Section 1: What is an Earthquake?
Activity #11: 
HOW TECTONIC FORCES AFFECT FAULTS
Concept: The orientation and sense of slip of faults in an
area is largely dependent upon the tectonic forces present in that area.
Materials:
Procedure:
On the tectonic scenarios page are several
examples of idealized fault-block diagrams. These diagrams are
transparent, so that the fault(s) within can be seen, but there is no
offset along the fault(s). It is your job to describe the nature of
the offset -- in terms of sense of slip -- that will result along each
fault, given a certain tectonic environment. These tectonic environments
will be described for you, next to each example, as a set of forces
working on different sides of the block.
North will always be toward the top of the diagram (and "away" from
you, since the blocks are seen at an angle). Also,
unless otherwise noted, you should assume that any force mentioned acts
upon the middle of the given face (north, south, east, or west) of the
block.
For each example, read the description of the tectonic environment,
and study the diagram. Once you have decided upon the proper
solution for the sense(s) of slip of the fault(s) depicted,
click on the image to see the correct answer. Then use the link
on the answer page to take you to the next example, and so on.
When you have finished all the examples, return to
this page, and read through the closing notes below.
If you haven't already, go now to the
tectonic scenarios page and start
working on the first example.
Closing Notes
The final example is a simple form of a horst and graben
structural environment. The words "horst" and "graben" come from
the German words for a high perch and a ditch, respectively.
Thus, the blocks that became uplands upon normal faulting
(click "Back" to inspect the diagram again if you need to)
would each be referred to as a "horst", and the down-dropped block of crust
making up the floor of the valley in between would be called a "graben".
If you imagine this fault geometry repeated over and over (diagrams like
the one shown placed edge-to-edge, along an east-west line), you would
have a terrain composed of alternating basins and mountain ranges,
similar to the Basin and Range province of the western United
States (which includes western Utah, most all of Nevada,
eastern California, and parts of Idaho, Oregon, and Arizona).
While the hypothetical horst and graben example and the Basin and Range
are both the result of extensional tectonics and produce similar
landforms, the faulting in the Basin and Range is actually dominated
by a different process, involving tilted blocks and listric
normal faults -- "listric" is a term used to describe the curved,
downward-flattening shape of these faults.
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