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Faults and earthquakes
Like most stories in geology, this one starts beneath the surface. As
you may know, the continents we live on are parts of moving plates.
Most of the action takes place where plates meet. Plates may collide,
pull apart, or scrape past each other.
All the stress and strain produced by moving plates builds up in
the Earth's rocky crust until it simply can't take it any more. All
at once, CRACK!, the rock breaks and the two rocky blocks move in
opposite directions along a more or less planar fracture surface called
The sudden movement generates an earthquake at
a point called the focus.
The energy from the earthquake spreads out as seismic
waves in all directions. The epicenter of
the earthquake is the location where seismic waves reach the surface
directly above the focus.
A normal fault. Click on diagram to see labels.
We classify faults by how the two rocky blocks on either side
of a fault move relative to each other. The one you
see here is a normal fault. A normal fault drops
rock on one side of the fault down relative
to the other side. Take a look at the side
that shows the fault and arrows indicating
movement. See the block farthest to the
right that is shaped kind of like a foot? That's the foot
Now look at the block on the other side of
the fault. See how it's resting or hanging on
top of the foot wall block? That's the hanging
Now, consider this: if we hold the foot wall
stationary, gravity will normally want
to pull the hanging wall down, right? Faults
that move the way you would expect gravity to move them normally
are called normal
faults! Not so hard, is it?
Take a look where the fault has ruptured
the Earth surface. Notice that movement
along the fault has produced an elongate cliff? That fault-generated
cliff is called
a fault scarp.
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A reverse fault. Click on diagram to see labels.
Compare this image with the normal fault above. Along a reverse fault
one rocky block is pushed up relative to rock on the other side.
Can you see the foot-shaped foot wall and
the hanging wall resting or hanging above
it? Think about this: if we hold the foot wall stationary,
where would the hanging wall go if we reversed gravity? The
hanging wall will slide upwards, right? When movement along a fault
is the reverse of
what you would expect with normal gravity we call them
Strike-slip faults have a different type of movement than normal and reverse
faults. You probably noticed that the blocks that move on either side
of a reverse or normal fault slide up or down along a dipping fault surface.
The rocky blocks on either side of strike-slip faults, on the other
hand, scrape along side-by-side. You can see in the illustration
that the movement
is horizontal and the rock layers beneath the surface
haven't been moved up or down on either side of the fault.
Take a look where the fault has ruptured the Earth surface.
Notice that pure strike-slip faults do not produce fault scarps.
There are other tell-tale changes in the landscape
that signal strike-slip faulting. As you might guess, where the two
massive blocks on either
side of a strike-slip fault grind against each other,
rock is weakened. Streams flowing across strike-slip faults are often
diverted to flow
along this weakened zone.
In real-life faulting is not such a simple picture! Usually faults do
not have purely up-and-down or side-by-side movement as we described above.
It's much more common to have some combination of fault movements
occurring together. For example, along California's famous San Andreas
strike-slip fault system, about 95% of the movement is strike-slip, but
about 5% of the movement is reverse faulting in some areas!
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