APPENDIX G
evaluate the possibility of active faulting beneath the
GEOLOGIC HAZARD SCREENING AND
site, a surface fault rupture hazard screening was
EVALUATION EXAMPLES
performed.
G-1.
Example 1 - Surface Fault Rupture Hazard
(1) The screening consisted of the three steps
Screening and Evaluation
outlined in paragraph F-3. First, geological maps from
the U.S. Geological Survey, the state geologic survey,
and the county were reviewed. These maps showed that
This example illustrates the steps involved in screening
the site is located on a gentle, west-plunging anticlinal
a site for a surface fault rupture hazard and a subsequent
fold that was not interpreted to be cut by faults. The
site evaluation using the criteria described in paragraphs
maps did reveal, however, the presence of an active,
F-3 and F-4. The example given below is based on a
northwest-trending fault within 0.6 miles (1 km) west of
case history study for an existing building.
the site and another potentially active fault within 0.6
a.
Review of available information
miles (1 km) north of the site. The potentially active
fault is not well expressed topographically, and it
appeared not to cut deposits interpreted to be of
The building site is located within a developing
Holocene age (last 11,000 years) but does displace
metropolitan area in a tectonically active region.
rocks of Quaternary age (last 1.8 million years).
Twelve moderate- to large-magnitude earthquakes have
Secondly, a review of topographic maps of the site and
occurred in the region surrounding the site within the
vicinity revealed no features suggestive of a fault-
last 160 years. The building is a 137-m (450-foot) long
related origin beneath the site. Third, black and white
by 61-m (200-foot) wide, one- to two-story structure.
aerial photographs of the building and vicinity, flown
prior to site modification by grading, were examined
(1) Geotechnical investigations indicated that the
stereographically. The photos confirmed the presence
site is underlain by volcanic (basalt) and sandstone
of the anticlinal fold, but soil cover at the site obscured
materials located within a meter of the ground surface.
any fault-related dislocations that might be present in
Based on these studies, the original building design
the volcanic layers beneath the site.
called for the building to be supported on shallow
foundations extending to bedrock, which was reported
(2) The screening process yielded no evidence that
to occur close to the surface beneath the building
faults were present beneath the site. However, the close
footprint. However, during construction of the
proximity of a known active fault west of the site having
foundations in the western part of the building, no rock
a nearly identical trend to the inferred soil/rock
or rock-like materials were encountered, requiring
boundary beneath the western part of the building, as
design and utilization of deep auger cast-in-place piles
well as the close proximity of a fault north of the site
in this part of the building.
that displaced Quaternary-aged sediments, meant that
the possibility of a surface fault rupture hazard could
(2) Examination of as-built construction
not be ruled out. Therefore, further evaluation of the
documents indicated the possible presence of an abrupt
potential for surface fault rupture at the site was
interface between the rock subsurface conditions to the
performed.
east and deep soil conditions to the west on the
property. When plotted on a map (Figure G-1), these
c.
Fault rupture hazard evaluation
data supported the presence of a steeply dipping
rock/soil contact that extended across the site on a
An exploratory trench was excavated to a depth of 6 to
north-northwesterly alignment. This trend is consistent
9 feet (1.8 to 2.7 m) across a portion of an open field
with the orientation of known active faults within the
site's tectonic environment and also with the direction of
the channel of a nearby river.
b.
Fault rupture hazard screening
Based on the data available in the geotechnical reports
and the construction documents, it was not possible to
judge whether the apparent soil/rock contact was a
buried erosional channel margin, reflecting a former
position of the river, the manifestation of geologically
young faulting with a down-on-the-west dip-slip
component, or the result of some other process. To
G-1
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