APPENDIX F
displacement at the ground surface for those cases
GEOLOGIC HAZARDS EVALUATIONS
where the fault slip extends to the ground surface.
Generally, fault rupture extends to the ground surface
only during moderate- to large-magnitude earthquakes
F-1. Introduction
(magnitudes equal to or greater than 6). However, not
all moderate-to large-magnitude earthquakes produce
fault slip at the ground surface. In some cases, the fault
This appendix describes guideline procedures for the
displacement may occur entirely at depth, with little or
evaluation of seismic-geologic site hazards, other than
no apparent permanent surface deformation (e.g., 1989
the ground shaking hazard. These hazards include: (a)
Loma Prieta, California earthquake of moment
surface fault rupture; (b) soil liquefaction; (c) soil
magnitude 7.0), or with more subdued or diffuse
differential compaction; (d) landsliding; and (e)
surface warping and fracturing (as may have
flooding. The evaluations of the hazards described in
accompanied the 1994 Northridge, California
this appendix should be carried out by qualified
earthquake of moment magnitude 6.7).
geotechnical professionals. Depending on the hazard,
disciplinary expertise in geotechnical engineering,
(1) Mode of fault movement. The mode of surface
geology, and seismology may be needed.
fault deformation is influenced by the type of faulting.
Different types of faults are illustrated in Figure F-1.
a. Overview of process for conducting geologic
These types are distinguished by the primary sense of
hazards evaluations. The process described herein for
relative displacement between the two sides of the
seismic-geologic hazards evaluation is a two-step
fault. Strike-slip faults are characterized by horizontal
process-- screening and evaluation. If a significant
movement; reverse or thrust faults involve relative
hazard is disclosed by this process, then hazard
upward movement of the crustal block above the fault
remediation should be developed.
plane; normal faults involve relative downward
movement of the block above the fault plane; and
b. Organization of remainder of this appendix.
oblique faults are characterized by both strike-slip and
Paragraph F-2 describes and illustrates the geologic
reverse or normal types of movement.
hazards. Screening procedures for these hazards are
presented in paragraph F-3. The intent in the screening
(2) Magnitude of displacements. Surface fault
process is to utilize readily available data and criteria to
displacements may range from a fraction of an inch to
ascertain whether a significant potential for any of the
several feet or more depending on the earthquake
hazards exists at the site. Paragraph F-4 presents
magnitude, steepness of the fault plane, type of
hazard evaluation procedures in the event that the
movement, and other factors. These same factors, as
screening process results in a conclusion that more
well as the nature of the surface geologic materials,
detailed evaluation is required to assess the hazard and
also influence how wide the zone of surface rupture is
its significance. Paragraph F-5 provides preliminary
likely to be. Because fault displacements tend to occur
information regarding hazard mitigation. Requirements
abruptly, often across a narrow zone, surface fault
for documentation of the evaluations of geologic
rupture can be catastrophic to structures situated
hazards are described in paragraph F-6. Examples of
directly astride the rupture zone. Figure F-2 illustrates
geologic hazard evaluations are presented in Appendix
surface fault rupture that occurred in the 1992 Landers,
G.
California earthquake. During this moment magnitude
7.3 earthquake, the displacement was mainly of the
strike-slip type (see Figure F-1) and the maximum
F-2. Description of Geologic Hazards
observed horizontal displacement along the fault was
5.5 m (18 feet). Figure F-3 illustrates damage to a
The following paragraphs provide brief descriptions of
structure astride the surface
the seismic-geologic hazards of surface fault rupture,
soil liquefaction, soil differential compaction,
landsliding, and flooding. Hazard significance in terms
of potential ground movements and effects on
structures are also summarized.
a. Surface fault rupture. Earthquakes are caused by
the sudden slip or displacement along a zone of
weakness in the earth's crust, termed a fault. Surface
fault rupture is the manifestation of the fault
F-1
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