By
using eRiskZone, the user will notice that the maximum PGA values on
rock for the return period of 475 years computed by our system may
differ to some extent from the reference PGA on rock reported in the
Italian Code (Ordinanza 3274/2003 - Attachment 1). In particular, it
can be observed that for areas characterized by high seismicity the
system computes lower PGA values than provided by the Code (the PGA
values computed by eRiskZone througout the entire Italian territory
are shown in Figure 1). This differences may depend on
different reasons:
Figure
1 - Peak ground accelerations on rock computed by eRiskZone
throughout the entire national territory
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First,
the
reference peak ground acceleration values indicated in the Code are
selected in a conventional way, and are assigned to areas where the
computed acceleration levels can be significantly lower. For
instance for Zone 2, where the PGA can have values between 0.15 a
0.25g, the Code prescribes a reference value equal to the higher of the
two, 0.25g.
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The
probabilistic methodologies adopted in our system imply working
assumptions and models that, although technically valid and plausible,
may differ from those adopted during the course of similar studies.
With
reference to the last item listed above, we propose in the following
a preliminary comparison between the PGA values computed by eRisKZone
and the PGA values computed by the National Institute for Geophysics
and Vulcanology (INGV). Both studies were carried out by means
of a similar probabilistic approach and are based upon the same
seismic catalogue and upon the same seismic zonation (ZS9).
Nevertheless
the two methodologies differ to some extent; for
instance different selection criteria were selected for the
attenuation laws (a single attenuation law was adopted by eRiskZone
for the online simplified system, whereas multiple
attenuation laws
were used by INGV), and different software was used in the
computations. A comparison in terms of difference (in
percent)
of the PGA computed over the entire national territory is shown in
Figure 2.
Figure
2 – Differences among the peak ground accelerations on rock
computed by eRiskZone and by INGV, respectively, over the
entire
Italian territory.
In
some areas characterized by high seismicity our model tends to
provide values lower by 10-15% with respect to the INGV study,
whereas the opposite occurs in areas of lower seismicity. In
Figure 3 the acceleration values computed in the two studies are
compare directly on a graphic. It can be noticed that the
maximum value computed in both studies are significantly lower that
the reference values indicated in the Italian Code for the areas of
highest seismicity. The values obtained by INGV are on
average
moderately higher than the values used by this portal.
Figure
3 – Comparison between the peak ground accelerations on rock
computed by eRiskZone and by INGV, respectively, over the entire
national territory.
In
conclusion it can be noticed that the comparison among the values
computed by eRiskZone over the entire national territory and those by
the National Institute of Geophysics and Vulcanology (INGV) are
reasonably similar, with differences of minor relevance for the
simplified analysis offered online. It is not possible, at the
moment, to compare also spectral acceleration as the INGV study does
not provide such information.
