Earthquakes - Geotechnical Engineering Investigation - page 985

Intensity distribution may be reduced and highly modified, as in the western
United States.

Great Plains and other large areas of reduced relief : Regional geology is likely to be more
uniform, but limestone and potential collapse conditions more prevalent, fault sys-
tems less pronounced and identifiable, and intensity distribution over very large
areas relatively uniform, as in the central United States (see Figure 11.18) .

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River valleys are possible flood areas with potentially liquefiable soils in the
floodway, pastoral, and estuarine zones; relatively poor foundation conditions;
and fault systems less pronounced and identifiable in broad, mature valleys.

●

Coastal areas exhibit great geological variations, but may contain liquefiable or
otherwise unstable soils, and, depending upon location and configuration, may
be subjected to the tsunami hazard.

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Climate
Climate relates to hazards and geologic conditions, as an aid or as a requirement for pre-
dictions. Some typical associations are the evaluation of the flood hazard, the type and
depth of residual soil development, and the potential for slope failures, collapsing soils,
and expansive soils.

Regional and Local Geologic Conditions
Regional conditions : include information on rock types and structures (faults, floods, etc.),
and the hazards of slope failures, ground subsidence or collapse from fluid or solid extrac-
tion or from natural causes, regional warping and tilting, and volcanoes.
Local conditions : include information on physiography and geology. Evaluation of con-
ditions may result in recommendations to abandon the site and select another location if
the constraints are judged to be too severe. The most severe constraints are local active
faulting and warping, high liquefaction potential, large-scale unstable or potentially
unstable slopes, volcanism, ground collapse, and tsunami potential.

11.5.3 Detailed Study of Regional and Local Geologic Conditions

Fault Studies
Prepare Geologic Structure Map
Data from the literature and terrain analysis techniques are used to prepare a map show-
ing all tectonic structures, including lineaments, within 200 mi of the site, with the primary
objective of locating all faults with the length-distance relationships given in Table 11.16.
If a known active or capable fault is within the 200 mi radius, and is a major fault, such as
the San Andreas Fault, then it is considered as the limit of the study area. The geologic
structure map is overlaid with the seismicity map and events correlated with lineaments
to identify potentially capable faults.
The NRC (2003) provides guidance in determining which faults may be of significance
in evaluating the safe-shutdown earthquake. In general, either capable or noncapable
faults with lengths less than those indicated in Table 11.16 need not be considered.

Investigate for Capable Faults
Investigations start near the site and extend outward to locate the design capable
fault. Judgment is required to evaluate the significance of lineament or fault length, site