Abstract
A multi-2-D non linear traveltime tomography of the shallow (3-4 km
deep) structure of Mt Vesuvius volcano was performed. Data have been
collected during two recent active seismic experiments using a total
of 17 on-land shots and about 140 three-component digital seismographs.
A newly developed technique for imaging the volcano velocity structure
has been applied, based on an adaptive model space investigation
where the number of grid nodes is progressively increased (multi-scale
approach). The optimal model parametrization is chosen according
to the minimum of the Akaike Information Criteria (AIC) parameter.
This corresponds to finding the best compromise between the data
misfit and simplicity of the model. The model parameter estimate
is performed through the computation of an a posteriori probability
density function (pdf), defined following the Bayesian approach.
The maximum likelihood model is searched by an optimization technique
which combines the genetic and simplex algorithms. The evaluation
of the a posteriori pdf is based on traveltime computations using
ray tracing techniques. Constraints on the model parameters are inserted
in the form of prior pdf and error maps are inferred from cross-sections
of the posterior probability around the found best fit solution.
The retrieved images of Mt Vesuvius volcano show variable P-velocities
in the range 1700-5800 m/s. A fairly detailed image of the top of
the Mesozoic carbonate rocks forming the basement of the volcanic
area is obtained. A 9 km long, 1 km deep depression was detected
at the N side of the volcano. The presence of a shallow high velocity
body is evidenced underneath the Mt Somma caldera and it can be interpreted
as a sub- or palaeovolcanic structure.
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