Civil Engineering Reference
In-Depth Information
5.6.2
Methods Used to Determine the Peak Ground Acceleration
The engineering geologist is often the best individual to determine the peak ground accelera-
tion
a
max
at the site based on fault, seismicity, and attenuation relationships. Some of the more
commonly used methods to determine the peak ground acceleration at a site are as follows:
●
Historical earthquake:
One approach is to consider the past earthquake history of the
site. For the more recent earthquakes, data from seismographs can be used to determine
the peak ground acceleration. For older earthquakes, the location of the earthquake and
its magnitude are based on historical accounts of damage.
Computer programs, such as the EQSEARCH computer program (Blake 2000b), have
been developed that incorporate past earthquake data. By inputting the location of the
site, the peak ground acceleration
a
max
could be determined. For example, Figs. B.1 to
B.11 (App. B) present an example of the determination of
a
max
based on the history of
seismic activity in the southwestern United States and northern Mexico.
The peak horizontal ground acceleration
a
max
should never be based solely on the his-
tory of seismic activity in an area. The reason is because the historical time frame of
recorded earthquakes is usually too small. Thus the value of
a
max
determined from his-
torical studies should be compared with the value of
a
max
determined from the other meth-
ods described below.
●
Code or other regulatory requirements:
There may be local building code or other reg-
ulatory requirements that specify design values of peak ground acceleration. For exam-
ple, by using Fig. 5.17 to determine the seismic zone for a given site, the peak ground
acceleration coefficient
a
max
g
can be obtained from Table 5.5. Depending on the distance
to active faults and the underlying subsoil profile, the values in Table 5.5 could underes-
timate or overestimate the peak ground acceleration.
●
Maximum credible earthquake:
The maximum credible earthquake (MCE) is often
considered to be the largest earthquake that can reasonably be expected to occur based
on known geologic and seismologic data. In essence, the maximum credible earthquake
is the maximum earthquake that an active fault can produce, considering the geologic
evidence of past movement and recorded seismic history of the area. According to
Kramer (1996), other terms that have been used to describe similar worst-case levels of
shaking include
safe shutdown earthquake
(used in the design of nuclear power plants),
maximum capable earthquake, maximum design earthquake, contingency level earth-
quake, safe level earthquake, credible design earthquake,
and
contingency design earth-
quake.
In general, these terms are used to describe the uppermost level of earthquake
forces in the design of essential facilities.
The maximum credible earthquake is determined for particular earthquakes or levels
of ground shaking. As such, the analysis used to determine the maximum credible earth-
quake is typically referred to as a
deterministic method.
●
Maximum probable earthquake:
There are many different definitions of the maximum
probable earthquake. The maximum probable earthquake is based on a study of nearby
active faults. By using attenuation relationships, the maximum probable earthquake mag-
nitude and maximum probable peak ground acceleration can be determined.
A commonly used definition of maximum probable earthquake is the largest predicted
earthquake that a fault is capable of generating within a specified time period, such as 50
or 100 years. Maximum probable earthquakes are most likely to occur within the design
life of the project, and therefore, they have been commonly used in assessing seismic risk
(Federal Emergency Management Agency 1994).
Another commonly used definition of a maximum probable earthquake is an earth-
quake that will produce a peak ground acceleration
a
max
with a 50 percent probability of
exceedance in 50 years (USCOLD 1985).
