Glossary of Terms5
Active fault—(1) A fault that has caused damage to manmade structures, or (2) a fault whose current rate of movement is sufficient to cause such damage, but where no manmade structures exist to document effects of movement.
Existing definitions of this term are both numerous and partly contradictory. By almost any of them, however, all faults that have been recognized at land surface in the Houston area are active, regardless of whether they have moved in historic times or not. We employ a more restricted and informal usage in discussing the Houston area, and use active fault only as a term of convenience to signify those faults that are, in effect, an immediate hazard to property.
Dip-slip movement—Slip of one fault block,
relative to the other block, by movement parallel to the dip of the fault. On
any inclined surface, such as a fault in the Houston area, it is possible to
draw an imaginary horizontal line, termed the strike of the fault, and to draw
a second line perpendicular to the strike, termed the dip line. The angle by
which the dip line deviates from horizontal is the dip of the fault; it corresponds
to the inclination of the fault in nonscientific terms. If movement of one fault
block relative to the other block occurs parallel to the dip line, it is termed
dip-slip movement.
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Downthrown side—The side of a fault that
appears to have moved downward relative to the other side (fig.
8). Also called downthrown fault block.
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Fault—A surface along which the nearly horizontal
sediment layers beneath the Houston area are broken and offset (fig.
8).
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Fault trace—The line defined by the intersection
of a fault surface with the land surface.
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Growth fault—A fault along which movement
occurs as sediments are deposited on and above the fault scarp. Continued movement
and sedimentation over an extended period of time causes the oldest and lowermost
sediments to be offset the most and causes the amount of offset to decrease
upward within younger deposits (fig. 8). Such faults
are common in the Gulf Coast.
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Point bar—A low, arcuate ridge of sand deposited
by a stream on the inside of a bend, or meander, in its course. A series of
such ridges, separated by intervening troughs, may be deposited along some meanders.
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Scarp—A relatively steep face or slope of
considerable linear extent, irrespective of origin. Some scarps are formed directly
by movement along a fault surface; the scarp separates two blocks of land that
were originally at the same or nearly the same elevation but which now are at
different levels due to faulting. A scarp represents the exposed surface of
a fault, usually modified to some extent by erosion on the upthrown side and
deposition on the downthrown side (figs. 5 and 8).
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Tiltbeam—A device for measuring the vertical
component of motion along an active fault. One end of the tiltbeam rests on
a base on the upthrown side, the other on the downthrown side, and the beam
is leveled. The vertical component of movement since the last releveling can
be calculated from the length of the beam and the amount by which it deviates
from a horizontal (level) line.
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Upthrown side—The side of a fault that
appears to have moved upward relative to the other side (fig.
8). Also called upthrown fault block.
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5 Some definitions presented here have been extracted and modified from the American Geological Institute “Glossary of Geology” (Gary and others, 1974).