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 surface of the Earth is modeled as being made up of many large tectonic plates. These plates very slowly move, owing to convection currents within the mantle below the crust. Because they do not all move in the same direction, plates often directly collide or move laterally along transform faults. Earthquakes are usually interplate earthquakes, that is, caused by the sudden movement at the edges of plates when stress is released. Intraplate earthquakes do not occur at plate boundaries, but often at fault zones ('cracks') in the middle of a plate. These often occur at the location of ancient fault zones or failed rifts due to adjustments on an old rupture.
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 , New York City in 1737 and 1884 (both quakes estimated at about 5.5 magnitude) and Charleston, SC earthquake in 1886 (estimated magnitude 6.5 to 7.3). 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).
The cause of these earthquakes is often uncertain. 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. 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. Cryoseisms can sometimes be mistaken for intraplate earthquakes.
