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The San Andreas fault in California is very distinct in the Carrizo Plain east of the city of San Luis Obispo, CA. Many faults can not be seen at the Earth's surface like this.
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USGS

Why Do Earthquakes Happen?

When giant blocks of rock are moved by plate tectonics, they are bound to bump into each other. Sometimes they slide smoothly past each other along faults. Other times the blocks of rock get stuck together and can't move smoothly. That might lead to an earthquake.

There might be no movement along a fault for a long time if the blocks of rock are hitched together. However, plate tectonic force continues to push the rocks and the energy builds up over many years.

Eventually the energy is released as an earthquake. The rock breaks and moves into a new position. Vibrations called seismic waves travel outward in all directions from the point where the energy was released. These seismic waves are what people on the surface of the Earth feel when they are in an earthquake.

There are different types of seismic waves. Some rumble the ground surface. Other types of seismic waves travel through the planet.

Last modified January 19, 2010 by Randy Russell.

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The Fall 2010 issue of The Earth Scientist, focuses on rocks and minerals, including articles on minerals and mining, the use of minerals in society, and rare earth minerals, and includes 3 posters!

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