STRIKE AND DIP
STRIKE AND DIPMaterials:
Procedure:
This activity is designed to familiarize you with the concepts of strike and dip, to make further studies of faults and fault models easier. In addition, you will be briefly introduced to a few key elements of seismology, which you will see again later in much greater detail.
The exercises on the Strike and Dip activity page will take you through some common examples of how strike and dip are found and applied. Before you go to the exercises, take a minute or two to study the fundamentals of strike and dip, below.
You may come back to this page at any time to review the fundamentals.
When you have completed all the exercises, use the link at the
bottom of the page to return to your place in the text.
At top right is a map view (an overhead view with north at top)
of the simplest symbol used to designate the strike and dip of a
planar geologic feature. While it is typically not
seen used in this form to denote the strike and dip of faults, it is
similar enough in most respects and so commonly used that it warrants
a brief study.
The longer line of the symbol represents the strike of the feature. You can simply measure the angle of this line right off the map to find the strike of whatever it represents. Here, that strike is 28° east of north, abbreviated N28E.
The shorter mark indicates the direction of dip. In this example, that
direction is to the southeast. The small number next to the symbol is
the angle of the dip -- 76°. Thus, the planar feature this symbol
describes can be modelled as a plane striking at N28E, and
dipping 76° to the southeast.
The top diagram at left shows a typically method for noting a fault's strike
and dip on a map. The long line is the surface trace of the fault,
drawn as it would normally be drawn on a map. The arrow coming off the
fault trace indicates the precise direction of dip. Remember that
strike and dip are always perpendicular. As shown in the bottom half
of the diagram, you can, by drawing a line perpendicular
to this dip arrow, determine the exact strike of the fault. In this
example, the strike is 64° east of north (N64E), with the dip
in a northwesterly direction.
As before, the small number next to the arrow is the value of the
angle of dip. Here, it is 83° -- this is a steeply-dipping
(near-vertical) fault. Note that the strike appears to be parallel
to the trace of the fault; this is not always the case, as you will
see below.
The figure at right shows the same symbols for marking the strike and
dip of a fault, applied to an example where the strike is obviously not
parallel to the surface trace of the fault. Other than that, everything
here is marked in the same manner as above. The dip is 52°
to the northeast, and the strike can be determined in the same manner
as before -- a line perpendicular to the dip arrow reveals that the strike
of this fault is 25° west of north, or N25W.
You may have noticed something odd about this example -- the surface trace of the fault is noticeably curved. There are two reasons this may occur, and from the map view at right, it is impossible to say which is true, here. One reason is that the fault itself may curve. In this case, the strike and dip we found above probably apply only to the immediate area along the fault at which they were marked, since at least the strike, if not the dip, will change if the fault is "bent".
Quite possibly, however, the fault is planar, but intersects the surface in an area with significant topography. Because the dip of this fault is far from vertical, the location of its intersection with the Earth's surface, and thus the appearance of its surface trace, is far more dependent upon the topography of the area. This can create some confusion, but it can also be a useful tool for identifying the direction of dip of non-vertical faults.
The method for determining the direction of dip of a fault
from its surface trace, using topography, is as follows:
Note which way the fault trace "bends" when it runs across,
in a roughly perpendicular manner,
a sudden linear change in elevation, like a stream channel or a ridge.
In such a case, the fault trace will form a crude "V" shape. When the fault
crosses a dip in elevation, the direction of the point of that "V"
is the same as the direction of dip. When the trace crosses a rise in
elevation, the opposite is true. We will see this method used in the
last example of this introduction.
The simplest case for finding a fault's strike and dip occurs when the
fault has a perfectly vertical dip. The symbol for a denoting the trace of
a fault with a vertical dip is shown at the top of the diagram at left.
A short, unlabelled mark is drawn across, and perpendicular to, the fault
trace.
Naturally, when the dip is vertical, there is no direction of dip to be specified. Since the numerical angle (90°) is obvious, it is generally not stated as a number, but simply as "vertical".
The strike of a vertical fault is also quite simple to find.
When a fault has a vertical dip, the trace of
the fault is equivalent (parallel) to the strike of the fault,
regardless of the surface topography it cuts across. Because
of this, vertical faults have very linear surface traces, and
any bends in the trace are actually bends within the fault itself
(we will look at fault bends much later in this section).
To find the strike of a vertical fault, then, simply measure the angle
its surface trace makes with a north-south line.
The map view at right shows the surface trace of a particular kind
of dipping fault -- a thrust fault. We will learn more about
these faults later, but for now, all you need to know is that the
"barbs" on the fault line indicate the direction of dip (as shown
just below the plain map view at right) and are always drawn on
the hanging wall of the fault.
The dip of any fault designated
as a thrust fault is always 45 degrees or less, but otherwise, this
symbol gives you no specific quantitative
information about the strike or dip of the fault.
Guessing the strike of a thrust fault from its trace can be very difficult, especially in an area of varied relief, unless you have a topographic contour map. The shallow dip of these faults means that their trends are affected by significant changes in elevation along strike. Only if you are looking at a thrust fault on a topographic contour map can you estimate its strike and dip in an area of high relief. This is done in the manner shown in the very bottom of the diagram, at right. This diagram shows the surface trace of a thrust fault (in black) crossing a region of significant relief. The "barbs" have been omitted from the fault trace for the sake of clarity.
The direction of dip of a thrust fault can easily be found using the direction the fault trace "bends" in response to local topography, in the manner mentioned earlier -- here the "V"s associated with stream channels point upstream (to the northeast), so this fault dips to the northeast, but we can double-check the truth of this claim by finding the strike.
To find the strike of a fault, mark all the points at which the fault crosses a particular contour line. Then, connect all these points with a single straight line. Here, that line strikes at N72W, which is compatible with a northeast dip. This should be the strike of the fault -- remember that the definition of strike is the line formed by the intersection of a (fault) plane with a horizontal plane. All the points of intersection you marked should lie on both the fault plane and a single horizontal plane, and so they should define the strike. To make sure, you can do this with a different contour line to double check the accuracy of the original.
Drawing another line of strike can also help you to find the angle of
the dip of the fault. Measure the horizontal distance between the
two lines of strike drawn. Here, it is 450 meters. The contour interval
here is 250 meters (as shown by the scale). This is the vertical separation
of these two lines. Since both lines are on the fault plane, calculating
the slope defined by these two distances yields the angle of dip for
the fault. In our example, you will find that the angle of dip
is roughly 29°.
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