GIULIANO F. PANZA 1,2 (corresponding author), FABIO ROMANELLI 1
1 Dipartimento di Scienze
della Terra – Università di Trieste
2 The Abdus Salam International
Centre for Theoretical Physics – Miramar,
Trieste
panza@dst.univ.trieste.it, romanel@dst.univ.trieste.it
FRANCO VACCARI
INGV-Oss. Vesuviano, c/o Dipartimento di Scienze della Terra – Università di
Trieste
vaccari@dst.univ.trieste.it
LUIS DECANINI, FABRIZIO MOLLAIOLI
Dipartimento di Ingegneria Strutturale e Geotecnica – Università di
Roma “La Sapienza”
Luis.Decanini@uniroma1.it, Fabrizio.Mollaioli@uniroma1.it
The input for the seismic risk analysis can be expressed with a description
of “ground shaking scenarios”, or with probabilistic maps of
perhaps relevant parameters.
The probabilistic approach, unavoidably based upon rough assumptions and models
(e.g. recurrence and attenuation laws), can be misleading, as it cannot take
into account, with satisfactory accuracy, some of the most important aspects
like rupture process, directivity and site effects. This is evidenced by the
comparison of recent recordings with the values predicted by the probabilistic
methods.
We prefer a scenario-based, deterministic approach in view of the limited seismological
data, of the local irregularity of the occurrence of strong earthquakes, and
of the multiscale seismicity model, that is capable to reconcile two apparently
conflicting ideas: the Characteristic Earthquake concept and the Self Organized
Criticality paradigm.
Where the numerical modelling is successfully compared with records, the synthetic
seismograms permit the microzoning, based upon a set of possible scenario earthquakes.
Where no recordings are available the synthetic signals can be used to estimate
the ground motion without having to wait for a strong earthquake to occur (pre-disaster
microzonation). In both cases the use of modelling is necessary since the so-called
local site effects can be strongly dependent upon the properties of the seismic
source and can be properly defined only by means of envelopes.
The joint use of reliable synthetic signals and observations permits the computation
of advanced hazard indicators (e.g. damaging potential) that take into account
local soil properties. The envelope of synthetic elastic energy spectra reproduces
the distribution of the energy demand in the most relevant frequency range
for seismic engineering. The synthetic accelerograms can be fruitfully used
for design and strengthening of structures, also when innovative techniques,
like seismic isolation, are employed.
For these reasons the skill of seismology to estimate realistic ground motions
at a particular site should be fully exploited by seismic engineers. The approach
has general validity and is particularly suitable to handle the special case
of the seismic hazard in Romania, and in particular in Bucharest, that is mostly
controlled by Vrancea sources. The application of these ideas to the assessment
of the seismic hazard, due to Vrancea sources, is described in other papers
in this issue.
Key words: seismic hazard assessment, synthetic seismograms, seismic engineering.