Geoscience Reference
In-Depth Information
Kyushu, Japan, earthquakes and the 2000 Iceland earthquake, all of them strong contri-
butors to the calibration dataset. Systematic numerical simulations were also carried out
using the
D
10
parameter.
The previous effects strongly depend on the fault-receiver configuration and their inclu-
sion is hardly feasible in the frame of a nation-wide probabilistic hazard analysis, not to
mention the uncertainties affecting the geometry and the slip rate of most active faults in
Italy. Nevertheless, a modified form of the attenuation relation (2.2) was developed for
D
10
, that includes radiation pattern terms as a function of the fault-to-receiver angle of
view.Thisshouldbeaptforapplicationsinscenariostudiesonasinglesite,withseismic
hazard controlled by the activity of a single fault.
3. Response of alluvium filled valleys and basins
While the influence of alluvium filled basins on site response has been the subject of
substantialresearch,theresultingmodificationsontheresponsespectraatthebasinssur-
face (especially for
T
1-2s) has not been as thoroughly investigated (see e.g. Chavez
Garcia and Faccioli, 2000) despite itsimportance in structuraldesign.
>
Significant previous studies tried to estimate basin amplification effects through the
analysisofstrong-motiondataandmostofthemquantifiedbasingeometryonlyinterms
of sediment depth (Trifunac and Lee, 1978), introducing such term in newly developed
attenuation models. Other studies tried to relate basin effects also to the relative location
of source and site position in the basin (Choi et al., 2005), or to the distance to the basin
edge (Joyner, 2000).
Herein, in order to identify how alluvium filled valleys and basins affect the
DRS
,we
consider both the wave field generated at the source and its propagation inside the basin
sediments by applying a recently developed
Domain Reduction Method
(DRM), which
allows to couple the separate analyses of source and basin response. This method, pro-
posedinBielaketal.(2003)onthebasisofearlierwork,wassubsequentlyextendedina
spectralelementenvironment(Facciolietal.,1997)andimplementedinnumericalcodes
suitedfor2D/3Dwavepropagationanalyses(Facciolietal.,2005),towhichwereferfor
details of implementation.
The main steps of the analyses are as follows. First a detailed numerical grid of the allu-
vium basin at study is prepared. Then the source mechanisms of interest are defined
and the complete 3D displacement field in the zone occupied by the basin is computed
using a horizontally layered local model of the Earth crust, through the efficient analyt-
ical method of Hisada and Bielak (2003). These displacements, properly manipulated,
are used to calculate the effective forces to be applied at the boundary of the numeri-
cal model of the basin as an equivalent, effective dynamic excitation. Wave propagation
insidethebasinisthenperformedwiththespectralcodeGeoELSE(Maggioetal.,2001;
Stupazzini, 2004) and the damped
DRS
are finally calculated and analysed at the surface
receivers of thebasin.
