Civil Engineering Reference
In-Depth Information
(Abrahamson, 2000). This information may be
either the peak ground acceleration (PGA) or
the acceleration-time history of the motions,
depending on the method of stability analysis
that is to be used. The process by which
the design motion parameters are established is
termed “seismic hazard” analysis, which involves
the following three steps (NHI, 1998;
than 11,000 years, and not all faults rupture
to the surface, lack of evidence that movement
has occurred in the Holocene is generally suffi-
cient evidence to dismiss the potential for ground
surface rupture. Most Holocene fault activity in
North America has occurred west of the Rocky
Mountains, and may be identified by detailed
mapping, followed by trenching, geophysics or
drilling. In regions where there is no surface
expression of fault rupture, seismic source char-
acterization depends primarily on micro-seismic
studies and the historic record of felt earthquakes.
Ground motion intensity.
Once the seismic
sources capable of generating strong ground
motions at a site have been identified and char-
acterized, the intensity of the ground motions
can be evaluated either from published codes
and standards, or from seismic hazard ana-
lysis as discussed in Section 6.5.3. The building
codes of countries with seismic areas publish
maps in which the country is divided into
zones showing, for example, the effective
peak acceleration levels (as a fraction of grav-
ity acceleration) with a 10% probability of
being exceeded in a 50-year period (Frankel
et al
., 1996). The information on these maps
may also be available on the internet. For
example, in the United States it is possible
to find acceleration levels for postal zip codes
(http:/geohazards.cr.usgs.gov/eq/). These pub-
lished accelerations can be used in geotechnical
design and have the value of promoting standard
designs within each zone.
Glass,
2000):
(a)
Identification of seismic sources capable of
producing strong ground motions at the site.
(b)
Evaluation of seismic potential for each
capable source.
(c)
Evaluation of the intensity of design ground
motions at the site.
Implementation of these three steps involves the
following activities.
Seismic sources.
Earthquakes are the result
of fault movement, so identification of seismic
sources includes establishing the types of faults
and their geographic location, depth, size and
orientation. This information is usually avail-
able from publications such as geological maps
and reports prepared by government geological
survey groups and universities, and any previ-
ous projects that have been undertaken in the
area. Also, the identification of faults can be
made from the study of aerial photographs, geo-
logical mapping, geophysical surveys and trench-
ing. On aerial photographs, such features as
fault scarplets, rifts, fault slide ridges, shutter
ridges and fault saddles, and off-sets in such fea-
tures as fence lines and road curbs (Cluff
et al
.,
1972) may identify active faults. In addition,
records of seismic monitoring stations provide
information on the location and magnitude of
recent earthquakes that can be correlated to fault
activity.
Seismic potential.
Movement of faults within
the Holocene Epoch (approximately the last
11,000 years) is generally regarded as the cri-
terion for establishing that the fault is active
(USEPA, 1993). Although the occurrence inter-
val of some major earthquakes may be greater
6.5.3 Ground motion characterization
As a complement to the published maps, a seismic
hazard analysis for a specific site can be conducted
by evaluating the magnitude of ground motions
from all capable sources with the potential for
generating strong ground motions at the site. The
value of seismic analysis, in contrast to the use
of published codes as discussed in the previous
paragraph, is the ability to incorporate the latest
developments in local seismicity. Furthermore, it
is possible to develop site-specific ground motions
