Civil Engineering Reference
In-Depth Information
According to Kramer (1996), other terms that have been used to describe earthquakes
of similar size are
operating basis earthquake, operating level earthquake, probable
design earthquake,
and
strength level earthquake.
●
USGS earthquake maps:
Another method for determining the peak ground acceleration
is to determine the value of
a
max
that has a certain probability of exceedance in a specific
number of years. The design basis ground motion can often be determined by a site-spe-
cific hazard analysis, or it may be determined from a hazard map.
An example of a hazard map for California and Nevada is shown Fig. 5.18. This map
was developed by the USGS (1996) and shows the peak ground acceleration for
California and Nevada. There are similar maps for the entire continental United States,
Alaska, and Hawaii. Note that the locations of the highest peak ground acceleration in
Fig. 5.19 are similar to the locations of the highest seismic zones shown in Fig. 5.17, and
vice versa. The USGS (1996) has also prepared maps that show peak ground acceleration
with a 5 percent and 2 percent probability of exceedance in 50 years. These maps are eas-
ily accessible on the Internet (see U.S. Geological Survey, National Seismic Hazard
Mapping Project).
The various USGS maps showing peak ground acceleration with a 10, 5, and 2 percent
probability of exceedance in 50 years provide the user with the choice of the appropriate
level of hazard or risk. Such an approach is termed a
probabilistic method,
with the
choice of the peak ground acceleration based on the concept of acceptable risk.
A typical ranking of the value of peak ground acceleration
a
max
obtained from the dif-
ferent methods described above, from the least to greatest value, is as follows:
1.
Maximum probable earthquake (deterministic method)
2.
USGS earthquake map: 10 percent probability of exceedance in 50 years (probabilistic
method)
3.
USGS earthquake map: 5 percent probability of exceedance in 50 years (probabilistic
method)
4.
USGS earthquake map: 2 percent probability of exceedance in 50 years (probabilistic
method)
5.
Maximum credible earthquake (deterministic method)
5.6.3
Example of the Determination of Peak Ground Acceleration
This example deals with the proposed W. C. H. Medical Library in La Mesa, California.
The different methods used to determine the peak ground acceleration for this project were
as follows:
●
Historical earthquake:
The purpose of the EQSEARCH (Blake 2000b) computer pro-
gram is to perform a historical search of earthquakes. For this computer program, the input
data are shown in Fig. B.1 (App. B) and include the job number, job name, site coordinates
in terms of latitude and longitude, search parameters, attenuation relationship, and other
earthquake parameters. The output data are shown in Figs. B.2 to B.11. As indicated in
Fig. B.4, the largest earthquake site acceleration from 1800 to 1999 is
a
max
0.189
g.
The EQSEARCH computer program also indicates the number of earthquakes of a
certain magnitude that have affected the site. For example, from 1800 to 1999, there were
two earthquakes of magnitude 6.5 or larger that impacted the site (see Fig. B.5).
●
Largest maximum earthquake:
The EQFAULT computer program (Blake 2000a) was
developed to determine the largest maximum earthquake site acceleration. For this
