Geoscience Reference
In-Depth Information
Table 2.3. Mean peak period and corresponding peak amplification fromH/V spectral ratios
Ground type
Peak period (s)
Peak amplification
B
0
.
44
±
0
.
44
4
.
55
±
2
.
38
C
0
.
72
±
0
.
56
5
.
69
±
2
.
90
D
0
.
80
±
0
.
33
6
.
38
±
2
.
58
8
5.5
5
4.5
4
3.5
3
2.5
2
1.5
1
D
7
6
D
5
D
4
C
D
3
C
C
C
2
B
B
B
1
B
0
0
0.5
1
Period [s]
1.5
2
0
0.5
1
Period [s]
1.5
2
Fig. 2.5. (Left)Comparison of D/A, B/A, and C/Aspectral amplification obtained using
two dummy variables for sitedescription (median values, thick black lines)or three
dummy variables (shaded zones, median
1 s.e.) for Japanese data only. (Right)H/V
spectralratios(thinlines)vsmedianD/A,C/A,andB/Aratiosobtainedfrom(2.2)using
threedummy variables
±
The mean H/V spectral ratios for different ground categories exhibit, however, a clear
site-dependent shape, where physically meaningful dominant periods for each ground
class can be identified (Figure 2.5, rhs). Starting from this observation, we recognised
that modifying (2.2) by introducing three dummy variables was necessary to identify
without ambiguity the influence of local ground conditions by means of the attenuation
relation. The results of introducing such a modification into (2.2) are shown by the three
shaded bands on the lhs of Figure 2.5, labelled as B, C, and D and representing the s.e.
bandsofthespectralratiosB/A,C/A,andD/A.Thecorrespondingmedianspectralratios
resulting from the use of only two dummy variables are represented by the black solid
curves.Whiletheresultsarecomparableintermsofmaximumspectralamplification,the
formulation with two dummy variables is clearly inadequate to segregate the dominant
vibration periods for each ground class. The 95% confidence interval for the coefficient
of
S
3
always contains the D/A spectral ratio obtained with two dummy variables, while
for the C and B ground classes the spectral ratio obtained by the two formulations are
statistically equivalent only in the 0.3-0.6s period range and for
T
7s. This means
thatintheintervalofperiodswherethepeakamplificationforgroundBandCoccurs,the
difference in pattern obtained with three dummy variables is statistically significant and
clearly preferable. For the same confidence level, the coefficient of
S
1
is not statistically
significant for
T
>
1
.
>
0
.
7s.
