READ: Earthquake Basics
Earthquake Basics
An earthquake is sudden ground movement caused by the release of energy stored in rocks. Almost all earthquakes happen at plate boundaries. Convection in Earth's interior cause the movement of tectonic plates, and as the plates move, stress builds in the rocks. When the stress becomes too much, the rocks break and release energy. The energy is transmitted as seismic waves, which we feel as an earthquake. During an earthquake the rocks usually move several centimeters.
The description of how earthquakes happen is called elastic rebound theory. Stress builds on both sides of a fault (break in the rock), causing the rocks to deform. When the stress becomes too great, the rocks break and end up in a different location. This releases energy and creates an earthquake.
1906 Earthquake in San Francisco, CA. Photo courtesy of CK-12.
Often, the greatest damage that results from an earthquake is not from the ground shaking, but from the effects of that shaking. For example, during the 1906 earthquake in San Francisco, shaking rupture gas mains and water pipes; this caused fires that were not able to be put out because there was no available water.
Liquefaction also causes large amounts of damage. Liquefaction occurs when water-saturated sandy soils are subjected to shaking from an earthquake of at least magnitude 5 on the Richter scale. The soil "liquefys" and loses its strength and the ability to support structures such as buildings, bridges and roadways. This may cause buildings to settle or topple, and landslides may also occur.