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19.1 Forces Within the Earth
Most earthquakes are the result of movement of Earth’s crust
produced by plate tectonics. Along the boundaries between two
plates, rocks in the crust often resist movement. Over time stress
builds up. Stress is the total force acting on crustal rocks per unit
of area. When stress overcomes the strength of the rock,
movement occurs along the fractures in the rock. The vibrations
caused by this sudden movement are felt as an earthquake.
There are three kinds of stress that act on Earth’s rocks:
compression, tension and shear. Compression is stress that
decreases the volume of material, tension is stress that pulls a
material apart, and shear is stress that causes a material to twist.
The deformation of materials in response to stress is strain.
Rocks can be twisted, stretched, and squeezed, but everything has a breaking point and when the stress and strain
on rock becomes to great, an earthquake happens. When stress on a rock is low, the material shows an elastic
deformation. This is when the material is compressed, bent, or stretched. Like a rubber band being stretched.
When the stress is released, the rubber band returns to its original shape. Plastic deformation is when stress
builds up past a certain point and permanent deformation occurs even when the stress is released.
Faults – A fault is a fracture or a system of fractures along which Earth moves. There are three types of
faults.
moves horizontally & vertically
because of compression
moves horizontally & vertically
because of tension
shear causes horizontal movement
Types of seismic waves
Seismic waves are vibrations produced in the ground by an earthquake. There are three types of seismic
waves.
Primary waves – also known as P-waves push and squeeze
rocks in the direction along which the waves are traveling.
The vibration passes through the ground parallel to the
direction of the movement.
Secondary waves also known as S-waves are named according
to their arrival time. These waves move slower than P-waves,
so they are the second set of waves to be felt. S-waves have a
motion that causes rocks to move at right angles in relation to
the direction of the waves.
Surface waves are the slowest of the three types of waves and
travel only along Earth’s surface. These waves can cause the
ground to move both sideways and up and down. These
waves cause the most destruction because they cause the
most movement of the ground and take the longest time to
pass.
Focus – is the point below the surface of the Earth where the waves
originate from.
Epicenter – is the point on Earth’s surface directly above the focus.