Mid- or intra-plate earthquakes are those that occur within the boundaries of the major crustal plates. Most (over 90%) earthquakes occur along the tectonically active plate boundaries, where the major crustal plates are moving past and/or away from each other. However, large and damaging earthquakes can and have occurred within the plates, far from the boundaries. For example, the New Madrid earthquakes of 1811–1812 occurred well within the North American Plate. Both intra-plate and plate boundary earthquakes occur along faults, zones of crustal weakness that have experienced and/or continue to experience relative movement and deformation associated with tectonic activity. Intra-plate earthquakes are difficult to explain because they occur in the relatively stable interiors of plates. Seismic data, however, indicate that faults and fractures are very common in the upper crust of the earth. These act as zones of weakness that can be reactivated if the in situ stress field becomes favorable. Studies of the New Madrid events indicate that the earthquakes occurred along fault zones of an ancient rift system within the North American Plate.
In the winter of 1811–1812, one of the largest sequences of earthquakes in recorded history occurred within the North American Plate near the town of New Madrid, Missouri. The magnitudes of at least three of those earthquakes are estimated to have been greater than 8.0 on the Richter scale . The earthquakes were felt over an area of millions of square miles and as far away as Boston. During the quakes, large areas of land rose and large areas of land fell several feet. Lakes were created and islands disappeared. The Mississippi River was disrupted, waterfalls formed, and large waves swamped many boats. During one of the events, the Mississippi River gave the illusion of flowing backwards. In 1895, another significant but smaller earthquake occurred in this region near the town of Charleston, Missouri. That quake was felt in 23 states.
Earthquakes along plate boundaries are relatively understandable given the tectonic activity localized there. Intra-plate earthquakes are much more enigmatic. One explanation offered for the New Madrid earthquakes are the increased stresses that were induced in the region by the unloading of the ice sheets that covered much of the northern United States until about 20,000 years ago. Geological evidence indicates that large earthquakes have occurred in this region before 1811. Some modeling suggests that the stresses that caused the earthquakes may persist for thousands of years in the future.
See also Plate tectonics; Rifting and rift valleys