Intraplate earthquake
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Although the theory of plate tectonics well describes the mechanisms for interplate earthquakes (earthquakes at plate boundaries), 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 in 1812 that were above magnitude 8 and were felt for hundreds of miles. 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. Historic examples of this occurred in Boston in 1755 (the largest U.S. earthquake ever recorded east of the New Madrid fault zone; some estimates put its magnitude as high as 7.0), New York City in 1737 and 1884 (both quakes estimated at about 5.5 magnitude) and Charleston, SC in 1886 (estimated magnitude 6.5). The Charleston quake was particularly surprising because unlike Boston and New York the area had almost no history of even minor earthquakes (to put in perspective, in addition to the three northeastern U.S. events previously mentioned, a more moderate magnitude 4 earthquake was recorded just north of New York City in 1985).
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, although this is more associated with interplate earthquakes. Some progress is being made in understanding the fault mechanics driving these earthquakes
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