structure. For the example illustrated in Fig. 2.14a
we would refer to the “strike of the fault,” and the
fact that the fault is not simply a plane in space
would be left unspoken. To understand what
someone means by the strike of a fault, both
the nature of the surface within the fault zone
and the size of the planar element should be
identified.
Knowing the strike of a planar element
enables one to identify the orientation of the line
of intersection of that element with a horizontal
plane. There are an infinite number of elements
that share the same strike, but vary in inclination,
so we have to define a second angle that is a
measure of this inclination in order to determine
uniquely the orientation of a particular planar
element relative to the local geographic coordinate system. This angle, d , is measured in a vertical plane that contains the dip direction, from the
dip direction to the planar element in question
(Fig. 2.15a). This angle is referred to as the dip of
the planar element that approximates the structure, and by convention two digits are used to
specify the dip. Thus, a dip of seven degrees would
be written 07Њ. Note that the angle measured from
the horizontal plane to the planar element in vertical planes of other orientations would be
smaller than the dip. Such an angle is referred to
as an apparent dip. Apparent dips are commonly
observed and measured in the field where exposures cut obliquely across structures.
Only two angles are necessary to reference a
planar element to the local geographic coordinate
system. Some use the strike and dip (␣ s , d ),
whereas others use the dip direction and dip (␣ d ,
d ). Because of the construction of some field
instruments it may be more convenient to
measure and record the strike. On the other
hand, the dip direction and dip require the
identification of only one direction and that direction corresponds more directly to the inclination
of the planar element that is measured. If the dip
direction and dip were used, the number pair
(085, 37) would indicate a planar element with a
dip direction just 5Њ to the north of east and an
inclination of 37Њ in that same direction. The
degree symbol is left off for recording convenience. The number pair (355, 37), plus the righthand rule, would identify the same planar
element using strike and dip. The relations
between dip direction and strike direction are:
(2.63)
Two special cases are noteworthy. For a horizontal
planar element the dip is zero, so the strike and
dip direction are undefined and we would write
(UDF, 00) for the strike (or dip) direction and dip:
(2.64)
For a vertical planar element the dip is 90Њ, but
there are two possible strike (and dip) directions,
and either one is suitable. For example, the strike
and dip of a vertical plane with line of strike
recorded in the field as (136, 90) could equally well
be recorded as (316, 90).
Next consider the orientations of linear elements that approximate structures such as the
slickenlines in Fig. 2.14b. These also are defined
with respect to the local geographic coordinate
system composed of east, north, and up (Fig.
2.15b). The first step is to imagine a vertical plane
that contains the linear element. Position yourself over the structure at the exposure (or imagine
positioning yourself over the line segment) such
that the structure (line segment) is inclined
downward in front of you. The plunge direction is
the direction of your view in the horizontal plane.
The azimuth of the plunge direction, ␣ p , is sometimes referred to as the trend of the linear
element and it is specified using three digits. The
angle p measured in the vertical plane from the
plunge direction down to the linear element is
defined as the plunge and it is specified using two
digits.
Two angles, the plunge direction and the
plunge (␣ p , p ) are necessary to reference a linear
element to the local geographic coordinate
system and these would be recorded, for example,
as (356, 58) indicating a linear trend just a few
degrees west of north and plunging 58Њ in that
direction. Again, two special cases are noteworthy.
For a vertical linear element the plunge is 90Њ, so
the plunge direction is undefined:
(2.65)
This line would be recorded as (UDF, 90). For a horizontal element the plunge is 0Њ, so there are two
possible plunge directions and either one can be
␣ p ϭ UDF, p ϭ 90°
␣ s ϭ UDF ϭ ␣ d , d ϭ 0°
␣ d ϭ ␣ s ϩ 90°, 0° Ͻ d Յ 90°
2.3 ORIENTATIONS OF STRUCTURAL ELEMENTS
55
we would refer to the “strike of the fault,” and the
fact that the fault is not simply a plane in space
would be left unspoken. To understand what
someone means by the strike of a fault, both
the nature of the surface within the fault zone
and the size of the planar element should be
identified.
Knowing the strike of a planar element
enables one to identify the orientation of the line
of intersection of that element with a horizontal
plane. There are an infinite number of elements
that share the same strike, but vary in inclination,
so we have to define a second angle that is a
measure of this inclination in order to determine
uniquely the orientation of a particular planar
element relative to the local geographic coordinate system. This angle, d , is measured in a vertical plane that contains the dip direction, from the
dip direction to the planar element in question
(Fig. 2.15a). This angle is referred to as the dip of
the planar element that approximates the structure, and by convention two digits are used to
specify the dip. Thus, a dip of seven degrees would
be written 07Њ. Note that the angle measured from
the horizontal plane to the planar element in vertical planes of other orientations would be
smaller than the dip. Such an angle is referred to
as an apparent dip. Apparent dips are commonly
observed and measured in the field where exposures cut obliquely across structures.
Only two angles are necessary to reference a
planar element to the local geographic coordinate
system. Some use the strike and dip (␣ s , d ),
whereas others use the dip direction and dip (␣ d ,
d ). Because of the construction of some field
instruments it may be more convenient to
measure and record the strike. On the other
hand, the dip direction and dip require the
identification of only one direction and that direction corresponds more directly to the inclination
of the planar element that is measured. If the dip
direction and dip were used, the number pair
(085, 37) would indicate a planar element with a
dip direction just 5Њ to the north of east and an
inclination of 37Њ in that same direction. The
degree symbol is left off for recording convenience. The number pair (355, 37), plus the righthand rule, would identify the same planar
element using strike and dip. The relations
between dip direction and strike direction are:
(2.63)
Two special cases are noteworthy. For a horizontal
planar element the dip is zero, so the strike and
dip direction are undefined and we would write
(UDF, 00) for the strike (or dip) direction and dip:
(2.64)
For a vertical planar element the dip is 90Њ, but
there are two possible strike (and dip) directions,
and either one is suitable. For example, the strike
and dip of a vertical plane with line of strike
recorded in the field as (136, 90) could equally well
be recorded as (316, 90).
Next consider the orientations of linear elements that approximate structures such as the
slickenlines in Fig. 2.14b. These also are defined
with respect to the local geographic coordinate
system composed of east, north, and up (Fig.
2.15b). The first step is to imagine a vertical plane
that contains the linear element. Position yourself over the structure at the exposure (or imagine
positioning yourself over the line segment) such
that the structure (line segment) is inclined
downward in front of you. The plunge direction is
the direction of your view in the horizontal plane.
The azimuth of the plunge direction, ␣ p , is sometimes referred to as the trend of the linear
element and it is specified using three digits. The
angle p measured in the vertical plane from the
plunge direction down to the linear element is
defined as the plunge and it is specified using two
digits.
Two angles, the plunge direction and the
plunge (␣ p , p ) are necessary to reference a linear
element to the local geographic coordinate
system and these would be recorded, for example,
as (356, 58) indicating a linear trend just a few
degrees west of north and plunging 58Њ in that
direction. Again, two special cases are noteworthy.
For a vertical linear element the plunge is 90Њ, so
the plunge direction is undefined:
(2.65)
This line would be recorded as (UDF, 90). For a horizontal element the plunge is 0Њ, so there are two
possible plunge directions and either one can be
␣ p ϭ UDF, p ϭ 90°
␣ s ϭ UDF ϭ ␣ d , d ϭ 0°
␣ d ϭ ␣ s ϩ 90°, 0° Ͻ d Յ 90°
2.3 ORIENTATIONS OF STRUCTURAL ELEMENTS
55