Why Do Earthquakes Happen?Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake. When two blocks of rock or two plates are rubbing against each other, they stick a little. They don't just slide smoothly; the rocks catch on each other. The rocks are still pushing against each other, but not moving. After a while, the rocks break because of all the pressure that's built up. When the rocks break, the earthquake occurs. During the earthquake and afterward, the plates or blocks of rock start moving, and they continue to move until they get stuck again. The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus (on top of the ground) is called the epicenter of the earthquake.
Earthquake-like seismic waves can also be caused by explosions underground. These explosions may be set off to break rock while making tunnels for roads, railroads, subways, or mines. These explosions, however, don't cause very strong seismic waves. You may not even feel them. Sometimes seismic waves occur when the roof or walls of a mine collapse. These can sometimes be felt by people near the mine. The largest underground explosions, from tests of nuclear warheads (bombs), can create seismic waves very much like large earthquakes. This fact has been exploited as a means to enforce the global nuclear test ban, because no nuclear warhead can be detonated on earth without producing such seismic waves.
Aftermath of EarthquakeIntense ground shaking can generate many sources of potential harm or loss. In the natural environment, such hazards include the following:
Landslides or avalanches.
Surface faulting, in which the surface of the ground along one side of a fault is displaced horizontally or vertically in relation to the ground on the other side.
Tsunamis, which can be triggered by earthquake-induced underwater landslides or by surface faulting that occurs on the floor of the ocean.
Liquefaction, in which loosely packed, water-logged soils temporarily lose strength and stiffness and behave like liquids, causing the ground to sink or slide.
Flash floods, which can be caused by liquefaction near rivers or lakes.
These hazards, as well as the ground shaking that may produce them, can also create a variety of hazards in the built environment. Buildings—or their components or contents—can be collapsed, toppled, broken apart, tossed around or rendered inoperable or unusable. The same can happen to lifeline infrastructure systems and their components, including those related to transportation, such as roads, bridges, railways, ports and airports and those related to utilities, such as distribution lines for water, waste water, electric power, telecommunications, natural gas and liquid fuels. Damage incurred among these hazards, such as broken gas or water pipes, can itself be hazardous, generating further damage by igniting fires or flooding buildings.
People can be affected in three major ways by earthquake hazards. They can be injured or killed by falling or collapsing objects, by objects thrown into the air or by earthquake-induced fires or flooding. They can incur direct economic losses, either personal or business-related, resulting from damage to existing property. Also, they can temporarily lose the ability to generate income, due to business and employment interruptions or terminations brought about by damage to private property or public infrastructure.
See more@:
http://www.geo.mtu.edu/UPSeis/why.html http://www.fema.gov/why-earthquakes-occur
