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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]].
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<ref>http://earthquake.usgs.gov/earthquakes/world/10_largest_world.php</ref>. 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..
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<ref>http://earthquake.usgs.gov/earthquakes/world/10_largest_world.php</ref>. 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==
'''Plate Boundaries'''
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.
'''Intra-Plate Earthquakes'''
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.
'''Vulcanism'''
The movement of magma within volcanoes can induce earthquakes. These are often an indicator of an approaching eruption, as at Mount St Helens.