field assessment might have been identified previously
the site, the following information is required to refine
during the geologic and topographic map review, aerial
definition of the hazard:
photographic interpretation, and/or during
conversations with geologists.
$ the location of fault traces (if any) with respect to
the site
(d) Subsurface exploration. Faults obscured by
overburden soils, site grading, and/or structures can be
$ the timing of most recent slip activity on the fault
potentially located by one or more techniques.
Geophysical techniques such as seismic refraction
$ the ground rupture characteristics for a design
surveying provide a remote means of identifying the
earthquake on the fault (e.g., type of faulting
location of steps in a buried bedrock surface and the
(Figure F-1), amount of slip and distribution into
juxtaposition of earth materials with different elastic
strike-slip and dip-slip components, and width of
properties. Geophysical surveys require specialized
the zone of ground deformation)
equipment and expertise, and their results may
sometimes be difficult to interpret. Trenching
(1) Fault location. There are several steps that can
investigations are commonly used to expose subsurface
be taken to confirm and define the location of faults.
conditions to a depth of 4.6 to 6.1 m (15 to 20 feet).
Further assessments will not be required if it can be
While expensive, trenches have the potential to locate
shown on the basis of the evaluation procedures
faults precisely and provide exposures for assessing
outlined below that there are no faults passing beneath
their slip geometry and slip history. Borings can also
the site.
be used to assess the nature of subsurface materials and
to identify discontinuities in material type or elevation
(a) Interpretation of aerial photographs. Aerial
that might indicate the presence of faults.
photographs can be an excellent supplementary
resource to geologic and topographic maps of the site
(2) Fault activity. If it is determined that faults pass
and vicinity for identifying faults. Older photographs
beneath the site, it is essential to assess their activity by
are particularly useful if they depict the site and/or its
determining the timing of the most recent slip(s). If it
environs prior to development activities that would
is determined, based on the procedures outlined below,
have altered or destroyed landforms that indicate the
that the faults are not active faults (see paragraph F-
presence of faults. For many parts of the country,
3a), then further assessments are not required.
stereo photographic coverage is available as far back as
the 1920s or 1930s. Aerial photographs are usually
(a) Assess fault relationship to young
available from several sources including private
deposits/surfaces. The most definitive assessment of
companies and from various governmental agencies
the recency of fault slip can be made in natural or
including the U.S. Geological Survey, U.S. Department
artificial exposures of the fault where it is in contact
of Agriculture (Soil Conservation Service), Bureau of
with earth materials and/or surfaces of Quaternary age
Land Management, Forest Service, etc. The USGS
(last 1.8 million years). Deposits might include native
maintains the repository for federal photographic
soils, glacial sediments like till and loess, alluvium,
resources at its EROS Data Center, Sioux Falls, South
colluvium, beach and dune sands, and other poorly
Dakota 57198.
consolidated surficial materials. Surfaces might
include marine, lake, and stream terraces, and other
(b) Contacting knowledgeable geologists. There
erosional and depositional surfaces. A variety of age-
probably are geologists/earth scientists familiar with
dating techniques, including radiocarbon analysis and
geologic and tectonic conditions in the site vicinity who
soil profile development, can be used to estimate the
will be willing to share their knowledge. These
timing of most recent fault slip.
geologists might work for governmental agencies
(federal, state, and local), teach and conduct research at
nearby colleges and universities, or practice as
consultants.
(c) Ground reconnaissance of site and vicinity.
Walkdown of the site and its vicinity should be
conducted to observe unusual topographic conditions
and to evaluate any geologic relationships visible in
cuts, channels or other exposures. Features requiring a
F-18
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