It's commonly assumed that earthquakes occur only near the boundaries of tectonic plates, and roughly 90% of earthquakes do happen in these areas. These boundaries include, for example, the San Andreas Fault, which runs roughly along the west coast of California, where the North American and Pacific plates meet.
But not all earthquakes occur along plate boundaries. For example, an earthquake near New Madrid, Missouri in the winter of 1811 was thousands of miles from the nearest fault, yet the magnitude 7.2 to 8.2 quake violently shook the region, triggering a series of powerful aftershocks collectively called the 1811-1812 New Madrid earthquakes.
