thrust and normal faults. An example of an evaluation
(b) Evaluate local seismicity. If stratigraphic data
of the potential for surface fault rupture following a
are not available for assessment of fault activity,
screening process is given in Appendix G.
historical seismicity patterns might provide useful
information. Maps and up-to-date plots depicting
historical seismicity surrounding the site and vicinity
c. Soil liquefaction. If a site has been filtered
can be obtained from the USGS at its National
through the screening criteria and liquefaction-
Earthquake Information Center in Golden, Colorado.
susceptible materials are identified, the potential for
Additional seismicity information may be obtained
liquefaction to occur due to earthquake ground shaking
from state geologic agencies and from colleges and
may be assessed by a variety of available approaches
universities that maintain a network of seismographs
(National Research Council, 1985). The most
(e.g., California Institute of Technology; University of
commonly utilized approach is the Seed-Idriss
California, Berkeley; University of Nevada, Reno;
simplified empirical procedure presented by Seed and
University of Washington; National Center for
Idriss (1971, 1982), as updated by Seed et al. (1985)
Earthquake Engineering Research, Buffalo, New York;
and Youd and Idriss (1997) that utilizes Standard
etc.). If the fault(s) that pass beneath the site are
Penetration Test (SPT) blowcount data. The latter
spatially associated with historical seismicity, and
citation refers to the Proceedings of the Workshop on
particularly if the seismicity and fault trends are
Evaluation of Liquefaction Resistance of Soils
coincident, the faults should probably be considered
conducted by the National Center for Earthquake
active.
Engineering Research (NCEER). The purpose of the
workshop was to update and augment the simplified
(c) Evaluate structural relationships. In the absence
liquefaction evaluation procedures. Where consensus
of both stratigraphic and seismological data, an
has been achieved by the workshop participants on
assessment of the geometric/structural relationships
changes and additions to the evaluation procedures,
between fault(s) at the site and faults of known activity
these changes and additions are incorporated herein.
in the region could be useful. Although less definitive
However, as of October 1998, workshop participants
than the two prior criteria, the probability that the site
are continuing to evaluate several aspects of the
fault is active increases if it is structurally associated
evaluation procedures.
with another active fault, and if it is favorably oriented
relative to stresses in the current tectonic environment.
The following paragraphs briefly summarize simplified
state-of-the-art approaches for evaluating liquefaction
(3) Fault rupture characteristics. If the evaluation
potential and its consequences. Guidance for
indicates one or more active faults are present beneath
liquefaction potential evaluations is also presented in
the site, the characteristics of future slip on the fault(s)
Navy Technical Report TR-2077-SHR (Ferritto,
can be estimated. Based on analysis of moderate and
1997b) and Department of Defense Handbook MIL-
large magnitude earthquakes worldwide, Wells and
HDBK-1007/3 (Department of Defense, 1997).
Coppersmith (1994) have developed empirical
Ferritto (1997b) also presents guidance for safety
relationships among earthquake moment magnitude
factors against liquefaction and allowable
and a variety of fault characteristics including
displacements for different facility types.
maximum displacement (Figure F-10) that are based on
fault type (e.g., strike-slip, reverse, and normal). These
(1) Seed-Idriss evaluation procedure. Peak ground-
curves provide a convenient means for assessing the
surface acceleration, earthquake magnitude, total and
amount of slip or displacement fault. Amounts of fault
effective overburden stresses at the point of interest,
displacement should be estimated assuming the
and the standardized SPT blowcount are needed to
occurrence of a maximum earthquake on the fault.
perform the evaluation using the Seed-Idriss simplified
Predicting the width of the zone of surface deformation
empirical procedure. The standardized blowcount
associated with a surface faulting event is more difficult
index used in the method is (N1)60, which represents the
because empirical relationships having general
SPT blowcount to advance a 51-mm (2-inch) O.D.
applicability have not yet been developed. The best
split-spoon sampler
means for assessing the width of faulting at the site is
site-specific trenching that crosses the entire zone. In
the absence of such information, the historical record
indicates that steeply dipping faults, such as vertical
strike-slip faults, tend to have narrower zones of
surface deformation than shallow dipping faults like
F-19
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