Environmental Engineering Reference
In-Depth Information
3
1
2B
2A
4
0
0
2B
3
1
1
3
4
3
2B
2A
2A
4
3
1
3
1
1
1
2A
2A
1
2B
0
0
Alaska
1
0
2B
2B
3
Hawai
4
2B
1
3
Factor
Factor
Factor
Zone
Zone
Zone
2B
2A
2B
%g, 10% Probability of exceedence in 50 years
0
1
0
0.15
3
4
0.3
0.4
3
3
4
Puerto
rico
0.075
0.20
Aleutian
islands
FIGURE 11.13
Seismic zone map of the United States. (From the International Conference of Building Officials, 1988.)
lines changed to incorporate new data. A recent seismic zone map of the Continental
United States is given in Figure 11.13.
A recent seismic risk map for the United States is given in Figure 11.14. It presents con-
tours of effective peak rock acceleration with a 90% probability of not being exceeded in a
50 year period.
11.2.5 Attenuation
Description
Attenuation is the decay or dissipation of energy or intensity of shaking with distance
from the source, occurring as the seismic waves travel through the Earth, and results in the
site intensity of rock excitation . In both deterministic and probabilistic seismic hazard analy-
ses calculations are made of the ground motion parameter of interest at a given site from
an earthquake of a given magnitude and site-to-source distance.
The epicentral area extends for some distance about the epicenter, in which there is no
attenuation, then with increasing distance there is wide regional variation in intensity dis-
tribution. It is affected by geology, topography, and length of fault rupture. Variations are
illustrated by isoseismal maps such as in Figure 11.9, Figure 11.15, or Figure 11.31; or by
seismicity maps such in Figure 11.10, which are used to develop attenuation relationships.
Figure 11.15 presents a comparison of intensity distributions from two earthquakes of dif-
ferent magnitudes but with fairly close epicenters.
Estimations
Theoretical Relationships
Previously, attenuation relationships were given in terms of intensity; at present most
attenuation “models” are based on strong ground motion. Theoretical relationships are
used to develop attenuation relations in areas where there are an insufficient number of
 
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