Earthquakes
An earthquake is the shaking of Earth’s crust caused by the release of energy. The cause of most
major earthquakes is the strain that builds up along faults at or near boundaries between lithospheric plates.
A fault is a break in the lithosphere along which movement has occurred.
The point at which an earthquake first occurs underground is called the focus. The epicenter is the
point on Earth’s surface directly above the focus.
The energy released in an earthquake travels in waves from the focus, called body waves. P waves or
primary waves are compressional waves that squeeze and stretch through rock materials as they pass through
Earth. S waves or secondary waves are shear waves, which cause particles of rock material to move at right
angles to the direction in which the waves are travelling. P waves can travel through any material. S waves
can travel through solid material, but not liquids or gases. P waves travel faster than S waves.
When P waves and S waves reach the surface, they cause surface waves. A love wave is a surface
wave that causes particles to move from side to side. A Rayleigh wave is a surface wave that travels slower
and cause particles of material to move in elliptical patterns such as ripples on a lake. Surface waves can
cause damage far from the epicenter.
An instrument called a seismograph detects and records earthquake waves. A seismogram is the
record sheet produced by a seismograph. The seismogram indicates the arrival time of P waves and S waves.
The difference in arrival times of the faster P waves and the slower S waves can determine the distance from
the seismograph to the epicenter. A seismograph tells the distance, but not the direction. 3 seismographs in
different locations are needed to determine the epicenter.
Seismograms can also be used to determine the strength or magnitude of an earthquake. Magnitude
is a measure of the amount of energy released by an earthquake. The Richter scale is used to measure
magnitude. A magnitude-7 earthquake is approximately 31 times more powerful than a magnitude-6 earth
quake and 961 (31 x 31) times more powerful than a magnitude-5 earthquake.
In an earthquake, ground shaking and foundation failure may cause buildings to collapse. Aftershocks,
fires and tsunamis can cause additional damage. Severe ground shaking can cause liquefaction of the loose
soil that temporarily takes on properties of a liquid, which can cause a building to collapse, because of lack of
support.
Aftershocks are a series of smaller earthquakes following a large one. Underwater earthquakes and
landslides can cause tsunamis.
http://www.discovery.com/video-topics/other/other-topics-earthquake-videos.htm
Japan Earthquake 1:44
http://www.youtube.com/watch?v=T32YvlEYS7I
New Zealand Earthquake 0:34
http://www.youtube.com/watch?v=L44AoCIl0gU
7.2 Quake Violently Shakes Water Out of Pool, California, 2010