Section 19.1 Earthquakes
STRESS – is the total amount of force that is placed upon crustal rocks that can cause a
change in the rock’s shape or volume (volume means the amount of space that an object
takes up). There are 3 main types of stress that crustal rocks can be subjected to:
compression, tension, and shearing. The movement of the Earth’s tectonic plates can
cause stress to build up in rock layers.
Compression – a type of stress that pushes crustal rocks together (the volume of
rocks will decrease) Rocks will get pushed higher up and deeper
down into the crust prior to breaking. Most commonly found at
convergent plate boundaries.
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Tension – a type of stress that pulls crustal rocks apart. (it causes materials to
lengthen) Rocks will become thinner in the middle prior to breaking.
Most commonly found at divergent plate boundaries.
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Shearing – a force that pushes rocks in 2 opposite horizontal directions.
(rocks will bend and twist prior to breaking) Most commonly
found at transform plate boundaries.
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When rocks are subjected to a low amount of stress they can still retain the ability to
spring back (or rebound back) to their original shape if the stress is removed. This is
known as elastic deformation. The stress can be removed by the rocks breaking or by
blocks of the earth crust shifting. If the rocks that are being subjected to stress do break,
they will spring back to their shape, this is known as the elastic rebound theory.
Click Here: Elastic Rebound Theory animation
If rocks get subjected to too much stress they reach their elastic limit and it is known as
strain. The rocks can undergo plastic deformation which means they lose their elastic
(rebound) quality and will be permanently bent, twisted, or maybe even break. When
rock layers are bent or twisted it can create folded rock.
If the rocks actually break, or undergo failure, it can create fractures or faults in the
rock layers. More on this in a moment.
The type of rock, temperature, and pressure all influence deformation. Some rock
types are more brittle or flexible and they will bend or break more easily. A rock layer
that is at a higher temperature will become more flexible and bend more easily. And as
pressure upon rock layers increases, the rocks will require greater stress levels to reach
the elastic limit.
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The Faulting of Rock Layers
Fracture – any break or system of breaks in rock layers where there has
been no movement.
Fault – any fracture (break) or system of fractures in rock layers along which
the Earth moves.
Is this a fracture or a fault? Which part is the hanging wall? Which part is the footwall?
Parts of a fault:
Fault plane – the surface along which the movement takes place.
Hanging wall – the block of crust that is found on top of the fault plane.
Footwall – the block of crust that is found on the bottom of the fault plane.
Fault Plane
Click here for information on: Types of faults
hanging wall
hanging wall
footwall
footwall
Shearing is the type of
stress that causes this
horizontal movement.
Tension is the type of stress
that causes the hanging wall
to drop down in relation to
the footwall.
Compression is the type of
stress that causes the hanging
wall to move upward in
relation to the footwall.
Earthquake Waves:
The name given to the energy released when slippage occurs along a fault plane is
known as seismic waves. This energy is released along the fault plane at the one
specific point where the slippage first occurs which is known as the focus. Directly
above the focus on the earth’s surface is point known as the epicenter.
Fault Plane
There are 3 types of seismic waves that are produced by every earthquake. They
are the primary waves (p-waves), secondary waves (s-waves), and the surface waves.
Click here to use the: The Mighty Wave Maker
(This demonstrates how seismic waves move through material)
Click here to see:
(This demonstrates how seismic waves move through material)
siesmic wave simulations
More examples of P-waves & S-waves: P-Wave & S-wave animations