 Fault
Normal faults
Reverse faults
Strike-slip fault
Fault scarp
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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 a fault.
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. |
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| A normal fault. |
Normal fault
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
wall. 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 wall.
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. |
Reverse fault
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 reverse faults! |
 Strike-slip
fault
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. |
