GLOSSARIES - Geotechnical Earthquake Engineering - page 519

Dip-Slip Fault A fault which experiences slip only in the direction of its dip. In other words, the
movement is perpendicular to the strike. Thus a fault could be described as a dip-slip normal fault,
which would indicate that it is a normal fault (see Fig. 2.12) with the slip only in the direction of its
dip.
Longitudinal Step Fault A series of parallel faults. These parallel faults develop when the
main fault branches upward into several subsidiary faults.
Normal Fault A fault where the hanging wall block has moved downward with respect to the
foot wall block. Figure 2.12 illustrates a normal fault. The hanging wall is defined as the overlying
side of a nonvertical fault.
Oblique-Slip Fault A fault which experiences components of slip in both its strike and dip
directions. A fault could be described as an oblique-slip normal fault, which would indicate that it is a
normal fault (see Fig. 2.12) with components of slip in both the strike and dip directions.
Reverse Fault A fault where the hanging wall block has moved upward with respect to the foot
wall block. Figure 2.13 illustrates a reverse fault.
Strike-Slip Fault A fault on which the movement is parallel to the strike of the fault. A strike-
slip fault is illustrated in Fig. 2.11.
Thrust Fault A reverse fault where the dip is less than or equal to 45°.
Transform Fault A fault that is located at a transform boundary (see Sec. 2.1). Yeats et al.
(1997) define a transform fault as a strike-slip fault of plate-boundary dimensions that transforms into
another plate-boundary structure at its terminus.
Fault Scarp Generally a portion of the fault that has been exposed at ground surface due to ground
surface fault rupture. The exposed portion of the fault often consists of a thin layer of fault gouge,
which is a clayey seam that has formed during the slipping or shearing of the fault and often contains
numerous slickensides.
First Arrival The first recorded data attributed to seismic waves generated by the fault rupture.
Flow Slide Phenomenon in which, if liquefaction occurs in or under a sloping soil mass, the entire
mass could flow or translate laterally to the unsupported side. Such slides tend to develop in loose, sat-
urated, cohesionless materials that liquefy during the earthquake.
Focal Depth The distance between the focus and epicenter of the earthquake.
Focus Also known as the hypocenter of an earthquake; the location within the earth that coincides
with the initial slip of the fault. In essence, the focus is the location where the earthquake was initiated.
Foreshock A small tremor that commonly precedes a larger earthquake or main shock by seconds
to weeks and that originates in or near the rupture zone of the larger earthquake.
Gouge The exposed portion of the fault often consisting of a thin layer of fault gouge, which is a
clayey seam that has formed during the slipping or shearing of the fault and often contains numerous
slickensides.
Graben The dropping of a crustal block along faults. The crustal block usually has a length that is
much greater than its width, resulting in the formation of a long, narrow valley. A graben can also be
used to describe the down-dropping of the ground surface, such as a graben area associated with a land-
slide.
Hazard A risk. An object or situation that has the possibility of injury or damage.
Hypocenter See Focus.
Inactive Fault Definitions vary, but in general an inactive fault that has had no displacement over
a sufficiently long time in the geologic past that displacements in the foreseeable future are considered
unlikely.
Intensity (of an Earthquake) A measure based on the observations of damaged structures and
the presence of secondary effects, such as earthquake-induced landslides, liquefaction, and ground
cracking. The intensity of an earthquake is also based on the degree to which the earthquake was felt
by individuals, which is determined through interviews. The most commonly used scale for the
determination of the intensity of an earthquake is the modified Mercalli intensity scale (see
Table 2.3).