An earthquake is when the Earth shakes, caused by tectonic activity in an area after the release of stored energy from the rock. Earthquakes most commonly occur on tectonic plate boundaries, and also occur along faults where previous movement has occurred. There are currently 500,000 detectable earthquakes in the world each year.
Earthquakes are a significant natural hazard to populations living in areas prone to them, and emergency responses to the hazard vary with economic development. The most powerful and destructive earthquakes occur at tectonic plate boundaries like the San Andreas Fault.
The severity of earthquakes is measured using the Moment Magnitude scale, which replaced the Richter scale in the 1970s. The largest quake ever recorded had a magnitude of 9.5; it struck Chile in 1960. Technically the Moment Magnitude scale only measures the absolute magnitiude; the Mercalli scale is used to measure the felt magnitude - essentially how much the ground shakes over the epicentre of the quake.
Causes of Earthquakes
Where tectonic plates are moving relative to each other, stress is created as friction prevents the edges of the plates from moving smoothly. When the stress has increased enough to overcome the friction, the plates will release the stored energy by moving abruptly, causing an earthquake.
Large movements of plates may cause stress zones that extend far from the boundaries. When this stress includes a weak area in the plate - known as a fault - the fault can release energy by slipping abruptly, again causing an earthquake.
The movement of magma within volcanoes can induce earthquakes. These are often an indicator of an approaching eruption, as at Mount St Helens.
Earthquakes and a young Earth
- Historical Earthquakes in China
- Latest Earthquakes in the World - Past 7 days.
- Latest Earthquakes in the North America Region.
- Geological Catalogs (Science Frontiers)