Geoscience Reference
In-Depth Information
20
0.06
6.3 <
M
< 6.7
0.05
DRS
= 2.5
a
g
S
(
T
C
T
) / (2
π
)
2
15
0.04
0.03
10
0.02
5
5.8 <
M
< 6.2
5.3 <
M
< 5.7
0.01
T
C
= 0.4 s
T
D
= 2 s
0
0
0
0 2 4 6 8 10
Period [s] Period [s]
Fig. 2.8. (Left)Observed median
DRS
(without distinction of ground type,
0.5
1
1.5
2
2.5
3
10
<
R
<
30km)as a function of magnitude. (Right)Displacement spectral shape
according toCEN (2004)
However, there are two major drawbacks to the bi-linear curve of CEN (2004). First, the
constant plateau region is proportional to the peak ground acceleration
a
g
, that does not
theoretically scale with magnitude as
D
10
, as shown by the observed median curves in
Figure 2.8 and as it can be deduced by the dependence on period of the
a
2
coefficient in
Table 2.2. Second, a corner period
T
D
2s is prescribed in CEN (2004) as the starting
pointoftheconstantdisplacementregion,whatevertheearthquakemagnitudegoverning
the seismic hazard of the region of interest. These points will be further discussed in
Section5, when displacement hazard representations will be introduced. At this stage,
we only point out that it seems more rational to approximate the displacement spectral
shape for design through the previous bi-linear curve with a constant level at
T
=
T
D
.
However, instead of fixing the value of such plateau as a function of
a
g
and
T
D
and of
arbitrarily setting
T
D
>
2s, both
D
10
and
a
g
should be independently determined and
T
D
obtained as theintersection of thetwo lines, i.e.
=
2
D
10
(
π)
2
T
D
=
(2.5)
.
2
5
a
g
ST
C
Theadvantageofusing(2.5)isthatthedependenceof
T
D
onmagnitudeisnowimplicitin
D
10
;thedrawbackofthissimplifiedspectralmodelisthatittendstobeoverconservative
for intermediate periods, closeto
T
D
,as it will become clearer inSection5.
2.6. OTHER ASPECTS
We did not consider so far the treatment devoted to source and near-field effects in
the S5 project. Briefly, there is only weak evidence of source directivity effects in the
large database discussed in Section2.1, and these were therefore neglected. On the other
hand,remarkableradiationpatterneffectsfor
T
valuesbetweenfewsandover10swere
detectedforstrike-slipeventsinthe
M
6.5to7.0range,notablythe2000Tottoriand2005
