Mitigation of seismic hazard of a megacity: the case of Naples

C. Nunziata, D. Fah, G. F. Panza

Abstract


The seismic ground motion of a test area in the eastern district of Naples was computed with a hybrid technique
based on the rnode surnrnation and the finite difference methods. This technique allowed the realistic
modelling of source and propagation effects, including local soil conditions. In the modelling, as seismic
source we considered the 1980 Irpinia earthquake, a good example of strong shaking for the area of Naples,
located about 90 km from the source. Along a profile through Naples, trencling N86°W, the subsoil is mainly
formed by alluvial (ash, stratified sand and peat) and pyroclastic materials overlying a pyroclastic rock (yellow
Neapolitan tuff) representing the Neapolitan bedrock. The detailed information available on the subsoil mechanical
properties and its geometry warrants the application of the sophisticated hybrid technique. For SH waves,
a comparison was made between a realistic 2-D seismic response and a standard I-D response, based
on the vertical propagation of waves in a plane layered structure. As expected the sedimentary cover caused an
increase in the signal's amplitudes and duration. If a thin uniform peat layer is present, the amplification effects
are reduced, and the peak ground accelerations are similar to those observed for the bedrock model. This
can be explained by the backscattering of wave energy at such a laqer. The discrepancies evidenced between
the l -D and the 2-D seismic response suggest that serious caution must be taken in the formulation of seismic
regulations. This is particularly true in the presence of the thin peat laqer where the misinatch between the l -D
and the 2-D amplification functions is particularly evident in correspondence of the dominant peak and of the
second significant peak.

Keywords


Seismic hazard;microzoning;numerical modelling;Naples

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References


DOI: https://doi.org/10.4401/ag-4055
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Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN: 2037-416X