has been detected
by the joint use of levelling, GPS and DIFSAR techniques. It indicates
subsidence, very concentrated in the crater area and in a narrow
strip all around the volcanic edifice, with maximum rates less than
0.01 m/yr. Static deformation in the crater area appears in agreement
with the mechanism of gravitational instability generating local
volcano-tectonic seismicity, while the peculiar pattern around the
volcanic edifice is probably due to the combination of extensional
stress and volcanic loading, generating a ring normal fault-like
structure. While the key results about structure and dynamics help
to define pre-eruptive scenarios, a new probabilistic procedure to
combine volcanological data and computer simulations has been used,
in this paper, to build hazard maps giving the probability, at each
location in the area, to be hit by a pyroclastic flow or to experience
a destructive fall-out deposit. The review and new results of this
work give then the first complete picture of the state of the art
in our knowledge about Somma-Vesuvius volcano.
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