Geoscience Reference
In-Depth Information
Fig. 2.10. Average 5% damped displacement response spectra (
DRS
)from2D
numerical simulations compared with theapplicable Eurocode 8 (CEN, 2004) spectrum,
median (
s.e.)values fromthe form(2.4) of the attenuation relation (withshading) and
corresponding strongmotion record at GBP acceleration station (fromsimulated event
26.09.97, 9:40h)
+
receivers), anchored at a 475 year peak acceleration
a
g
23
g
(yielded by regional
probabilistic hazard analysis). The
DRS
from the horizontal strong motions recorded at
GBPisalsodisplayed(projectedinthedirectionparalleltothecross-section).Themean
+
=
0
.
1 s.e. spectral band predicted by (2.4) is also included in the figure for comparison.
The observed
DRS
has amplitudes that largely exceed the standard spectrum of the code
from 1.5s to 4.5s period. These amplitudes, moreover, cannot be matched by either the
numericalanalysisortheattenuationrelation.Thiscastsproperlightonthestrongampli-
fication effects affecting the response of alluvium valleys at long periods. Note that the
fundamental 1D resonant period inthe deepest portion of the valley isabout 3s.
The Gubbio observations (and similar ones in other parts of the world) prompted us to
planandperformaparametricanalysisofalluviumfilledvalleyssubjectedtotheradiation
ofdifferenttypesofearthquakesources.A2DmodelborrowedfromtheEuropeanProject
Sismovalp(www-lgit.obs.ujf-grenoble.fr/sismovalp)waschosenforvalleyshape,i.e.the
so-calledA0modelrepresentativeofthetypicalvalleyoftheEuropeanAlps.Theprofile
and mechanical properties of the layers inside the basin were defined as an average over
a number of basins located in the Italian, French, and Swiss Alps. In Table 2.4 are given
the velocity model of the basin and of the rock outside and beneath it. Ground category
C identifies nearly all the sitesat the surfaceof the basin.
