Geological-geomorphological framework and site effects at San Giuliano di
Puglia (CB) and othter localities struck by the seismic sequence of
October-November 2002.
Since Oct. 31 2002 a seismic sequence has affected a sector of the Molise
Apennines between the Fortore and Biferno river valleys.
Although the magnitude of the two main events (Ml 5.4 and 5.3) is lower
than that of the large historical earthquakes of the Apennines, some
villages experienced significant damage and casualties have been recorded
at San Giuliano di Puglia due to the first shock.
The results of the first surveys made by the CNR-IGAG personnel,
immediately after the first event, permitted to hypothesise a significant
inhomogeneity of the damage distribution. This inhomogeneity is not only
evident from the comparison of the damage of adjacent villages, but also
within single localities. This suggested that some local factors, i.e. the
geological characteristics of the substratum, instability phenomena due to
landsliding and/or the different vulnerability of the edifices,
conditioned the amount and distribution of the damage.
On this basis, researchers of CNR-IGAG, in collaboration with researchers
of the Universities of Napoli and Chieti, began detailed surveys aimed at
defining the actual role that local factors have had in controlling the
damage distribution at San Giuliano di Puglia and in other villages
significantly struck by the earthquakes (i.e. Ripabottoni, Morrone del
Sannio, Pietramontecorvino and Castellino sul Biferno). Detailed
geological surveys permitted to reconstruct the geological structure of
the different areas and to define the lithological characteristics of the
foundation soils of the damaged localities.
The study of the slope instability permitted to define and classify the
main landslides affecting the investigated areas. Direct observations on
the characteristics of the buildings (height and age of the buildings,
type and quality of the walls, modifications to the original
structure, ...) permitted to subdivide the villages in areas statistically
characterised by a homogeneous type of edifices.
Gathered data, transferred on large-scale maps, indicated that in the case
of San Giuliano di Puglia the structural-geological conditions evidently
caused a significant amplification of the ground strong motion. In the
other investigated villages, the site effects are more moderate and the
increase of the damage level is conditioned by gravitational phenomena or
by the co-seismic re-activation of pre-existing landslides.
The described geological analyses represent the basis for the seismic
microzonation. These studies are, therefore, fundamental to define the
local seismic response within a certain village. In conclusion, the
seismic microzonation represents the basis for a correct urbanistic
development.
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