Monday, December 1, 2008

An earthquake is a natural phenomenon in which masses of rock shift below Earth's surface, releasing enormous amounts of energy and sending out shock waves, causing the surface ground to shake dramatically. Not all earthquakes are enormous, but they can become one of Earth's most destructive forces. The shock waves are called seismic waves, which are recorded with a seismograph, or seismometer. The magnitude of an earthquake is conventionally recorded on the Richter scale, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale.

Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, and nuclear experiments. An earthquake's point of initial rupture is called its hypocenter. The term epicenter refers to the point at ground level directly above this. At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides.

There are three main types of fault that may cause an earthquake: normal, reverse (thrust) and strike-slip. Normal and reverse faulting are examples of dip-slip, where the displacement along the fault is in the direction of dip and movement on them involves a vertical component. Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary. Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary. Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other ; transform boundaries are a particular type of strike-slip fault. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip.