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...