Geophysical data acquisition - OGS

(SPEM) and the Wavenumber Integration Method (WIM), which have very
different features.
The 2-D Chebyshev spectral element method (SPEM) is a high-order finite element
technique, which solves the variational formulation of the equation. It has been
entirely developed at OGS by E. Priolo and G. Seriani. The use of irregular meshes,
as well as the high computational accuracy, which derives from the use of highorder
Chebyshev polynomials, make the SPEM particularly suitable to solve
numerically the seismic wave propagation through complex geological structures.
The WIM solves the 3-D elastic full-wave equation in a plane layer medium. It has
been developed by R. Herrmann at St. Louis University. Synthetic seismograms
contain all wavefield phases, in both the near- and the far-fields. Furthermore,
simulations can be set up very simply, since the method needs only few parameters to
be defined. These properties make WIM suitable for performing predictions at a regional
scale, as well as for kinematic modelling of the rupture process along the fault.
The methodological development has been focused on improving the capability of
the SPEM for building complex models. In the standard implementation, the
spectral element method decomposes the whole model into a patch of sub-regions
having constant physical properties. This approach is inadequate to represent geomodels,
since they usually feature small scale heterogeneities and medium
properties that change continuously in the space. This fact motivated the
development of a different approach, in which 1) the geological structure is builtup
through an interpolator which handles discontinuities, and using a minimum
number of control points and lines, and 2) the mesh size adapts continuously to
the medium properties and is controlled by few geometric constraints.
In general, the activity has been developed mainly within a number of national
research projects funded or co-ordinated by the GNDT1 and GNV2. During year
2000, one three-years project – the Catania Project – finished its activity, while five
new research projects started. In particular, the latter include a one-year project –
the Marche Microzonation Project – and four three-years projects, namely:
1) Development and Comparison between Methodologies for the Evaluation of
Seismic Hazard in Seismogenic Areas: Application to the Central and Southern
Apennines; 2) Damage Scenarios in the Veneto-Friuli Area; 3) Detailed Scenarios
and Actions for Seismic Prevention of Damage in the Urban Area of Catania; and
4) Integrated Seismic Methods Applied to the Investigation of the Active Volcano
Structure. An Application to the Phlegrean Fields Caldera. We will shortly present
some of those projects in the following.
The Catania Project aimed at evaluating the seismic risk of a highly urbanised area
typical of the Mediterranean region. The OGS contribution of the last year was
twofold. In a first part, we simulated a recent event, the M 5.8 earthquake, which
struck Eastern Sicily on December 13, 1990, and was recorded by the Catania
ENEA-ENEL accelerometric station. Seismograms computed using the SPEM
agree very well with those recorded by the accelerometric station. On the contrary,
any plane layer representation, which simplifies the complex structure used by
SPEM, does not provide a comparable agreement. In this case, the synthetic
seismograms are computed by the WIM. This study has several outcomes. Firstly,
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