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An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The distribution of earthquakes on the Earth is not random, but concentrates in belts which correspond to the three existing kinds of lithospheric plate boundaries: compressional (subduction zones, continent-continent-collision), extensional (mid-ocean ridges) and transforms. In terms of seismic energy, approximately 98% are released at plate boundaries and thereof 80% originate from subduction zones, 15% from continent-continent-collisions and up to 5% from mid-ocean ridges. The distribution of seismicity worldwide from 1973 to 2007 is plotted in Figure 1, the earthquake depth is indicated by the color scale.
World seismicity map for years 1973-2007, color scale indicates depth range.
Data provided by USGS. Figure plotted using GMT.
At the Earth's surface, earthquakes manifest themselves by a shaking and sometimes displacement of 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 and occasionally volcanic activity. Seismic waves released by earthquakes are generally recorded with seismometers and analyzed to report for example the moment magnitude of an earthquake. The Richter magnitude was developed in 1935 by Charles Richter and Beno Gutenberg at the California Institute of Technology. Since it is a logarithmic scale, an increase of one magnitude implies a tenfold increase in shaking amplitude. The intensity of shaking is generally measured on the modified Mercalli scale.
In its most generic sense, the word earthquake is used to describe any seismic event - whether a natural phenomenon or an event caused by humans - that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, nuclear experiments and production/injection in hydrocarbon reservoirs. The approximate energy equivalents of earthquakes in terms of TNT explosive force can be found here.
More information about earthquakes can be found on http://www.jordskjelv.no (in Norwegian) or http://earthquake.usgs.gov/learning (in English).
At NORSAR we study the whole range of earthquakes, from acoustic emissions and microseismic events to regional earthquakes, mining blasts and nuclear explosions up to devastating earthquakes like the 2004 Great Sumatra-Andaman earthquake that generated the Boxing day tsunami.
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