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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Seismic Expression of Normal Faults
and Associated Structures
By
Normal faults and associated secondary structures are
common features in continental rifts. Fault
dip and displacement
stratal dip, and fold position and size vary considerably.
Synthetic seismic-reflection profiles show that each of these
structural variables, as well as rock velocity, influence the
seismic expression of rift-related structures.
The observed dip and curvature of any fault
on an
unmigrated seismic section depend, not only on the dip and
curvature of the actual
fault
surface, but also on the velocity
and dip of the overlying strata. The observed dip of a
fault
decreases as the velocity of the strata directly overlying the
fault
increases. Thus, planar normal faults in rocks whose
velocities increase with depth may appear to flatten with depth
on seismic sections. The observed dip of a
fault
decreases as
the acute angle between the
fault
surface and the overlying
strata decreases. Consequently, on unmigrated seismic sections,
normal faults dipping in the opposite direction as the
strata may appear to have steeper dips than identical normal
faults dipping in the same direction as the strata; planar
normal faults active during deposition may appear to steepen
with depth.
The appearance of secondary structures associated with
normal faulting on unmigrated seismic sections depends on
the position and size of the secondary structures. A greater
thickness of low-velocity rocks on the downthrown aide of a
normal fault
may disrupt and bend the reflections on the
upthrown side. Depth, rock-velocity distribution, and
fault
displacement affect the severity of the distortion. This distortion
may obscure secondary structures on the upthrown side
of faults, and can be interpreted erroneously as secondary
faulting and folding. Synclines produced by drag on the
downthrown sides of normal faults generally have small radii
of curvature relative to their burial depths. This relationship
makes them difficult to identify on unmigrated seismic sections.
In contrast, forced folds in rifts are gentle, shallow
structures overlying normal faults. These folds are easier to identify because they are unaffected by the distortion beneath
faults, and their synclines have large radii of curvature
compared to their burial depths.
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