i changed the first line as it was a poor defintion of an earthquake, the current one is more accurate but might need some more explanation. i removed this comment from the end: 
  "Comments: The build-up of stress in the tectonic plates, as the main cause for earthquakes, is a well-known modell that is constantly worked by the media. It may be wrong, especially for earthquakes with magnitude > 7 or so. The temperature in the Earth's crust increases by about 30 Degree Celsius with every kilometre depth. This has been established with deep drilling projects to a depth of about 10 kilometres. The material in the depth of the origin of an earthquake, which may well be 10, 50 or even several hundred kilometres, is thus either too hot to store an enormous amount of stress, or the material is molten to begin with. So the modell of a "build-up of stress" may be a nice one, but not the right one. The cause for large earthquakes is yet unknown." 

 beacuse it is largely inaccurate. Whilst the cause of deep focus earthquakes i.e those in the Benioff zone is unclear as it mentions in the article. this has nothing to do with the large magnitude quakes.