Geoscience Reference
In-Depth Information
Table 2.4. Adopted sediment and crustal velocity model for parametric analyses of 2D alluvium
valleys
Valley sediments
Crustalmodel
Thickness (m)
V
s
(
ms
−
1
Thickness (m)
V
s
(
ms
−
1
)
)
15
250
30
1200
15
350
70
2000
70
450
250
2400
250
600
2350
2800
100
800
2000
3300
-
-
-
3500
-
-
30
1200
Fig. 2.11. Valley model withnumerical grid(spectral nodes). Notezone subdivision
(I,II,III,IV,V)
In Figure 2.11 the valley geometry with the spectral nodes of the 2D numerical grid is
shown. Notethesubdivisionof thevalley intofour zones, aimed atcapturing differences
in the site response features. The boundary zones of the model are within a distance
of respectively 200m and 400m from basin edges. The grid step allows to propagate
frequencies up to about 3Hz, with a sampling rate of 2.5 points per wavelength. The
computational mesh contains 1126 spectral elements (with 5
5 integration points) and
1140 spectral nodes. Absorbing boundaries were introduced on all the vertical sides of
the model and on itsbottomface.
×
Source analysis, carried out with the Hisada and Bielak (2003) code, involved normal
(heredenotedasN),reverse(R)andstrikeslip(SS)focalmechanism.Thedipangleofthe
fault was chosen to correspond to typical earthquake sources of Italy Alpine region: 60
◦
forN,20
◦
forRand85
◦
forSS.Arectangularfault
withbilateralpropagation
(hypocenter at 10km depth) was arbitrarily placed with strike 0
◦
at the origin of the
coordinate system. Rupture velocity on thefault was taken as 0
(
L
=
2W
)
75
V
s
.
The first cycle of simulations regarded an
M
5.2 event, the main source parameters of
which are summarised in Table 2.5. From
M
,
M
o
and rupture area were estimated after
Wells and Coppersmith (1994). From
M
o
and area, the slip was estimated, while the rise
time was obtained fromGeller (1976).
.
