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Although the theory of plate tectonics well describes the mechanisms for interplate earthquakes (earthquakes at plate boundaries), there is the fact that very large intraplate earthquakes (earthquake within plates) can inflict heavy damage on towns and cities.
With plate tectonics the world is modeled as a collection of 'dinner plates' sliding past each other on the giant table of the earth, these are, in fact, cracked dinner plates, under high stress. Nearly all the relative motion takes place at the edges of the plates, but there are still the 'creaks and groans of an ancient crust'. At times, motions along these interior weak zones produce rather large earthquakes.
A series of famous intraplate earthquakes occurred on the New Madrid fault zone. A similar large earthquake devastated the region of Gujarat, India, in 2001, resulting in a large loss of life. Many cities in North America and elsewhere live with the seismic risk of a rare, large intraplate earthquake.
Nobody is exactly sure what causes these earthquakes. In many cases, the causative fault is deeply buried, and sometimes cannot even be found. Under these circumstances it is difficult to calculate the exact seismic hazard for a given city, especially if there was only one earthquake in historical times. An especially dangerous form of earthquake, which has been involved in many deaths is the blind thrust earthquake.
Scientists continue to search for the causes of these earthquakes, and especially for some indication of when they will strike next. The best success has come with detailed micro-seismic monitoring, involving dense arrays of seismometers. In this manner, very small earthquakes associated with a causative fault can be located with great accuracy, and in most cases these line up in patterns consistent with faulting.
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External link A very nice explanation of Intraplate earthquakes http://www2.bc.edu/~kafka/Why_Quakes/why_quakes.html | 
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