Download 4. Earthquakes PPT

3d - Students know why and how earthquakes
occur and the scales used to measure their
intensity and magnitude.
• We have learned that
Earth is constantly
changing.
• The surface of Earth
changes everyday due
to dynamic processes,
such as weather and
plate
tectonics.
• Remember…Plate
Tectonics states,
“Earth’s
crust is broken into
plates that move in
different directions
and at different rates”.
• The surface of Earth is in constant motion because
of forces inside the planet (heat – convection)
• These forces cause sections (plates) of Earth’s
surface to move
• This movement puts stress on the rocks near the
plate edges
•To relieve this
stress – rocks tend
to bend, compress,
or stretch
• Earthquake – vibrations produced when rock
snaps and breaks under force
Objects can only withstand so much force:
- A stick will snap when bent too far.
- A rubber band will break if pulled too tight
• Elastic limit –
maximum force an
object can withstand
before breaking
• Rocks have an elastic limit.
• Rocks will actually bend until they reach their
elastic limit.
• When rocks bend too far, they will break.
• Fault - break or crack in the rock of Earth’s crust.
• Faults form when
Earth’s plates move
and place stress on
one another…..
Eventually forcing
the crust to snap.
• Earthquakes occur when rocks
move past each other along a
fault
• Rocks have jagged edges
• The jagged edges along a fault get stuck
• Stress builds up in the stuck rock as the plates try
to move
• The rocks bend until they reach their elastic limit
and break
• The rocks snap back to their original position
(elastic rebound)
• The breaking of the rock creates an earthquake
Stress and Strain
• Stress is the forces per unit area acting on a
Material.
3 kinds of stress (forces) that act on Earth’s rocks:
1)
•
2)
•
3)
•
Compressional – pressing together
Decreases volume of material
Tensional – pulls apart
Extension of crust
Shear – sliding past
Causes rock to twist
•Fractures form when stress exceeds the strength of
the rocks involved.
•The surface along which the movement takes places
is called the fault plane.
Tensional Strain
Shear Strain
• The three types of stress create the 3 main types of faults.
•Foot wall – block of rock below the fault line
(imagine being able to walk on it)
•Hanging wall – block of rock above the fault line
(imagine being able to hang from it)
1)
•
•
•
Reverse faults
caused by compression
Hanging wall moves up
shortens the crust
2)
•
•
•
Normal faults
caused by tension
Hanging wall moves down
lengthens the crust
3) Strike-slip
• caused by shear
• horizontal offset
Reverse Fault
If hanging wall is above the
footwall it is a reverse fault!
Normal Fault
The red line marks equivalent
layers on opposite side of the
fault. Since the hanging wall
dropped relative to the footwall,
this is clearly a normal fault.
Imperial Valley, California
October 15, 1979.
Furrows in a plowed field were offset about 11 inches by a magnitude 6.5
earthquake.
Offset river system on the San Andreas!