Environmental Engineering Reference
In-Depth Information
1.0
0.1
0.01
0.005
2
5
10
20
50
100 200
500
Distance, km
FIGURE 11.16
Acceleration attenuation curves for the United States. The solid lines are curves for the eastern region
(east of longitude 105
). The dashed lines together with solid lines at close distances are the attenuation curves
used for the western United States and are taken from Schnable and Seed (1973). It is to be noted that under
certain conditions the area of shaking in the eastern United States is very much larger than in the western
regions under similar earthquake conditions (see
Figure 11.18)
.
(From Algermissen, S.T. and Perkins, D.M.,
U.S. Geological Survey, Open File Report 76-416, 1976. With permission.)
°
with those in the western states, it can be shown that those in the east often affect areas
100 times greater than those in the west as illustrated in Figure 11.18. In addition, those in
the east are not often associated with evidence of surface faulting (Nuttli, 1979).
11.2.6
Amplification
Description
Ground Amplification Factor
Site intensity is often amplified by soil conditions. An increase in ground acceleration with
respect to base rock excitation is termed the ground amplification factor.
Stable Soil Conditions
Under conditions where the soils are stable (nonliquefiable), the influence of local soil con-
ditions on ground motions can take the form of dynamic amplification, which can result in
an increase in peak amplitudes at the surface or within a specific layer. The duration of shak-
ing may also be increased. The factors influencing the occurrence are not well understood,
although, in general, amplification is a function of soil type (densities and dynamic proper-
ties are most significant), depth, and extent. Attenuation may occur under certain conditions.
Unstable (liquefiable)
soils are described in
Section 11.3.3.
Influencing Factors
Soil Type and Thickness
Zeevaert (1972) concluded that in the valley of Mexico City, acceleration in the lacustrine
soils is approximately two times larger than in the compact sand and gravel surrounding
